(I posted this in comp.dsp a couple of weeks ago, but most of the actual
DSP people seem to have disappeared, so the discussion petered out
fairly fast. Trying again here.)

Hi, all,

Hoping there are still some DSP folks round here despite the evil Google
ban. (But I repeat myself.)

I'm working on a completely noninvasive sensor for fetal blood oxygen,
using optical sensing through the mom's abdomen. It's a very low SNR
measurement on account of all the attenuation.

The mom's heartbeat modulates her pulse-ox signal, which is much
stronger than the fetus's on account of the scattering and absorption in
maternal tissue.

The data are several time series. The main issue is the variability of
both pulses, which smear out the spectra and therefore knock the peak
heights way down towards the noise. There are weak multiplicative
effects between maternal and fetal signals, as you'd expect.

What I'm looking to do is something like:

1. Use a digital PLL to find the time-dependent maternal pulse rate.

2. Resample the data accordingly, and notch out the first 5 or so mom
harmonics.

3. Do the PLL thing on the fetal pulse, and signal average to pull out
the average fetal pulse ox signal.

Extra credit: sometimes the baby's pulse can cross the first or second
harmonic of the mom's, and it would be good to preserve both pulse
shapes accurately.

Resampling a noisy signal isn't necessarily the most well-conditioned
operation, so I'd welcome suggestions for just how to do this.

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:
> Resampling a noisy signal isn't necessarily the most well-conditioned
> operation, so I'd welcome suggestions for just how to do this.

Not a guru at this stuff, but two points come to mind:

1) Is it really necessary to constrain the problem to optical detection?
Given the opportunity, I'd try to use acoustical cues as well. If both the
fetal and maternal heartbeats are detectable with a microphone, perhaps
one that's integrated in the sensor right next to the optical hardware, that
might be a good source of data for cross correlation.

2) I wouldn't worry about occasional harmonic coincidence between the
fetal and maternal pulses, except to the extent needed to reject those
pulses. Why ask for trouble? You don't need every pulse, do you? The
maternal pulse is a strong signal, so I'd use it to time-gate the fetal pulse
detection process.

-- john, KE5FX

John Miles, KE5FX wrote:
> On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:
>> Resampling a noisy signal isn't necessarily the most
>> well-conditioned operation, so I'd welcome suggestions for just how
>> to do this.
>
> Not a guru at this stuff, but two points come to mind:
>
> 1) Is it really necessary to constrain the problem to optical
> detection? Given the opportunity, I'd try to use acoustical cues as
> well.

That's how it's done today, an ultrasound technique called
cardiocotography. It's very sensitive, but not very selective--fetal
heart rate responds to a very wide range of effects unrelated to
respiratory distress.

The result is a lot of unnecessary C-sections, with serious health
consequences to both mums and babies.

We really need to measure differential optical absorption in the child's
blood to get its O2 sat level.

> If both the fetal and maternal heartbeats are detectable with a
> microphone, perhaps one that's integrated in the sensor right next to
> the optical hardware, that might be a good source of data for cross
> correlation.
>
> 2) I wouldn't worry about occasional harmonic coincidence between
> the fetal and maternal pulses, except to the extent needed to reject
> those pulses. Why ask for trouble? You don't need every pulse, do
> you? The maternal pulse is a strong signal, so I'd use it to
> time-gate the fetal pulse detection process.

It's a serious problem for signal detection. The pulse rates are only
1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

søndag den 6. juni 2021 kl. 20.45.47 UTC+2 skrev Phil Hobbs:
> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
> DSP people seem to have disappeared, so the discussion petered out
> fairly fast. Trying again here.)
>
> Hi, all,
>
> Hoping there are still some DSP folks round here despite the evil Google
> ban. (But I repeat myself.)
>
> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> using optical sensing through the mom's abdomen. It's a very low SNR
> measurement on account of all the attenuation.
>
> The mom's heartbeat modulates her pulse-ox signal, which is much
> stronger than the fetus's on account of the scattering and absorption in
> maternal tissue.
>
> The data are several time series. The main issue is the variability of
> both pulses, which smear out the spectra and therefore knock the peak
> heights way down towards the noise. There are weak multiplicative
> effects between maternal and fetal signals, as you'd expect.
>
> What I'm looking to do is something like:
>
> 1. Use a digital PLL to find the time-dependent maternal pulse rate.

can't you get the maternal pulse rate from a regular pulse rate monitor?

On Sunday, June 6, 2021 at 1:04:45 PM UTC-7, Phil Hobbs wrote:
> That's how it's done today, an ultrasound technique called
> cardiocotography. It's very sensitive, but not very selective--fetal
> heart rate responds to a very wide range of effects unrelated to
> respiratory distress.

So there's unlikely to be a way to cross-correlate results from both?
Never mind ultrasound, if you hold an ordinary microphone to the
mother's abdomen, maybe with some conduction gel, wouldn't you
get both sounds? Obviously they'll be at or near the noise floor
but there might be enough of a signal to enhance the SNR in the
photodetection process you're running at the same time.

> It's a serious problem for signal detection. The pulse rates are only
> 1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.

Exactly, which is why multispectral sensing comes to mind.

-- john, KE5FX

Lasse Langwadt Christensen wrote:
> søndag den 6. juni 2021 kl. 20.45.47 UTC+2 skrev Phil Hobbs:
>> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
>> DSP people seem to have disappeared, so the discussion petered out
>> fairly fast. Trying again here.)
>>
>> Hi, all,
>>
>> Hoping there are still some DSP folks round here despite the evil Google
>> ban. (But I repeat myself.)
>>
>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>> using optical sensing through the mom's abdomen. It's a very low SNR
>> measurement on account of all the attenuation.
>>
>> The mom's heartbeat modulates her pulse-ox signal, which is much
>> stronger than the fetus's on account of the scattering and absorption in
>> maternal tissue.
>>
>> The data are several time series. The main issue is the variability of
>> both pulses, which smear out the spectra and therefore knock the peak
>> heights way down towards the noise. There are weak multiplicative
>> effects between maternal and fetal signals, as you'd expect.
>>
>> What I'm looking to do is something like:
>>
>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>
> can't you get the maternal pulse rate from a regular pulse rate monitor?
>
>
Sure. We get it for free anyway.

The measurement scheme is to use two or three LED wavelengths, and
interrogate the optical intensity field inside the child's head using a
few photodetectors spaced out along a line.

In a lossless but highly scattering medium, the optical intensity obeys
Laplace's equation, so the typical penetration depth you interrogate is
of the order of the separation of the source and detector.

The detectors closest to the LEDs thus get almost pure maternal signal.
One can do adaptive noise cancellation (ANC) tricks by taking the
combined fetal/maternal signal from the more distant detectors and
subtracting off an adjustable amount of the maternal signal until the
cross-correlation is minimized. Folks have been doing that for probably
20 years, but no commercial instrument has resulted.

So what I want to do is to concentrate all of the fetal pulse ox signal
into a single frequency bin so as to maximize the SNR. That's what all
the resampling is about.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

John Miles, KE5FX wrote:
> On Sunday, June 6, 2021 at 1:04:45 PM UTC-7, Phil Hobbs wrote:
>> That's how it's done today, an ultrasound technique called
>> cardiocotography. It's very sensitive, but not very selective--fetal
>> heart rate responds to a very wide range of effects unrelated to
>> respiratory distress.
>
> So there's unlikely to be a way to cross-correlate results from both?
> Never mind ultrasound, if you hold an ordinary microphone to the
> mother's abdomen, maybe with some conduction gel, wouldn't you
> get both sounds? Obviously they'll be at or near the noise floor
> but there might be enough of a signal to enhance the SNR in the
> photodetection process you're running at the same time.

The acoustic signal doesn't carry the O2 saturation information that we
care about. The acoustic properties don't change measurably with O2
saturation.

>
>> It's a serious problem for signal detection. The pulse rates are only
>> 1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.
>
> Exactly, which is why multispectral sensing comes to mind.

Not sure what you mean there.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>(I posted this in comp.dsp a couple of weeks ago, but most of the actual
>DSP people seem to have disappeared, so the discussion petered out
>fairly fast. Trying again here.)
>
>Hi, all,
>
>Hoping there are still some DSP folks round here despite the evil Google
>ban. (But I repeat myself.)
>
>I'm working on a completely noninvasive sensor for fetal blood oxygen,
>using optical sensing through the mom's abdomen. It's a very low SNR
>measurement on account of all the attenuation.
>
>The mom's heartbeat modulates her pulse-ox signal, which is much
>stronger than the fetus's on account of the scattering and absorption in
>maternal tissue.
>
>The data are several time series. The main issue is the variability of
>both pulses, which smear out the spectra and therefore knock the peak
>heights way down towards the noise. There are weak multiplicative
>effects between maternal and fetal signals, as you'd expect.
>
>What I'm looking to do is something like:
>
>1. Use a digital PLL to find the time-dependent maternal pulse rate.
>
>2. Resample the data accordingly, and notch out the first 5 or so mom
>harmonics.
>
>3. Do the PLL thing on the fetal pulse, and signal average to pull out
>the average fetal pulse ox signal.
>
>Extra credit: sometimes the baby's pulse can cross the first or second
>harmonic of the mom's, and it would be good to preserve both pulse
>shapes accurately.
>
>Resampling a noisy signal isn't necessarily the most well-conditioned
>operation, so I'd welcome suggestions for just how to do this.
>
>Thanks
>
>Phil Hobbs

How long do you plan to sample for? And do you expect either pulse
rate to change much during that time?

I'm thinking about saving a lot of samples and doing a sort of
adpative tracking correlation on the saved data, essentially locking
to a changing non-sine signal. If you do the dominant mom signal
first, you could subtract it from the baby signal.

Several whacko time-domain algorithms come to mind. Not FFT based,
since the frequencies may drift, but there might be a freq domain
equivalent.

If you were here, we could try my new Amazon whiteboard markers.

Imagine a polynomial that essentially time modulates the saved
samples. Now do a polynomial curve fit to maximize the signal
autocorrelation. Use that to subtract the time drift, and then
subtract out the mom waveform. Then do the babywave.

Or something even worse.

One of our scopes can do a software PLL on stored samples to recover
the clock and payload of serial data.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sun, 6 Jun 2021 16:04:32 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Miles, KE5FX wrote:
>> On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:
>>> Resampling a noisy signal isn't necessarily the most
>>> well-conditioned operation, so I'd welcome suggestions for just how
>>> to do this.
>>
>> Not a guru at this stuff, but two points come to mind:
>>
>> 1) Is it really necessary to constrain the problem to optical
>> detection? Given the opportunity, I'd try to use acoustical cues as
>> well.
>
>That's how it's done today, an ultrasound technique called
>cardiocotography. It's very sensitive, but not very selective--fetal
>heart rate responds to a very wide range of effects unrelated to
>respiratory distress.
>
>The result is a lot of unnecessary C-sections, with serious health
>consequences to both mums and babies.
>
>We really need to measure differential optical absorption in the child's
>blood to get its O2 sat level.
>
>> If both the fetal and maternal heartbeats are detectable with a
>> microphone, perhaps one that's integrated in the sensor right next to
>> the optical hardware, that might be a good source of data for cross
>> correlation.
>>
>> 2) I wouldn't worry about occasional harmonic coincidence between
>> the fetal and maternal pulses, except to the extent needed to reject
>> those pulses. Why ask for trouble? You don't need every pulse, do
>> you? The maternal pulse is a strong signal, so I'd use it to
>> time-gate the fetal pulse detection process.
>
>It's a serious problem for signal detection. The pulse rates are only
>1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.
>
>Thanks
>
>Phil Hobbs

Suppose you poke a needle into mom's belly, with some sort of
fiberoptical things. Stop just short of the baby's hide.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

jlarkin@highlandsniptechnology.com wrote:
> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
>> DSP people seem to have disappeared, so the discussion petered out
>> fairly fast. Trying again here.)
>>
>> Hi, all,
>>
>> Hoping there are still some DSP folks round here despite the evil Google
>> ban. (But I repeat myself.)
>>
>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>> using optical sensing through the mom's abdomen. It's a very low SNR
>> measurement on account of all the attenuation.
>>
>> The mom's heartbeat modulates her pulse-ox signal, which is much
>> stronger than the fetus's on account of the scattering and absorption in
>> maternal tissue.
>>
>> The data are several time series. The main issue is the variability of
>> both pulses, which smear out the spectra and therefore knock the peak
>> heights way down towards the noise. There are weak multiplicative
>> effects between maternal and fetal signals, as you'd expect.
>>
>> What I'm looking to do is something like:
>>
>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>>
>> 2. Resample the data accordingly, and notch out the first 5 or so mom
>> harmonics.
>>
>> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
>> the average fetal pulse ox signal.
>>
>> Extra credit: sometimes the baby's pulse can cross the first or second
>> harmonic of the mom's, and it would be good to preserve both pulse
>> shapes accurately.
>>
>> Resampling a noisy signal isn't necessarily the most well-conditioned
>> operation, so I'd welcome suggestions for just how to do this.
>>
>> Thanks
>>
>> Phil Hobbs
>
> How long do you plan to sample for? And do you expect either pulse
> rate to change much during that time?

Anywhere from 10 minutes to some hours, depending if it's an office
visit or childbirth. The rates go all over the place--2:1 is not unknown.

> I'm thinking about saving a lot of samples and doing a sort of
> adpative tracking correlation on the saved data, essentially locking
> to a changing non-sine signal. If you do the dominant mom signal
> first, you could subtract it from the baby signal.

Yup, that's the plan. My current tentative scheme is to use a DPLL to
measure the instantaneous frequency and resample to make it appear
harmonic. Not entirely sure about the variation of pulse shape with
pulse rate--it might be invariant, proportional, or some tertium quid.

>
> Several whacko time-domain algorithms come to mind. Not FFT based,
> since the frequencies may drift, but there might be a freq domain
> equivalent.
>
> If you were here, we could try my new Amazon whiteboard markers.

That would be fun. Maybe later this summer.
> Imagine a polynomial that essentially time modulates the saved
> samples. Now do a polynomial curve fit to maximize the signal
> autocorrelation.

OK, sort of an implicit version of the DPLL/resampling thing.

Use that to subtract the time drift, and then
> subtract out the mom waveform. Then do the babywave.

To get good enough rejection for cases where the fundamental or first
few harmonics collide, we might do something like bootstrap statistics,
where you use random selection with replacement to generate many
synthetic distributions from your single observed one. Picking times
when the phase relations are favourable might make it easier.

I'd sort of like to generate ensemble-averaged pulse waveforms and use
them to strobe the input data. That ought to be be good for 5-10 dB of
SNR in tough situations.

>
> Or something even worse.

In the words of Arlo Guthrie, "I'm not proud. Or tired." (Just wait for
it to come around again on the gee-tar.)
>
> One of our scopes can do a software PLL on stored samples to recover
> the clock and payload of serial data.

For a 2-Hz signal, a Cortex M33 ought to be able to do amazing things.
The question is, what things exactly?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

jlarkin@highlandsniptechnology.com wrote:
> On Sun, 6 Jun 2021 16:04:32 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> John Miles, KE5FX wrote:
>>> On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:
>>>> Resampling a noisy signal isn't necessarily the most
>>>> well-conditioned operation, so I'd welcome suggestions for just
>>>> how to do this.
>>>
>>> Not a guru at this stuff, but two points come to mind:
>>>
>>> 1) Is it really necessary to constrain the problem to optical
>>> detection? Given the opportunity, I'd try to use acoustical cues
>>> as well.
>>
>> That's how it's done today, an ultrasound technique called
>> cardiocotography. It's very sensitive, but not very
>> selective--fetal heart rate responds to a very wide range of
>> effects unrelated to respiratory distress.
>>
>> The result is a lot of unnecessary C-sections, with serious health
>> consequences to both mums and babies.
>>
>> We really need to measure differential optical absorption in the
>> child's blood to get its O2 sat level.
>>
>>> If both the fetal and maternal heartbeats are detectable with a
>>> microphone, perhaps one that's integrated in the sensor right
>>> next to the optical hardware, that might be a good source of data
>>> for cross correlation.
>>>
>>> 2) I wouldn't worry about occasional harmonic coincidence
>>> between the fetal and maternal pulses, except to the extent
>>> needed to reject those pulses. Why ask for trouble? You don't
>>> need every pulse, do you? The maternal pulse is a strong signal,
>>> so I'd use it to time-gate the fetal pulse detection process.
>>
>> It's a serious problem for signal detection. The pulse rates are
>> only 1-2 Hz, so you don't get that many pulses in a measurement of
>> a few minutes.
>>

>
> Suppose you poke a needle into mom's belly, with some sort of
> fiberoptical things. Stop just short of the baby's hide.

During childbirth, that's done already--you can get a good measurement
as long as the cervix is dilated by 2 cm or so.

Amniocentesis does the same sort of thing, but results in a ~1% fatality
rate for the baby IIRC, so you wouldn't want to do that routinely.

(BITD Mo and I decided against amniocentesis, as we weren't going to
abort regardless.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 6/6/2021 4:50 PM, Phil Hobbs wrote:
> jlarkin@highlandsniptechnology.com wrote:
>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
>>> DSP people seem to have disappeared, so the discussion petered out
>>> fairly fast.  Trying again here.)
>>>
>>> Hi, all,
>>>
>>> Hoping there are still some DSP folks round here despite the evil Google
>>> ban. (But I repeat myself.)
>>>
>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>> using optical sensing through the mom's abdomen.  It's a very low SNR
>>> measurement on account of all the attenuation.
>>>
>>> The mom's heartbeat modulates her pulse-ox signal, which is much
>>> stronger than the fetus's on account of the scattering and absorption in
>>> maternal tissue.
>>>
>>> The data are several time series.  The main issue is the variability of
>>> both pulses, which smear out the spectra and therefore knock the peak
>>> heights way down towards the noise.  There are weak multiplicative
>>> effects between maternal and fetal signals, as you'd expect.
>>>
>>> What I'm looking to do is something like:
>>>
>>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>>>
>>> 2. Resample the data accordingly, and notch out the first 5 or so mom
>>> harmonics.
>>>
>>> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
>>> the average fetal pulse ox signal.
>>>
>>> Extra credit: sometimes the baby's pulse can cross the first or second
>>> harmonic of the mom's, and it would be good to preserve both pulse
>>> shapes accurately.
>>>
>>> Resampling a noisy signal isn't necessarily the most well-conditioned
>>> operation, so I'd welcome suggestions for just how to do this.
>>>
>>> Thanks
>>>
>>> Phil Hobbs
>>
>> How long do you plan to sample for? And do you expect either pulse
>> rate to change much during that time?
>
> Anywhere from 10 minutes to some hours, depending if it's an office
> visit or childbirth.  The rates go all over the place--2:1 is not unknown.
>
>> I'm thinking about saving a lot of samples and doing a sort of
>> adpative tracking correlation on the saved data, essentially locking
>> to a changing non-sine signal. If you do the dominant mom signal
>> first, you could subtract it from the baby signal.
>
> Yup, that's the plan.  My current tentative scheme is to use a DPLL to
> measure the instantaneous frequency and resample to make it appear
> harmonic.  Not entirely sure about the variation of pulse shape with
> pulse rate--it might be invariant, proportional, or some tertium quid.
>
>>
>> Several whacko time-domain algorithms come to mind. Not FFT based,
>> since the frequencies may drift, but there might be a freq domain
>> equivalent.
>>
>> If you were here, we could try my new Amazon whiteboard markers.
>
> That would be fun.  Maybe later this summer.
>> Imagine a polynomial that essentially time modulates the saved
>> samples. Now do a polynomial curve fit to maximize the signal
>> autocorrelation.
>
> OK, sort of an implicit version of the DPLL/resampling thing.
>
> Use that to subtract the time drift, and then
>> subtract out the mom waveform. Then do the babywave.
>
> To get good enough rejection for cases where the fundamental or first
> few harmonics collide, we might do something like bootstrap statistics,
> where you use random selection with replacement to generate many
> synthetic distributions from your single observed one.  Picking times
> when the phase relations are favourable might make it easier.

This is a paper about inferring behavioral patterns from noisy data from
e.g. radio trackers attached to sea creatures, but might be relevant to
the current problem (sorry, couldn't find non-paywalled version):

<https://link.springer.com/article/10.1007/s13253-019-00366-2>

On 6/6/2021 5:03 PM, bitrex wrote:
> On 6/6/2021 4:50 PM, Phil Hobbs wrote:
>> jlarkin@highlandsniptechnology.com wrote:
>>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> (I posted this in comp.dsp a couple of weeks ago, but most of the
>>>> actual
>>>> DSP people seem to have disappeared, so the discussion petered out
>>>> fairly fast.  Trying again here.)
>>>>
>>>> Hi, all,
>>>>
>>>> Hoping there are still some DSP folks round here despite the evil
>>>> Google
>>>> ban. (But I repeat myself.)
>>>>
>>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>>> using optical sensing through the mom's abdomen.  It's a very low SNR
>>>> measurement on account of all the attenuation.
>>>>
>>>> The mom's heartbeat modulates her pulse-ox signal, which is much
>>>> stronger than the fetus's on account of the scattering and
>>>> absorption in
>>>> maternal tissue.
>>>>
>>>> The data are several time series.  The main issue is the variability of
>>>> both pulses, which smear out the spectra and therefore knock the peak
>>>> heights way down towards the noise.  There are weak multiplicative
>>>> effects between maternal and fetal signals, as you'd expect.
>>>>
>>>> What I'm looking to do is something like:
>>>>
>>>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>>>>
>>>> 2. Resample the data accordingly, and notch out the first 5 or so mom
>>>> harmonics.
>>>>
>>>> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
>>>> the average fetal pulse ox signal.
>>>>
>>>> Extra credit: sometimes the baby's pulse can cross the first or second
>>>> harmonic of the mom's, and it would be good to preserve both pulse
>>>> shapes accurately.
>>>>
>>>> Resampling a noisy signal isn't necessarily the most well-conditioned
>>>> operation, so I'd welcome suggestions for just how to do this.
>>>>
>>>> Thanks
>>>>
>>>> Phil Hobbs
>>>
>>> How long do you plan to sample for? And do you expect either pulse
>>> rate to change much during that time?
>>
>> Anywhere from 10 minutes to some hours, depending if it's an office
>> visit or childbirth.  The rates go all over the place--2:1 is not
>> unknown.
>>
>>> I'm thinking about saving a lot of samples and doing a sort of
>>> adpative tracking correlation on the saved data, essentially locking
>>> to a changing non-sine signal. If you do the dominant mom signal
>>> first, you could subtract it from the baby signal.
>>
>> Yup, that's the plan.  My current tentative scheme is to use a DPLL to
>> measure the instantaneous frequency and resample to make it appear
>> harmonic.  Not entirely sure about the variation of pulse shape with
>> pulse rate--it might be invariant, proportional, or some tertium quid.
>>
>>>
>>> Several whacko time-domain algorithms come to mind. Not FFT based,
>>> since the frequencies may drift, but there might be a freq domain
>>> equivalent.
>>>
>>> If you were here, we could try my new Amazon whiteboard markers.
>>
>> That would be fun.  Maybe later this summer.
>>> Imagine a polynomial that essentially time modulates the saved
>>> samples. Now do a polynomial curve fit to maximize the signal
>>> autocorrelation.
>>
>> OK, sort of an implicit version of the DPLL/resampling thing.
>>
>> Use that to subtract the time drift, and then
>>> subtract out the mom waveform. Then do the babywave.
>>
>> To get good enough rejection for cases where the fundamental or first
>> few harmonics collide, we might do something like bootstrap
>> statistics, where you use random selection with replacement to
>> generate many synthetic distributions from your single observed one.
>> Picking times when the phase relations are favourable might make it
>> easier.
>
>
> This is a paper about inferring behavioral patterns from noisy data from
> e.g. radio trackers attached to sea creatures, but might be relevant to
> the current problem (sorry, couldn't find non-paywalled version):
>
> <https://link.springer.com/article/10.1007/s13253-019-00366-2>
>

Sounds like the application there is they're trying to figure out which
"small signal" movements of the seal imply it's resting or mating or
feeding but the position sensor also records the seal being shoved
around by the tides and currents which is a larger, but irrelevant signal

On Sun, 6 Jun 2021 16:04:32 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Miles, KE5FX wrote:
>> On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:
>>> Resampling a noisy signal isn't necessarily the most
>>> well-conditioned operation, so I'd welcome suggestions for just how
>>> to do this.
>>
>> Not a guru at this stuff, but two points come to mind:
>>
>> 1) Is it really necessary to constrain the problem to optical
>> detection? Given the opportunity, I'd try to use acoustical cues as
>> well.
>
>That's how it's done today, an ultrasound technique called
>cardiocotography. It's very sensitive, but not very selective--fetal
>heart rate responds to a very wide range of effects unrelated to
>respiratory distress.
>
>The result is a lot of unnecessary C-sections, with serious health
>consequences to both mums and babies.
>
>We really need to measure differential optical absorption in the child's
>blood to get its O2 sat level.
>
>> If both the fetal and maternal heartbeats are detectable with a
>> microphone, perhaps one that's integrated in the sensor right next to
>> the optical hardware, that might be a good source of data for cross
>> correlation.
>>
>> 2) I wouldn't worry about occasional harmonic coincidence between
>> the fetal and maternal pulses, except to the extent needed to reject
>> those pulses. Why ask for trouble? You don't need every pulse, do
>> you? The maternal pulse is a strong signal, so I'd use it to
>> time-gate the fetal pulse detection process.
>
>It's a serious problem for signal detection. The pulse rates are only
>1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.

As I recall, there are math models (more complicated than a single
polynomial) of the pulse EKG waveforms.

It ought to be possible to have a pair of windowed batch processors
that lock onto the two EKGs independently. Probably one would start
with the mother's EGG, and then subtract that and look in the residue
for the baby's EKG.

Given that one is looking for full waveforms, one will be able to fit
with many samples per heartbeat cycle, sidestepping the 1 to 2 Hz
issue.

Joe Gwinn

On Sunday, June 6, 2021 at 11:45:47 AM UTC-7, Phil Hobbs wrote:

> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> using optical sensing through the mom's abdomen. It's a very low SNR
> measurement on account of all the attenuation.
>
> The mom's heartbeat modulates her pulse-ox signal, which is much
> stronger than the fetus's on account of the scattering and absorption in
> maternal tissue.
>
> The data are several time series. The main issue is the variability of
> both pulses, which smear out the spectra and therefore knock the peak
> heights way down towards the noise. There are weak multiplicative
> effects between maternal and fetal signals, as you'd expect.
>
> What I'm looking to do is something like:
>
> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>
> 2. Resample the data accordingly, and notch out the first 5 or so mom
> harmonics.
>
> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
> the average fetal pulse ox signal.

The quick/dirty way would be to consider only the two fundamentals,
run two PLLs with quadrature (sine and cosine) outputs. Then
do the multiply-accumulate of two components of mother signal
and two components of fetal signal, and hope they don't synchronize
too often. Getting more harmonics than one, while useful, is a
less quick way to get the project wrapped up.

Two harmonics? Six? No obvious right number, to my knowledge.

On 2021-06-06, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
> jlarkin@highlandsniptechnology.com wrote:
>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>> using optical sensing through the mom's abdomen. It's a very low SNR
>>> measurement on account of all the attenuation.

Interesting article on racial bias in regular pulse oximeters:
https://www.wired.com/story/pulse-oximeters-equity/

--
Jasen.

mandag den 7. juni 2021 kl. 03.00.56 UTC+2 skrev Jasen Betts:
> On 2021-06-06, Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
> > jla...@highlandsniptechnology.com wrote:
> >> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
> >> <pcdhSpamM...@electrooptical.net> wrote:
> >>
> >>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> >>> using optical sensing through the mom's abdomen. It's a very low SNR
> >>> measurement on account of all the attenuation.
> Interesting article on racial bias in regular pulse oximeters:
> https://www.wired.com/story/pulse-oximeters-equity/
>

they make it sound like they were intentionally designed to not work as well on dark skin.
I guess that gets more clicks

On 6/6/2021 8:34 PM, Jasen Betts wrote:
> On 2021-06-06, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> jlarkin@highlandsniptechnology.com wrote:
>>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>>> using optical sensing through the mom's abdomen. It's a very low SNR
>>>> measurement on account of all the attenuation.
>
> Interesting article on racial bias in regular pulse oximeters:
> https://www.wired.com/story/pulse-oximeters-equity/
>
>

Hmm, so stuff really was better in the old days. But it sounds like it's
a skin-tone bias, not a "racial" bias per se.

There is or used to be some exhibit at the Museum of Science in Boston
that incidentally returned your "race" according to your Fitzpatrick
skin type determined by a similar kind of measurement. During the summer
when I've got a farmer's tan it repeatedly returned "Middle Eastern"
despite that I'm not the slightest bit Middle Eastern.

and all AI-based systems to determine a person's "race" from physical
characteristics will make the same kind of gross errors from time to
time, guaranteed, because the concept is bunkum, the "artificial
intelligence" is only a mechanical Turk for returning the biases of
their designers. Unfortunately like polygraphs it is something the
government spends money on, anyway...

On Sun, 6 Jun 2021 16:50:43 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>jlarkin@highlandsniptechnology.com wrote:
>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
>>> DSP people seem to have disappeared, so the discussion petered out
>>> fairly fast. Trying again here.)
>>>
>>> Hi, all,
>>>
>>> Hoping there are still some DSP folks round here despite the evil Google
>>> ban. (But I repeat myself.)
>>>
>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>> using optical sensing through the mom's abdomen. It's a very low SNR
>>> measurement on account of all the attenuation.
>>>
>>> The mom's heartbeat modulates her pulse-ox signal, which is much
>>> stronger than the fetus's on account of the scattering and absorption in
>>> maternal tissue.
>>>
>>> The data are several time series. The main issue is the variability of
>>> both pulses, which smear out the spectra and therefore knock the peak
>>> heights way down towards the noise. There are weak multiplicative
>>> effects between maternal and fetal signals, as you'd expect.
>>>
>>> What I'm looking to do is something like:
>>>
>>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
>>>
>>> 2. Resample the data accordingly, and notch out the first 5 or so mom
>>> harmonics.
>>>
>>> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
>>> the average fetal pulse ox signal.
>>>
>>> Extra credit: sometimes the baby's pulse can cross the first or second
>>> harmonic of the mom's, and it would be good to preserve both pulse
>>> shapes accurately.
>>>
>>> Resampling a noisy signal isn't necessarily the most well-conditioned
>>> operation, so I'd welcome suggestions for just how to do this.
>>>
>>> Thanks
>>>
>>> Phil Hobbs
>>
>> How long do you plan to sample for? And do you expect either pulse
>> rate to change much during that time?
>
>Anywhere from 10 minutes to some hours, depending if it's an office
>visit or childbirth. The rates go all over the place--2:1 is not unknown.
>
>> I'm thinking about saving a lot of samples and doing a sort of
>> adpative tracking correlation on the saved data, essentially locking
>> to a changing non-sine signal. If you do the dominant mom signal
>> first, you could subtract it from the baby signal.
>
>Yup, that's the plan. My current tentative scheme is to use a DPLL to
>measure the instantaneous frequency and resample to make it appear
>harmonic. Not entirely sure about the variation of pulse shape with
>pulse rate--it might be invariant, proportional, or some tertium quid.
>
>>
>> Several whacko time-domain algorithms come to mind. Not FFT based,
>> since the frequencies may drift, but there might be a freq domain
>> equivalent.
>>
>> If you were here, we could try my new Amazon whiteboard markers.
>
>That would be fun. Maybe later this summer.
>> Imagine a polynomial that essentially time modulates the saved
>> samples. Now do a polynomial curve fit to maximize the signal
>> autocorrelation.
>
>OK, sort of an implicit version of the DPLL/resampling thing.

Mom's rate vs time should be obvious. Use that as the first whack at
the polynomial (or whatever function) rubber-bands the sample timing.
Then fine tune.

I'd imagine that unexpected events could make mom's pulse rate jump
around.

I remember a certain lady screaming at the nurse GIVE ME DRUGS YOU
BITCH.

The nurse said she was used to that sort of thing.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On 6/6/2021 9:18 PM, Lasse Langwadt Christensen wrote:
> mandag den 7. juni 2021 kl. 03.00.56 UTC+2 skrev Jasen Betts:
>> On 2021-06-06, Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
>>> jla...@highlandsniptechnology.com wrote:
>>>> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
>>>> <pcdhSpamM...@electrooptical.net> wrote:
>>>>
>>>>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
>>>>> using optical sensing through the mom's abdomen. It's a very low SNR
>>>>> measurement on account of all the attenuation.
>> Interesting article on racial bias in regular pulse oximeters:
>> https://www.wired.com/story/pulse-oximeters-equity/
>>
>
> they make it sound like they were intentionally designed to not work as well on dark skin.
> I guess that gets more clicks
>

Depends on how you interpret the phrase "built in", I don't think that
automatically implies it was done intentionally, the text certainly
doesn't as far as I can tell.

It's a colloquial usage like "built to fail" if it were an article about
that early airliner with square windows, obviously its designers didn't
intend that.

On Monday, June 7, 2021 at 6:24:23 AM UTC+10, Phil Hobbs wrote:
> John Miles, KE5FX wrote:
> > On Sunday, June 6, 2021 at 1:04:45 PM UTC-7, Phil Hobbs wrote:
> >> That's how it's done today, an ultrasound technique called
> >> cardiocotography. It's very sensitive, but not very selective--fetal
> >> heart rate responds to a very wide range of effects unrelated to
> >> respiratory distress.
> >
> > So there's unlikely to be a way to cross-correlate results from both?
> > Never mind ultrasound, if you hold an ordinary microphone to the
> > mother's abdomen, maybe with some conduction gel, wouldn't you
> > get both sounds? Obviously they'll be at or near the noise floor
> > but there might be enough of a signal to enhance the SNR in the
> > photodetection process you're running at the same time.
>
> The acoustic signal doesn't carry the O2 saturation information that we care about. The acoustic properties don't change measurably with O2 saturation.

Sure. But the O2 saturation information for maternal blood is going to be modulated by the mother's heart beat, so you can use the mother's heart rate to subtract it out.

The O2 saturation for the fetal blood is going to be modulated by the mother's heart rate too - that is what is driving her blood through the palcenta - but it is also going to be modulated by the fetal heart rate, so if you can extract the fetal heart rate you can cross-correlate the O2 saturation signal with that, and pull out something that might be the fetal O2 saturation signal, if you were looking at the right place.

> >> It's a serious problem for signal detection. The pulse rates are only 1-2 Hz, so you don't get that many pulses in a measurement of a few minutes.

So you have to measure for more than a few minutes. If you had a wearable monitoring setup, you might get the mother to wear it for a day or two.

Go for it. The hospitals will have to buy more of them if you can persuade them that this is the right way to get reliable results.

> > Exactly, which is why multispectral sensing comes to mind.
>
> Not sure what you mean there.

The signal you want to look at is being modulated by the mother's heart rate and the fetal heart rate. You are going to have to pull out both.

--
Bill Sloman, Sydney

On Monday, June 7, 2021 at 6:50:52 AM UTC+10, Phil Hobbs wrote:
> jla...@highlandsniptechnology.com wrote:
> > On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
> > <pcdhSpamM...@electrooptical.net> wrote:
> >
> >> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
> >> DSP people seem to have disappeared, so the discussion petered out
> >> fairly fast. Trying again here.)
> >>
> >> Hi, all,
> >>
> >> Hoping there are still some DSP folks round here despite the evil Google
> >> ban. (But I repeat myself.)
> >>
> >> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> >> using optical sensing through the mom's abdomen. It's a very low SNR
> >> measurement on account of all the attenuation.
> >>
> >> The mom's heartbeat modulates her pulse-ox signal, which is much
> >> stronger than the fetus's on account of the scattering and absorption in
> >> maternal tissue.
> >>
> >> The data are several time series. The main issue is the variability of
> >> both pulses, which smear out the spectra and therefore knock the peak
> >> heights way down towards the noise. There are weak multiplicative
> >> effects between maternal and fetal signals, as you'd expect.
> >>
> >> What I'm looking to do is something like:
> >>
> >> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
> >>
> >> 2. Resample the data accordingly, and notch out the first 5 or so mom
> >> harmonics.
> >>
> >> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
> >> the average fetal pulse ox signal.
> >>
> >> Extra credit: sometimes the baby's pulse can cross the first or second
> >> harmonic of the mom's, and it would be good to preserve both pulse
> >> shapes accurately.
> >>
> >> Resampling a noisy signal isn't necessarily the most well-conditioned
> >> operation, so I'd welcome suggestions for just how to do this.
> >
> > How long do you plan to sample for? And do you expect either pulse
> > rate to change much during that time?

Both pulse rates are going to change with time - the adult human pulse rate is constant over a roughly seven minute interval.

When I was working at EMI Central Research the body-scanner crew wanted to synchronously sample the heart position data, and the scheme fell down on exactly that point.

You are are going to need to have long strings of actual pulse signals - both for the mother and fetus over the same period - as well as an equally long string of O2 saturation data for the same period, and cross-correlate after you have collected it.

This may look clumsy, but it is the only approach that has a hope of working.

> Anywhere from 10 minutes to some hours, depending if it's an office
> visit or childbirth. The rates go all over the place--2:1 is not unknown.
>
> > I'm thinking about saving a lot of samples and doing a sort of
> > adaptive tracking correlation on the saved data, essentially locking
> > to a changing non-sine signal. If you do the dominant mom signal
> > first, you could subtract it from the baby signal.

Forget the adaptive part. Just get the actual data and work with that.

> Yup, that's the plan. My current tentative scheme is to use a DPLL to
> measure the instantaneous frequency and resample to make it appear
> harmonic.

That doesn't strike me as a good idea. The pulse data isn't all that regular.

> Not entirely sure about the variation of pulse shape with
> pulse rate--it might be invariant, proportional, or some tertium quid.

Just measure what it is, and correlate with that.
> > Several whacko time-domain algorithms come to mind. Not FFT based, since the frequencies may drift, but there might be a freq domain equivalent.

The pulses aren't going to have a neat frequency spectrum

> > If you were here, we could try my new Amazon whiteboard markers.
>
> That would be fun. Maybe later this summer.
>
> > Imagine a polynomial that essentially time modulates the saved samples.

Why waste the time. The samples aren't going to be regular enough to make this a useful step.

<snip>

> I'd sort of like to generate ensemble-averaged pulse waveforms and use them to strobe the input data. That ought to be be good for 5-10 dB of SNR in tough situations.

Probably not a good idea. Cross-correlate the raw data

> > One of our scopes can do a software PLL on stored samples to recover the clock and payload of serial data.

Sadly, the human pulse has got short term frequency stability.

> For a 2-Hz signal, a Cortex M33 ought to be able to do amazing things.
> The question is, what things exactly?

You do need to think harder about that - and the nature of the pulse signals which you need to cross-correlate.

--
Bill Sloman, Sydney

On Monday, June 7, 2021 at 11:30:34 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sun, 6 Jun 2021 16:50:43 -0400, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
> >jla...@highlandsniptechnology.com wrote:
> >> On Sun, 6 Jun 2021 14:45:35 -0400, Phil Hobbs
> >> <pcdhSpamM...@electrooptical.net> wrote:
> >>
> >>> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
> >>> DSP people seem to have disappeared, so the discussion petered out
> >>> fairly fast. Trying again here.)
> >>>
> >>> Hi, all,
> >>>
> >>> Hoping there are still some DSP folks round here despite the evil Google
> >>> ban. (But I repeat myself.)
> >>>
> >>> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> >>> using optical sensing through the mom's abdomen. It's a very low SNR
> >>> measurement on account of all the attenuation.
> >>>
> >>> The mom's heartbeat modulates her pulse-ox signal, which is much
> >>> stronger than the fetus's on account of the scattering and absorption in
> >>> maternal tissue.
> >>>
> >>> The data are several time series. The main issue is the variability of
> >>> both pulses, which smear out the spectra and therefore knock the peak
> >>> heights way down towards the noise. There are weak multiplicative
> >>> effects between maternal and fetal signals, as you'd expect.
> >>>
> >>> What I'm looking to do is something like:
> >>>
> >>> 1. Use a digital PLL to find the time-dependent maternal pulse rate.

Won't work because maternal pulse rate isn't a constant frequency signal.

> >>> 2. Resample the data accordingly, and notch out the first 5 or so mom harmonics.

See above.
> >>> 3. Do the PLL thing on the fetal pulse, and signal average to pull out the average fetal pulse ox signal.

The fetal pulse rate probably isn't going to be a constant frequency signal either it might be a lot more stable than the mother's, but it is probably regulated by the same kind of slow feedback loop

<snip>

--
Bill Sloman, Sydney

On Monday, June 7, 2021 at 4:45:47 AM UTC+10, Phil Hobbs wrote:
> (I posted this in comp.dsp a couple of weeks ago, but most of the actual
> DSP people seem to have disappeared, so the discussion petered out
> fairly fast. Trying again here.)
>
> Hi, all,
>
> Hoping there are still some DSP folks round here despite the evil Google
> ban. (But I repeat myself.)
>
> I'm working on a completely noninvasive sensor for fetal blood oxygen,
> using optical sensing through the mom's abdomen. It's a very low SNR
> measurement on account of all the attenuation.
>
> The mom's heartbeat modulates her pulse-ox signal, which is much
> stronger than the fetus's on account of the scattering and absorption in
> maternal tissue.
>
> The data are several time series. The main issue is the variability of
> both pulses, which smear out the spectra and therefore knock the peak
> heights way down towards the noise. There are weak multiplicative
> effects between maternal and fetal signals, as you'd expect.
>
> What I'm looking to do is something like:
>
> 1. Use a digital PLL to find the time-dependent maternal pulse rate.

Probably a bad idea. The maternal pulse rate isn't constant over periods shorter than about seven minutes.

An ecg will give you the exact time the maternal heart start contracting, and if you bin your optical data for specific intervals after that spike, you may be able to build a picture of the average transient in the maternal blood oxygenation.
> 2. Resample the data accordingly, and notch out the first 5 or so mom
> harmonics.

Probably not a good idea.
> 3. Do the PLL thing on the fetal pulse, and signal average to pull out
> the average fetal pulse ox signal.

Again the fetal pulse rate probably isn't going to be constant either.

Getting the fetal ecg isn't easy, but it can be done. If you can pick up the spike generated when the fetal heart starts contacting you could bin the optical data in a separate set of bins corresponding to a set of delays from the fetal heart contraction.

> Extra credit: sometimes the baby's pulse can cross the first or second harmonic of the mom's, and it would be good to preserve both pulse shapes accurately.
>
> Resampling a noisy signal isn't necessarily the most well-conditioned operation, so I'd welcome suggestions for just how to do this.

Sample the data separately - or at least set up two different banks of samples from the same data stream - based on what the maternal heart was doing at sampling time and and what the fetal heart was doing at that instant.

--
Bill Sloman, Sydney

On 07/06/21 04:47, Bill Sloman wrote:
> Both pulse rates are going to change with time - the adult human pulse rate is constant over a roughly seven minute interval.

The key word might be "roughly".

I believe the inter-beat interval is normally chaotic, and a "too"
constant interval indicates heart problems. Presumably the chaos
reduces the problems of resonances etc.

I suspect that will significantly reduce the usefulness of
FFT-based techniques.

Also be aware that you may have to work with normally
low heart rates. My resting rate is normally ~52, and
if I am very relaxed it can be 45. Fortunately the block
doesn't have any (visible) symptoms.