"Brian Gaff (Sofa)" <briang1@blueyonder.co.uk> wrote in message
news:svn6up$3k8$1@dont-email.me...
> Who knows the logic of all this. Back in the early days of flat displays,
> some screens had oblong pixels which made the setting up of a picture on
> them rather interesting.

SD, especially in widescreen, has very oblong pixels. For 576 rows of
pixels, you'd expect either 4/3 * 576 = 768 or else 16/9 * 576 = 1024 pixels
across the width. I believe the reduction from 768 to 720 is because of the
Kell Factor in analogue days: an interlaced analogue picture looks less
sharp than a non-interlaced one (assuming you're not distracted by the extra
flicker of non-interlaced) and they reduced the bandwidth of the signal to
give comparable resolution in the horizontal direction to the perceived
resolution in the vertical direction. And then that carried over into
digital standards and widescreen stretched everything out even further. In
contrast, HD is 1920x1080 which is 16/9, so that has square pixels.

"Brian Gregory" <void-invalid-dead-dontuse@email.invalid> wrote in message
news:j87l20FpshmU1@mid.individual.net...
> On 01/03/2022 20:51, NY wrote:
>> I've never heard of 528x576. Is that used for any sub-SD channels on
>> DVB-T or DVB-S in the UK? Or is it more commonly found outside the UK?
>> Unlike 704/720 full-SD, I've only ever seen 544 sub-SD.
>
> "That's TV (UK)" Freeview channel 91 is 528x576.
>
> The satellite version of it seems to be the same 528x576 padded with black
> out to 544x576.

Ah, I can't get That's TV on terrestrial because the signal is too weak
where I live, due to a combination of factors: distance, a hill in the way,
wrong direction (it's aimed at Hull or Grimsby, rather than north to
Bridlington), possibly wrong aerial (*), possibly different propagation and
cable attenuation with higher frequency. I have occasionally received the
LOC multiplex when reception conditions have been abnormal: for That's TV, a
TSReader scan shows 128 kb/s audio but the video stream could not be
decoded.

(*) I suspect my aerial dates from analogue days, maybe even before Channel
4, when everything was lower than 530 MHz. Certainly COM4 (546 MHz) is
noticeably worse reception than COM 5 (490) or COM6 (514) even though all
three are almost the same power, and COM7 (746 MHz) is undetectable (though
the lower power is probably a big cause too). Maybe one day I'll get it
looked at, though I tend to use satellite for most TV, only using
terrestrial as a fall-back if there are two or more things I want to record
at the same time.

"Mark Carver" <mark.carver@invalid.invalid> wrote in message
news:j88og1F1jl0U1@mid.individual.net...
> On 01/03/2022 17:12, Robin wrote:
>> On 01/03/2022 15:23, Mark Carver wrote:
>>> On 01/03/2022 15:13, Brian Gregory wrote:
>>>> On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
>>>>> Why did the 405 line use positive video, as this put the sync
>>>>> pulses at
>>>>> the lowest signal level and hence more prone to am interference like
>>>>> cars
>>>>> and vacuum cleaner interference.
>>>>
>>>> I assume they simply made the decision too soon, before they knew all
>>>> the full implications of it.
>>>>
>>> Indeed, there weren't that many vehicles around in 1936. Although those
>>> that were had absolutely no electrical suppression.
>>>
>>> In fact, wasn't ignition suppression mandated in the 1950s, when both
>>> cars and TVs increased in number and it was all becoming a nuisance ?!
>>
>> yes; well remembered :)
>>
> I'm struggling to remember the death of Queen Victoria however

Was unsuppressed ignition only a problem for TV? Did it not cause problems
with radio reception, especially VHF which is more similar to 405-line TV?
Or is VHF radio less susceptible because it is FM rather than AM/VSB?

On 02/03/2022 09:57, NY wrote:
> "Mark Carver" <mark.carver@invalid.invalid> wrote in message
> news:j88og1F1jl0U1@mid.individual.net...
>> On 01/03/2022 17:12, Robin wrote:
>>> On 01/03/2022 15:23, Mark Carver wrote:
>>>> On 01/03/2022 15:13, Brian Gregory wrote:
>>>>> On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
>>>>>>    Why did the 405 line use positive video, as this put  the sync
>>>>>> pulses at
>>>>>> the lowest signal level and hence more prone to  am interference
>>>>>> like cars
>>>>>> and vacuum cleaner interference.
>>>>>
>>>>> I assume they simply made the decision too soon, before they knew
>>>>> all the full implications of it.
>>>>>
>>>> Indeed, there weren't that many vehicles around in 1936. Although
>>>> those that were had absolutely no electrical suppression.
>>>>
>>>> In fact, wasn't ignition  suppression mandated in the 1950s, when
>>>> both cars and TVs increased in number and it was all becoming a
>>>> nuisance ?!
>>>
>>> yes; well remembered :)
>>>
>> I'm struggling to remember the death of Queen Victoria however
>
>
> Was unsuppressed ignition only a problem for TV? Did it not cause
> problems with radio reception, especially VHF which is more similar to
> 405-line TV? Or is VHF radio less susceptible because it is FM rather
> than AM/VSB?

I recall being driven mad by next door's electric kitchen knife when
trying to listen to FM radio,  it didn't affect MF/LF reception at all.

Also didn't some early (1980s vintage) FM car radios have impulse noise
filters (by use of a delay to mute the audio when there was a pulse ?)
Seem to recall the same devices were available for record decks too ?

It was far more of a problem at the bottom end of UHF Band IV, than at
the top of Band V too

On 02/03/2022 09:54, NY wrote:
> Ah, I can't get That's TV on terrestrial because the signal is too weak
> where I live, due to a combination of factors: distance, a hill in the
>...

It's marginal here. On my aerial some receivers can manage it most of
the time, others can't.

--
Brian Gregory (in England).

Op 27-2-2022 om 9:52 schreef Ian Jackson:
> In message <sve6n4$gl7$2@dont-email.me>, NY <me@privacy.invalid> writes
>> "Mark Carver" <mark.carver@invalid.invalid> wrote in message
>> news:j7rqloFh7u3U1@mid.individual.net...
>>> Wasn't the reason negative mod was adopted for 625/UHF because it
>>> pushed  the sync pulses (rather than high luminance picture content)
>>> into the non  linearity 'zone' of klystrons ?
>>> Anyway negative mod any better though for reception and impulse
>>> rejection;  overshoot and all that ?
>>
>> I'd always understood that the main reason for negative mod was better
>> rejection of impulse interference from poorly-regulated car and
>> motorbike (especially motorbike!) ignition systems - probably more of
>> an issue with spark-gap/coil ignition than with modern electronic
>> ignition.
>
> For several reasons, positive modulation is a real PITA. One of them is
> that there is no easily-obtainable measurement of the RF signal level.
>
> With negative mod, it's simply sync tip level, which is steady and
> constant, and easy to measure with a peak-detecting meter, or as might
> be displayed on a spectrum analyser.
>
> With positive mod, it can vary between 30% (black level) and 100% (peak
> white). OK, there might be a few lines of VITs, but these are
> essentially transient, and make peak level difficult to 'catch'.

And positive modulation needs more power, because almost the whole line
is between 30% and 100%.
While with negative modulation almost the whole line is between 0% and 70%.
Only the syncs are at 100%.

In message <t005cj$phh$1@dont-email.me>, Rink
<rink.hof.haalditmaarweg@planet.nl> writes
>Op 27-2-2022 om 9:52 schreef Ian Jackson:
>> In message <sve6n4$gl7$2@dont-email.me>, NY <me@privacy.invalid> writes
>>> "Mark Carver" <mark.carver@invalid.invalid> wrote in message
>>>news:j7rqloFh7u3U1@mid.individual.net...
>>>> Wasn't the reason negative mod was adopted for 625/UHF because it
>>>>pushed  the sync pulses (rather than high luminance picture content)
>>>>into the non  linearity 'zone' of klystrons ?
>>>> Anyway negative mod any better though for reception and impulse
>>>>rejection;  overshoot and all that ?
>>>
>>> I'd always understood that the main reason for negative mod was
>>>better rejection of impulse interference from poorly-regulated car
>>>and motorbike (especially motorbike!) ignition systems - probably
>>>more of an issue with spark-gap/coil ignition than with modern
>>>electronic ignition.
>> For several reasons, positive modulation is a real PITA. One of them
>>is that there is no easily-obtainable measurement of the RF signal
>>level.
>> With negative mod, it's simply sync tip level, which is steady and
>>constant, and easy to measure with a peak-detecting meter, or as might
>>be displayed on a spectrum analyser.
>> With positive mod, it can vary between 30% (black level) and 100%
>>(peak white). OK, there might be a few lines of VITs, but these are
>>essentially transient, and make peak level difficult to 'catch'.
>
>
>And positive modulation needs more power, because almost the whole line
>is between 30% and 100%.
>While with negative modulation almost the whole line is between 0% and 70%.
>Only the syncs are at 100%.
>
No. With PAL I and B/G, with -ve mod sync tip is 100% of envelope
amplitude, black is 76% (-2.4dB) and white is 20% (-14dB). Fully
saturated yellow takes the downward excursion down to 12.5% (leaving
just enough to facilitate the inter-carrier mix to give a sound IF of 6
(5.5 for B/G) MHz. [NTSC is very similar, but slightly different.]
--
Ian