Daft question: why is it that some channels and/or some platforms (satellite
versus terrestrial) broadcast standard-definition as 704x576 and some as
720x576. There are even cases where the same channel is 704 on satellite and
720 on terrestrial (or vice versa). It looks as if the picture area is the
same, but differently scaled, rather than the 704 version being the 720
version with 8 pixels either side being cropped off.

Both numbers are exact multiples of 16 (44x16 versus 45x16): I imagine the
compression algorithms work best with widths and heights that are exact
multiples of 16 (*). But why the two different standards which are so close?
I can understand why some lower-budget channels are sub-SD 544x576 because
that reduces bandwidth and therefore transmission costs. But the saving of
704 rather than 720 is minuscule.

(*) Was it pure chance that the analogue 625/25 format had 575 active lines
which is extremely close to 576 (a multiple of 16)? Was the multiple-of-16
factor relevant when 625/25 was specced - eg to derive the frame frequency
by a cascade of divide-by-2 analogue frequency-division stages from the line
frequency.

On 24/02/2022 21:15, NY wrote:
> But the saving of 704 rather than 720 is minuscule.
I think the BBC changed their DTT channels from 720 to 704, as although
tiny, it liberated enough bandwidth to squeeze local radio into PSB1

On 25/02/2022 08:12, Mark Carver wrote:
> On 24/02/2022 21:15, NY wrote:
>> But the saving of 704 rather than 720 is minuscule.
> I think the BBC changed their DTT channels from 720 to 704, as although
> tiny, it liberated enough bandwidth to squeeze local radio into PSB1

Fair enough, although since then they've had to make much bigger
economies to fit in BBC Three. I suppose they designed the resolution
and bit rates when BBC Three was present, then benefited from a time
when BBC Three was internet-only and the remaining channels could have
higher bit rates, and have now reverted to the original state.

Reducing 720 to 704 is a saving of 16/720 = 2% - but for several
channels I can see that it would be enough to fit in several radio channels.

I assume that is why its in mono then. Very few of the stations on radio on
Freeview are stereo. The main bbc ones, Asian network, Classic FM and not
much else it seems.

As for the frame refresh rate, Many early 405 line receivers assumed that
the frame was synchronised to the mains, since when this stopped being the
case many needed extra smoothing in the power supply to stop hum bars
rolling slowly over the screen.
When you say active lines on 625, I'm assuming you mean that sync pulses
and other data like teletext were in the ones not officially seen. In the US
they used 525 and their mains was, and still is 60hz of course, I'd guess
for much the same reasons as our relationship.
History of TV is pretty full of strange decisions.
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.
Brian

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"Mark Carver" <mark.carver@invalid.invalid> wrote in message
news:j7rhb9FffruU1@mid.individual.net...
> On 24/02/2022 21:15, NY wrote:
>> But the saving of 704 rather than 720 is minuscule.
> I think the BBC changed their DTT channels from 720 to 704, as although
> tiny, it liberated enough bandwidth to squeeze local radio into PSB1

Why did they bring back BBC 3? Did the streaming fail, I mean did nobody
watch it. To be honest its full of crap now, but some is probably better
crap than the current crap on bbc1.
All depends on your tastes I'd expect. One assumes they have cut back on
the kiddies channels again then.
Brian

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"NY" <me@privacy.net> wrote in message
news:0cKdnRcT3aYtBIX_nZ2dnUU7-dHNnZ2d@brightview.co.uk...
> On 25/02/2022 08:12, Mark Carver wrote:
>> On 24/02/2022 21:15, NY wrote:
>>> But the saving of 704 rather than 720 is minuscule.
>> I think the BBC changed their DTT channels from 720 to 704, as although
>> tiny, it liberated enough bandwidth to squeeze local radio into PSB1
>
> Fair enough, although since then they've had to make much bigger economies
> to fit in BBC Three. I suppose they designed the resolution and bit rates
> when BBC Three was present, then benefited from a time when BBC Three was
> internet-only and the remaining channels could have higher bit rates, and
> have now reverted to the original state.
>
> Reducing 720 to 704 is a saving of 16/720 = 2% - but for several channels
> I can see that it would be enough to fit in several radio channels.

On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
> I assume that is why its in mono then. Very few of the stations on radio on
> Freeview are stereo. The main bbc ones, Asian network, Classic FM and not
> much else it seems.
It's not worth having the radio stations in stereo on DTT. I suspect
they are aimed at consumption based on kitchen, bedroom, and hotel
tellies, where listening is a secondary activity.
> When you say active lines on 625, I'm assuming you mean that sync pulses
> and other data like teletext were in the ones not officially seen.
Blanking consumes about 25+25 lines on analogue.

Digital platforms do not transmit the blanking period, it would be a
pointless waste of bandwidth

> 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.

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 ?

Well on the old dual standard set from Redifusion I had it was clear that
the negative video was better and more stable, of course to find out, both
would have had to have been transmitted on vhf. You could however find that
as Europe went 625 negative, except France, that negative was better at
rejecting impulse noise that otherwise gave rise to picture jitter on the
French stations. It was pretty bad on their old 819 line service too, which
we modified the set to handle though the audio needed a second receiver as
did the am sound on 625 of course.
I remember that when I could see, looking at colour TVs in France one
could not really tell it was not negative mod and used Seam colour. You
occasionally heard am interference though on some stations.
Brian

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"Mark Carver" <mark.carver@invalid.invalid> wrote in message
news:j7rqloFh7u3U1@mid.individual.net...
> On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
>> I assume that is why its in mono then. Very few of the stations on radio
>> on
>> Freeview are stereo. The main bbc ones, Asian network, Classic FM and not
>> much else it seems.
> It's not worth having the radio stations in stereo on DTT. I suspect they
> are aimed at consumption based on kitchen, bedroom, and hotel tellies,
> where listening is a secondary activity.
>> When you say active lines on 625, I'm assuming you mean that sync
>> pulses
>> and other data like teletext were in the ones not officially seen.
> Blanking consumes about 25+25 lines on analogue.
>
> Digital platforms do not transmit the blanking period, it would be a
> pointless waste of bandwidth
>
>> 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.
>
> 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 ?
>
>

"Mark Carver" <mark.carver@invalid.invalid> wrote in message
news:j7rqloFh7u3U1@mid.individual.net...
> On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
>> I assume that is why its in mono then. Very few of the stations on radio
>> on
>> Freeview are stereo. The main bbc ones, Asian network, Classic FM and not
>> much else it seems.
> It's not worth having the radio stations in stereo on DTT. I suspect they
> are aimed at consumption based on kitchen, bedroom, and hotel tellies,
> where listening is a secondary activity.

Interesting assumption. I don't have any FM radio, apart from a) an
all-in-one hifi system and b) a separate tuner, neither of which I CBA to
set up, especially since the tuner requires a 300 ohm aerial and doesn't
have a telescopic one.

I use DTT or DSat for recording radio programmes (mostly Radio 4), in
exactly the same way as for TV programmes.

"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.

"Roderick Stewart" <rjfs@escapetime.myzen.co.uk> wrote in message
news:9e4k1h9ndt62ghledil4qq31k5ga07fo12@4ax.com...
> On Fri, 25 Feb 2022 10:30:53 -0000, "Brian Gaff \(Sofa\)"
> <briang1@blueyonder.co.uk> 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.
>
> And why didn't 405 line use equalising pulses?

What was the purpose of equalising pulses? I could never find a really good
explanation.

On 26/02/2022 21:37, NY wrote:
> Interesting assumption. I don't have any FM radio, apart from a) an
> all-in-one hifi system and b) a separate tuner, neither of which I CBA to
> set up, especially since the tuner requires a 300 ohm aerial and doesn't
> have a telescopic one.

Never had any problem with just a piece of wire connected to the back of
the VHF FM tuner though use DAB most of the time?

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'.
--
Ian

On Sat 26/02/2022 21:37, NY wrote:
> "Mark Carver" <mark.carver@invalid.invalid> wrote in message
> news:j7rqloFh7u3U1@mid.individual.net...
>> On 25/02/2022 10:30, Brian Gaff (Sofa) wrote:
>>> I assume that is why its in mono then. Very few of the stations on
>>> radio on
>>> Freeview are stereo. The main bbc ones, Asian network, Classic FM and
>>> not
>>> much else it seems.
>> It's not worth having the radio stations in stereo on DTT. I suspect
>> they are aimed at consumption based on kitchen, bedroom, and hotel
>> tellies, where listening is a secondary activity.
>
> Interesting assumption. I don't have any FM radio, apart from a) an
> all-in-one hifi system and b) a separate tuner, neither of which I CBA
> to set up, especially since the tuner requires a 300 ohm aerial and
> doesn't have a telescopic one.
>
> I use DTT or DSat for recording radio programmes (mostly Radio 4), in
> exactly the same way as for TV programmes.

If your tuner does not have a 75R FM aerial socket just connect the
aerial with core to one of the 300R terminals and the screen to the
earth (or an earth) terminal - may be one of the AM aerial connections
if the tuner is AM/FM. You will loose a little sensitivity but it will
match well and certainly work.

In message <t0km1htg218ft7suu43dbtossa17db9ise@4ax.com>, Roderick
Stewart <rjfs@escapetime.myzen.co.uk> writes
>On Sun, 27 Feb 2022 08:52:43 +0000, Ian Jackson
><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>
>>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.
>
>You can sample the black level on the back porch (as some of the
>better homemade receivers did to obtain an AGC voltage), but you only
>get a measure of 30% of peak, which is not as good as a 100% reference
>that can be sampled with simpler circuitry. Designers of domestic
>electronics being cheapskates is not a new thing.
>
There were also some commercial sets that did have a 'proper' gated
black-level AGC system and black-level clamp (especially in the earlier
days), but an awful lot had that ghastly mean-level AGC, with AC-coupled
video drive to the CRT. [Around 1967 the Wireless World carried an
article on how to do a simple add-on modification to at least stabilise
black level. I did the mod to my Ferguson 1500, and it did sort-of
work.]
--
Ian

Yes well another oddity of old black and white sets, not the early ones but
those around the time of dual standard was the AC coupled video that made
blacks lighter when not much white was on the screen.
I did modify one with a dc restore circuit once, but then found I needed to
modify the eht to stop the regulation affecting picture size and focus when
you got a sudden flash from something.
Brian

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"Roderick Stewart" <rjfs@escapetime.myzen.co.uk> wrote in message
news:giim1hlosd2gi4ifdgf329qh0kgk328mvm@4ax.com...
> On Sat, 26 Feb 2022 21:40:36 -0000, "NY" <me@privacy.invalid> wrote:
>
>>"Roderick Stewart" <rjfs@escapetime.myzen.co.uk> wrote in message
>>news:9e4k1h9ndt62ghledil4qq31k5ga07fo12@4ax.com...
>>> On Fri, 25 Feb 2022 10:30:53 -0000, "Brian Gaff \(Sofa\)"
>>> <briang1@blueyonder.co.uk> 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.
>>>
>>> And why didn't 405 line use equalising pulses?
>>
>>What was the purpose of equalising pulses? I could never find a really
>>good
>>explanation.
>
> Equalisation between fields of the residual charge on the integrating
> capacitor in the field sync separator. Without them, the line sync
> pulse that immediately precedes a field sync pulse will precede it by
> a whole line on alternate fields, and only half a line on the others.
> The timing of the start of field scan depends on the voltage on the
> integrating capacitor, and if it's not equal between field scans, it
> can trigger alternately late and early, and cause the lines displayed
> on the screen to bunch in pairs. ("Early" translates to up, and "late"
> to down). Using two or three lines of half width pulses twice as often
> doesn't alter the average signal voltage, so it won't interfere with
> anything else, but it gives enough time for the capacitor to discharge
> to a level that doesn't matter, so when the broad pulses begin, the
> integrating circuit has the same initial conditions on every field.
>
> There are other ways of detecting field sync, but most early TVs used
> fairly simple circuitry (essentially a resistor and a capacitor) that
> didn't always work perfectly. The extra complication of adding
> equalising pulses in the broadcasters' sync pulse generators obviates
> the need for extra care or complication in millions of receivers, so
> you might expect that as soon as the idea had been thought of it would
> have been implemented everywhere, but our 405 line system never did.
>
> Rod.

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.

--
Brian Gregory (in England).

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 ?!

On 24/02/2022 21:15, NY wrote:
> Daft question: why is it that some channels and/or some platforms
> (satellite versus terrestrial) broadcast standard-definition as 704x576
> and some as 720x576. There are even cases where the same channel is 704
> on satellite and 720 on terrestrial (or vice versa). It looks as if the
> picture area is the same, but differently scaled, rather than the 704
> version being the 720 version with 8 pixels either side being cropped off.

It's worse than that. There are three different formats.

1) 704x576, all pixels used, scales to 16:9 with correct aspect ratio.

2) 720x576, only the middle 704 pixels are needed to make 16:9 in the
exact correct aspect ratio. The extra 8 pixels each side may have
picture info in them from just outside the 16:9 shaped picture, or they
may be black.

3) 720x576, all pixels used, scales to 16:9 with correct aspect ratio.

--
Brian Gregory (in England).

On 01/03/2022 15:58, Brian Gregory wrote:
> It's worse than that. There are three different formats.
>
> 1) 704x576, all pixels used, scales to 16:9 with correct aspect ratio.
>
> 2) 720x576, only the middle 704 pixels are needed to make 16:9 in the
> exact correct aspect ratio. The extra 8 pixels each side may have
> picture info in them from just outside the 16:9 shaped picture, or they
> may be black.
>
> 3) 720x576, all pixels used, scales to 16:9 with correct aspect ratio.

Oh and it's worth mentioning that there is the same mess with 544x576
and 528x576.

--
Brian Gregory (in England).

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 :)

--
Robin
reply-to address is (intended to be) valid

On 01/03/2022 15:23, Mark Carver wrote:
> On 01/03/2022 15:13, Brian Gregory wrote:

>> 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 ?!

As someone who was driving 1950s cars in the 1960s, I can confirm that
the early cars had copper conductors in the HT leads, and that would
have radiated the spark current. Later 1950s cars had HT leads with
carbon cores and they didn't create interference. One early car I
bought had copper cored leads terminated in "antiference" spark plug
connectors and that didn't interfere with TVs either.

I never explored what was in the spark plug connectors because they were
a sealed unit and I couldn't see what was inside without wrecking it,
and I had no idea where I might find replacements.

Jim

"Brian Gregory" <void-invalid-dead-dontuse@email.invalid> wrote in message
news:j86u9bFlh8fU2@mid.individual.net...
> On 01/03/2022 15:58, Brian Gregory wrote:
>> It's worse than that. There are three different formats.
>>
>> 1) 704x576, all pixels used, scales to 16:9 with correct aspect ratio.
>>
>> 2) 720x576, only the middle 704 pixels are needed to make 16:9 in the
>> exact correct aspect ratio. The extra 8 pixels each side may have picture
>> info in them from just outside the 16:9 shaped picture, or they may be
>> black.
>>
>> 3) 720x576, all pixels used, scales to 16:9 with correct aspect ratio.
>
> Oh and it's worth mentioning that there is the same mess with 544x576 and
> 528x576.

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.

Thinking of "[the pixels] may have picture info in them [...] or they may be
black", why is it that some SD broadcasts (particularly BBC regional news)
have a black or white half-line at the top right of the picture? I could
understand a black half-line at the top left or the bottom right of an
archive programme made in analogue, but why would modern all-digital
production introduce a half-line?

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.

--
Brian Gregory (in England).

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.
Brian

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"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.
>
> --
> Brian Gregory (in England).

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