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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 10th Dec 2022, 11:11 am   #1
inaxeon
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Default Group delay (in analogue modulation / transmission systems)

Hello

This is a subject I came across in context of modulators, it seems that some countries i.e. Australia/Sweden/Norway specify different levels of acceptable group delays.

Does anyone know what the actual concern is with group delay? How come we even cared about it at all?

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Old 10th Dec 2022, 1:15 pm   #2
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Default Re: Group delay (in analogue modulation / transmission systems)

Non-flat group delay in the context of an FM IF filter translates into distortion of any FM signal passing through it.

Flat group delay (IE all frequencies take the same time to pass through) is the same thing as 'linear phase'

Some modulators also exhibit non-flat group delay. Same result.

David
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Old 10th Dec 2022, 1:20 pm   #3
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Default Re: Group delay (in analogue modulation / transmission systems)

Getting consistent and flat group-delay was an essential pre-requisite for the implementation of Ceefax/Oracle on the back of the existing PAL TV signal; you couldn't easily achieve it with traditional L-C filters in the IF strip but the coming of SAW filters was the solution we all needed.
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Old 10th Dec 2022, 1:26 pm   #4
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Default Re: Group delay (in analogue modulation / transmission systems)

We used to use terrestrial analogue microwave links for PAL television outside broadcast relays and the visible effect of group delay in the system was that the colour no longer 'sat' on the luminace image. For example, on a wide angle shot of a football player as a small red figure on a green background the red from the players shirt would be seen to be displaced to the right of the players body.
Needless to say that we went to great lengths to keep such delays within strict limits so no such effect would have been noticed on our output.

Steve.
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Old 10th Dec 2022, 4:07 pm   #5
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Default Re: Group delay (in analogue modulation / transmission systems)

Quote:
Originally Posted by G6Tanuki View Post
you couldn't easily achieve it with traditional L-C filters in the IF strip but the coming of SAW filters was the solution we all needed.
Tradition LC filters operate on the principle of reactance and resonance in classic network theory.

Transversal filters operate on tapped delay lines, multipliers and summers. They are a lot more complex, but give you some independence in choosing amplitude and phase/time responses.

You can make an LC transversal filter using L-C ladders as an approximation to a tapped delay line. Saws are naturally delay based elements, and transversal filters lie at the root of most digital/software filters.

Analogue transversal filters were used, but only where there was no real alternative. They were common as equalisers for TV and data modem systems.

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Old 10th Dec 2022, 8:33 pm   #6
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Default Re: Group delay (in analogue modulation / transmission systems)

At BBC Transmitter Capital Projects Department (TCPD) in the mid-'80s, there was a reference document (written by a senior TCPD engineer by the name of Nick Davies) titled "Group Delay, Ceefax and I.F. Mod Drives". This was pretty much the 'bible' on the subject ... and of course, other ex-Beeb transmitter engineers on here will remember their Wood Norton training - especially the legendary Tony Larkham, whose particular 'baby' was the on-site Pye-TVT 20kW pulsed klystron system which employed an I.F. modulation drive system. Tony made use of a 'Heucke' (pronounce 'Hoyker') test set for group delay alignment; needless to say, the overall performance of that particular system was considered a benchmark by many, both in Transmitter Operations and T.C.P.D.

Best wishes
Guy

(postscript: https://www.vintage-radio.net/forum/...25&postcount=9 )
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Last edited by Nymrod121; 10th Dec 2022 at 8:58 pm. Reason: 9-year old link reference added (yup, thought I'd mentioned this topic before :) )
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Old 10th Dec 2022, 9:43 pm   #7
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Default Re: Group delay (in analogue modulation / transmission systems)

Thanks for all of the detailed answers. In particular it's good to relate what (frankly) is quite an specialised subject to a real world phenomenon.

So in conclusion we don't overly care about group delay which is entirely linear in nature, it's when it becomes nonlinear that there's a problem then some black magic is required to equalise it.

The modulator in question has a group delay equaliser in it - a bit of brain bending item. Was really interested to know what problem it was intended to solve, and now I do!
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Old 10th Dec 2022, 10:25 pm   #8
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Default Re: Group delay (in analogue modulation / transmission systems)

Not quite.

You want group delay to be constant across the bandwidth. If you had a linear slope it would be bad. Any non-flatness means that different frequency components of a signal will arrive at the end of the system at different times, so waveforms get mangled and harmonic/intermod distortion is introduced to FM signals after they are demodulated.

So the ideal group delay characteristic is a straight line, but only one of zero slope.

The confusion lies in that perfect, flat group delay is the same thing as a linear phase characteristic across the bandwidth, a straight line, but of any slope.

Group delay is the rate of change of phase, phase is the integral of group delay.

You can see how it confuses many people.

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Old 10th Dec 2022, 11:56 pm   #9
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Default Re: Group delay (in analogue modulation / transmission systems)

Group Delay. I haven’t heard that term in a decade or so, mostly associated- in the telecoms world- with FDM(Frequency Division Multiplexing) analogue trunk circuits. It’s been a while but basically a band of a few hundred kHz was “ sliced” up into segments for transmission and re-combined at the distant end. There were whole suites of equipment to correct for group delay, as some of the frequencies presumably travel( or at least reach the other end) at different times.
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Old 11th Dec 2022, 4:48 am   #10
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Default Re: Group delay (in analogue modulation / transmission systems)

When I joined HP at South Queensferry, the big product of the division was the Microwave Link Analyser, MLA. For 70 MHz IF links, and then recently introduced, a model for 140MHz. Ivan and his gang were working on the development of the dual range 70/140MHz system. Group delay 'W' curves were everywhere. The designers of the earlier systems were all still around. It was an interesting place to be.

Years later, having some time in manufacturing engineering for a bit of variety, I wrote the PCO (production change order) which ended the last MLAs. No new FDM stuff was being done out in the real world, and there was enough test gear along with it.

I used to explain FDM as sending multiple phone channels as SSB signals on stacked frequency plans either by cable directly, or subsequently FM'd onto microwave carriers.

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Old 11th Dec 2022, 10:44 am   #11
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Default Re: Group delay (in analogue modulation / transmission systems)

Quote:
Originally Posted by Radio Wrangler View Post
Not quite
I got it. I just used the wrong terminology.

In the context of television, would be fair to say the attached image is an accurate representation of group delay? (this is just a software simulation, based on the description by fetteler)
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Old 11th Dec 2022, 4:38 pm   #12
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Default Re: Group delay (in analogue modulation / transmission systems)

You can viewstep and pulse waveforms as having many frequency components. With variable group delay, they no longer all arrive at the right times, so things like black-white and white-black transitions will lose crispness and grow ripples in the time domain.

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Old 12th Dec 2022, 6:18 am   #13
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Default Re: Group delay (in analogue modulation / transmission systems)

Some transmission authorities included group delay pre-correction (pre-distortion) in their analogue TV transmitter specifications in order to counter the expected group delay distortion in typical domestic receivers. I think that originally, it was done at low-to-mid video frequencies for monochrome reception, with correction at higher video frequencies (around the colour subcarrier) in the colour era.

Commentary on the matter showed up in the CCIR/ITU-R TV System reports from 1974 Geneva onwards. I have attached a couple of examples – group delay curves are shown towards the end. (The whole set may be found at the ITU site.)

CCIR 1974 Geneva Report 624 TV Systems.pdf ITU-R BT.1701-1 200508 TV Systems.pdf

Evidently post-demodulation video group delay correction was sometimes included in higher performance receiving systems. I have attached a couple of examples, one with L-C and the other with SAW vision IF filtering.

BBC RC1-511 Block Schematic.pdf WW TV Tuner Block & Vision Schematics.pdf


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Old 12th Dec 2022, 7:31 am   #14
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Default Re: Group delay (in analogue modulation / transmission systems)

It is common in active circuits for group delay characteristics to be level dependent, so if you're doing more than compensating the effect of a passive filter, life gets interesting.

Also. levels change due to the transition regions of filters, and through the evil magic which is AM to PM conversion, the group delay suffers further.

This probably is liveable with a simple TV signal, but when you've got 2600 FDM channels you need to control the intermods of, and want good NPR, then it bites!

David
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Old 12th Dec 2022, 9:28 am   #15
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Default Re: Group delay (in analogue modulation / transmission systems)

Ah, group delay.

In 1978, I spent many months as a student apprentice with the Marconi Company grappling system level group delay corrections. It was for Goonhilly 4 which was the Orbital Test Satellite ground station for BT and ran 14 GHz up, 11Ghz down with IFs of 140MHz for increased FDM, TDM and TV bandwidth/capacity. The group delay problems in the up/down chains were horrendous with multiple correctors spread across the 140 MHz IF bandwidth in sections about 30MHz wide. They were really tricky to adjust whilst still keeping the amplitude response flat. The HP MLAs weren't really up to the job and we were using a specialist piece of test equipment made by US company Rantec to produce system test data that BT was happy with. The correction system was very complicated having send/receive chains and also one for the actual satellite which added its own weird problems. Once set up and with BT happy, it all stayed put no matter what the 2kW Travelling Wave Tube ((TWT) HPAs (High Power Amplifiers) were set up to for output power. That was fortunate. As a young student apprentice I had been given one heck of a task and it was very exciting being down at Goonhilly working on such a prestigious project. For quite a while, group delay seemed to take over my life!

Best regards,

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Old 12th Dec 2022, 10:20 am   #16
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Default Re: Group delay (in analogue modulation / transmission systems)

When Pye TVT in Cambridge became Varian TVT, some schoolfriends and I scavenged vast quantities of scrap gear from their labs. It was all to do with television broadcast, all the way from the camera to the transmitter itself. I remember an awful lot of various PCBs, front panels and other bits and pieces among the junk bearing the label "group delay". What I learned is that it was clearly a big issue in analogue TV broadcast all the way along the transmission chain!

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Old 15th Dec 2022, 1:09 am   #17
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Default Re: Group delay (in analogue modulation / transmission systems)

Here is the group delay correction curve for the WW TV tuner mentioned upthread. Flattening the net curve up to 5 MHz was the evident objective, achieved by a pair of post-demodulator all-pass filters.

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As far as I know, UK system I was transmitted without any group delay precorrection for expected receiver distortion, so a net flat receiver curve would seem to have been the objective.


The BBC TV receiver also mentioned had a special quality SAWF that probably had a better group delay curve than typical domestic receiver types, but even so, post-demodulation group delay correction was provided for the 4.5 to 5.5 MHz range.


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Old 15th Dec 2022, 2:01 am   #18
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Default Re: Group delay (in analogue modulation / transmission systems)

Quote:
Originally Posted by Radio Wrangler View Post
Non-flat group delay in the context of an FM IF filter translates into distortion of any FM signal passing through it.

Flat group delay (IE all frequencies take the same time to pass through) is the same thing as 'linear phase'
Perhaps illustrated by the Revox A76 FM tuner case. The IF filter had a flat group delay curve to +/- 100 kHz. On the other hand, the IF bandpass curve was very narrow by the conventional wisdom, only 130 kHz wide as compared with the 210 kHz or so more common for stereo. (Although it had a Gaussian, rather than a flat-topped shape.) But the A76 was one of the early solid-state "supertuners".

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Old 15th Dec 2022, 6:28 am   #19
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Default Re: Group delay (in analogue modulation / transmission systems)

The real disappointment of having a supertuner, is the paucity of listenable material now broadcast. There's Radio 3, and then the rest is over-compressed junk.

Playing with this sort of equipment is fun in itself and the challenge can be satisfying, but there isn't much to do with it afterwards.

One escape would be a high quality material source into a high quality pantry transmitter, but anyone pragmatic enough to engineer a super tuner would be pragmatic enough to 'cut out the middle man' and connect the source directly to the amplifier.

David
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