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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 Oct 2022, 1:34 pm   #21
Radio Wrangler
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

I did indeed.

The thing with a modular system is that you don't have to have all the modules available from the start that it might eventually use. They may never have done a NICAM module, but it is quite likely that they left the door open for one if they later changed their mind.

I've spent a lot of time designing modular stuff, and even did a PAL colour synthesiser as my undergrad project many years ago.

Doing something modular involves a fair amount of otherwise unnecessary metalwork and connectors. Things could have been made cheaper and simpler. The payoff from the modular approach is when you want to play mix and match with variants to create custom units or just a wide spread of variants. Some things never thought of in the beginning may get done, some things that were a twinkle in the eye at the beginning may never come to pass. Without being party to the initial discussions and seeing how things went, it can be very difficult to piece together from the outside and after the event.

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Old 10th Oct 2022, 1:46 pm   #22
inaxeon
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

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Originally Posted by Radio Wrangler View Post
I did indeed.

The thing with a modular system is that you don't have to have all the modules available from the start that it might eventually use. They may never have done a NICAM module, but it is quite likely that they left the door open for one if they later changed their mind.

David
I've done extensive teardowns and analysis of Philips NICAM modulation system from the same era as the PM5680 on my website. It would be physically impossible to cram their solution into a single card slot like that. It actually has three cards that size, and that is not including the CPU board, clock generator and directional IF coupler.

Probably by the late 1990s such a development would be possible with the help of LSIs and surface mount components but there is still a problem as the audio inputs on the rear of the unit are such that it's one input per card as would be typical for Zweiton so physical modification would be required. By that time PTV were working on DVB-T solutions anyhow.

For the benefit of erasing any doubt here is a spec sheet for the PM5680:

https://xpander.mattmillman.com/file...5680_Specs.pdf

All of the options are listed.
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Old 10th Oct 2022, 2:49 pm   #23
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

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As to where these weightings would be applied, I am not sure, as I was only a humble TV repair person, and not a broadcast engineer, but I would suspect it was done inside your PM5680.
Well that is an interesting thing to point out. I actually just managed to get a copy of the manual for the previous generation PM5580 which looks to have quite similar functionality. I can't see anything about adjustment of weightings in there but I'll probably have to read through it a few more times to be sure.
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Old 10th Oct 2022, 4:18 pm   #24
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

Would it need any adjustment? Couldn't it be hardwired with precision components?

It's analogous to FM pre-emphasis, is there a control for that?
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Old 10th Oct 2022, 5:30 pm   #25
inaxeon
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

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Would it need any adjustment? Couldn't it be hardwired with precision components?

It's analogous to FM pre-emphasis, is there a control for that?
Yes there is an on/off switch.
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Old 11th Oct 2022, 8:02 pm   #26
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

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Originally Posted by inaxeon View Post
Quote:
Originally Posted by rambo1152 View Post
As to where these weightings would be applied, I am not sure, as I was only a humble TV repair person, and not a broadcast engineer, but I would suspect it was done inside your PM5680.
Well that is an interesting thing to point out. I actually just managed to get a copy of the manual for the previous generation PM5580 which looks to have quite similar functionality. I can't see anything about adjustment of weightings in there but I'll probably have to read through it a few more times to be sure.
These weighting factors to the R-Y and B-Y signals were applied at source.

A typical bit of broadcast tv kit would produce RGB outputs but obviously we needed PAL (eventually) so the RGB signals were applied to a PAL coder which transformed these signals to Y, R-Y and B-Y, the R-Y and B-Y signals were then weighted (by the factors already given) and modulated onto a colour subcarrier to give the U and V signals. these were then 'multiplexed' together to give us the familiar PAL signal. This process of weighting the R-Y and B-Y signals was universal inside PAL coders and as such was not something to concern transmitter engineers. Broadcast kit that worked entirely in the PAL domain, such as a VTR machine for example, would obviously be working with PAL signals throughout so these considerations were of no concern.

The process of weighting the R-Y and B-Y signals to derive the U and V signals inside a PAL coder was a 'run of the mill' consideration and as such not something to concern your average broadcast engineer because it was 'built in' and not really anything you thought about because it was so fundamental to the whole signal chain at source.

They would be nothing to stop someone running transmitter site putting in additional signal limiting if they chose to, but I am not aware that they did. But who knows ..
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Old 11th Oct 2022, 9:06 pm   #27
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

Thanks for the detailed explanation. So much useful info for this project coming from these forums! But it sounds like this is wandering outside the boundaries of my project!

Out of interest - would this be an example of such a PAL encoder: https://www.vintage-radio.net/forum/...9&d=1488833342 ?

Last edited by Radio Wrangler; 12th Oct 2022 at 6:17 am. Reason: looks like a mix-up with a quote
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Old 11th Oct 2022, 9:53 pm   #28
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

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Originally Posted by inaxeon View Post

Thanks for the detailed explanation. So much useful info for this project coming from these forums! But it sounds like this is wandering outside the boundaries of my project!

Out of interest - would this be an example of such a PAL encoder: https://www.vintage-radio.net/forum/...9&d=1488833342 ?
Well it refers to itself as a PAL encoder and it appears to have the expected ins and outs so yes I guess it must be - not seen one of those myself (looks a bit vintage to me). Personally I was more familiar with Link and Cox encoders.

Last edited by Radio Wrangler; 12th Oct 2022 at 6:18 am. Reason: quoting someone quoting themselves looks like there was a mix-up!
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Old 22nd Oct 2022, 8:40 pm   #29
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Default Re: Does anyone know anything about "Modulation Depth"? (Philips PM5680)

Finally back to actually work this out. The assertion I made about 90% modulation depth turns out to be completely incorrect.

What I did in the end is flipped through all of the patterns on a Philips PM5631 pattern generator (it has a hundred) and selected the one which has the highest amplitude baseband signal I could see on my scope i.e. highest luma + chroma value (I didn't actually look at it).

I then connected a second input on the scope to the output of a receiver tuned to the setup: https://www.vintage-radio.net/forum/...d.php?t=195298 winding the modulation depth control downwards until I start to see it distorted on the receiver. Once I hit this point I then wound it up a few percent again just to be safe.

I found the optimum setting is actually around 10%.
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