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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 23rd Feb 2019, 1:34 am   #1
radiograham
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Question Output of UHF modulators.

The 625 line signal consisted of vision and sound carriers,vision being amplitude modulated and the sound was fm.When using a vcr rf output does the vcr modulator output consist of the same ie vision am modulated and sound fm?also what is the output from a uhf modulator when fed from say a dvd player,is this the same? Regards Graham.
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Old 23rd Feb 2019, 3:22 am   #2
rambo1152
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Default Re: Output of uhf modulators

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The 625 line signal consisted of vision and sound carriers,vision being amplitude modulated and the sound was fm.When using a vcr rf output does the vcr modulator output consist of the same ie vision am modulated and sound fm?also what is the output from a uhf modulator when fed from say a dvd player,is this the same? Regards Graham.

The analogue transmitters (the main ones not necessarily the relay stations) combined the output of two separate UHF transmitters, the FM sound transmitter being 5.996MHz higher than the AM vision.

The little modulators in VHS machines simplified this, the vision modulated the UHF carrier and it is mixed with a 6MHz oscillator with the FM sound applied. This mixing causes the sound carrier to appear in the expected place, and also an unwanted sound carrier to appear below the vision as well, but the vision itself will be double-side-band anyway, in a transmitter the vision was vestigial side-band.

The receiver doesn't care too much about these fudges and produces a reasonable picture and sound.

I hope that goes some way to answer your question.
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Last edited by rambo1152; 23rd Feb 2019 at 3:36 am.
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Old 23rd Feb 2019, 9:45 pm   #3
Pieter H
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Default Re: Output of uhf modulators

Two remarks to Graham's inputs:

It is true that in the early TV broadcast transmitters the vision and sound signals were transmitted by separate RF transmitters. However, I'm pretty sure that later the vision and sound intercarrier were first combined and then in one step modulated a single transmitter.

The same was essentially true for the VCR UHF modulators, see e.g. https://www.maximus-randd.com/tv-tun...tml#vcrmodules. Otherwise a modulator module should contain two UHF oscillators.

Due to this method indeed a double sided RF signal was generated, occupying roughly 2 RF channels. When tuning the TV to these signals only one would give correct picture and sound, the other one only a distorted picture or sound-only.

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Old 24th Feb 2019, 12:59 am   #4
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Default Re: Output of uhf modulators

For high power, it was always separate sound/vision transmitters. The reason is inter-modulation distortion due to non-linearity in the output PA, whether it's a klystron, a valve (common at VHF) or semiconductor. For the same reason, at least in IF modulated transmitters (which became the norm), there was always the risk of sideband regeneration with the unwanted (and carefully filtered) lower sideband HF components re-appearing. The spectral purity of the VSB (Vestigial Side Band) transmission and its sound sub-carrier depended upon the absolute linearity of the PA.

At low powers, yes, you could combine the sound/vision and back-off on the output power to prevent all the LSB HF components coming back to haunt you, but at high power, you needed really good linearity correction and separate sound/vision chains to meet the specifications. Even then, high-power filters (cavity based) were often needed to meet specific local demands for band-plan conformance.

All gone now with digital, but the analogue wasn't half 'fun'!

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Old 24th Feb 2019, 1:15 am   #5
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Default Re: Output of uhf modulators

Quote:
Originally Posted by Pieter H View Post
Two remarks to Graham's inputs:

The same was essentially true for the VCR UHF modulators, see e.g. https://www.maximus-randd.com/tv-tun...tml#vcrmodules. Otherwise a modulator module should contain two UHF oscillators.

Pieter
The balanced modulator IC's used, cited as the part number N343-OM are very interesting. For example it says here that they are the same as the TCA240, or that this IC is a development of them (which is an error)

https://www.radiomuseum.org/forum/n1...t_n343_om.html

The N343-OM is the TCA820 I think (not a TCA240), if you check the data sheet for these , pin outs etc you will see its the same part. These are spectacularly good balanced modulator IC's that work well into the UHF spectrum.

The TCA820's were also used inside the modulator unit of Philip's PM5519 Televsion pattern generator. The TCA820 went out of production during the course of that unit, so Philips had to replace them with the TDA0820T with different pin outs and as a result Philips had to change the pcb inside the modulator unit, and a customer had to buy an entire unit for repairs. Due to this I collected some of these IC's so I could make sure to be able to repair my PM5519's if required.
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Old 24th Feb 2019, 10:27 pm   #6
radiograham
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Default Re: Output of UHF modulators.

Hi I was wondering what is the case when the outputs from say a DVD player are fed to a modulator such as the Maplin's one ,are the two carriers generated ie vision AM modulated, sound FM modulated?
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Old 25th Feb 2019, 12:09 am   #7
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Default Re: Output of UHF modulators.

Normally you feed them baseband audio and video. The modulator won't know or care if it's a DVD, camera, VHS, or something else, it will still produce the same RF signals.
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Old 25th Feb 2019, 2:00 am   #8
radiograham
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Default Re: Output of UHF modulators.

Yes I do get that, the thing I can' t understand is the sound carrier output from the modulator still FM or not.
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Old 25th Feb 2019, 3:27 am   #9
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Default Re: Output of UHF modulators.

Yes.
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