26-01-2020, 06:14 PM
Wondering why it was always positive mod & AM sound, & negative mod & FM sound? AFAIK there were no positive mod FM or negative mod AM systems...
Positive Modulation
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26-01-2020, 06:14 PM
Wondering why it was always positive mod & AM sound, & negative mod & FM sound? AFAIK there were no positive mod FM or negative mod AM systems...
26-01-2020, 07:30 PM
Doesn't fully answer the question. You can't use +ve mod with intercarrier sound because the carrier falls to zero at sync tips. I think that AM and intercarrier are a difficult combination. So if you want to use intercarrier sound (to help with tuner drift for example) then you have to use -ve vision and FM.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
29-01-2020, 12:54 PM
Agree - intercarrier AM sound would be impossible, whether the vision is +ve or -ve modulation, because the intercarrier signal is heavily AM'd by the video. It needs a good limiter before the FM sound discriminator.
So if you had AM sound, you'd just strip off all your sound info in the process. Unless... of course, the VISION was FM in which case no AM of the intercarrier beat would arise. But, let's not go there, nobody was quite radical enough to have tried it AFAIK!
29-01-2020, 01:48 PM
Terrestrial microwave links (originally valves with frequency multipliers and TWTs) and later video senders and Analogue Satellite all used FM modulation of the entire baseband. The multiple FM subcarriers were added to the video baseband, up to 6 channels on some, though the higher audio channels tended to have poorer S/N and also if the receiver IF wasn't brilliant you got video crosstalk on the audio too.
I repaired the valve frequency multiplier on what must have been obsolete BBC gear in the mid 1970s. All the Video senders seem to use 6.0 & 6.5 MHz for the audio, which is very good if the video in simply has a 75R resistor and no signal. The so called Digisenders are the same Analogue FM as other makes. The 2.4 GHz models more range than the 5.x. While they might have a choice of 4 video channels, most use a fixed 433MHZ OOK transmitter driven by the sort of IR RX in a VHS (3 pin TTL out) and a simple super-regen rx for the reverse IR remote channel. So few worked with setboxes that used IRDA. The very early illegal "video senders" were just the sort of modulator in a game console or VHS, with an amplifier. They were mono and also dreadful.
29-01-2020, 02:13 PM
I had no real interest in TV's as a hobby apart from way back when I did a bit of DX, I did TV's as job (TV servicing) Mainly single or dual standard valve TV's back then, shortly to be followed by dual standard hybrid TV's, then single standard hybrid etc, then colour TV, then colour TV with chips, then chips with more in 'em + T/Text, back around the mid '80's ITT introduced the Digivision chassis....Yuk...me thought the writing was on the wall...it was for me, I got out of the trade, the demand for service engineers was dropping, only a matter of time.
Game changers for me and many others when I was in the TV game were Sony, Hitachi etc, I was in at the beginning, UK producers on their complacent bums, easy meat for the Japanese, UK tried to make things difficult for them but there was no stopping. Lawrence.
29-01-2020, 03:21 PM
You can modulate any combination of vision polarity and AM/FM sound using intercarrier methods. It's simple. Just modulate the sound on to the difference frequency between S and V carriers, then add it to the vision signal ready to be modulated on the RF carrier. It places more stringent linearity requirements on everything after the signals are added.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
29-01-2020, 04:30 PM
The arguments for vision polarity and sound modulation can get very detailed. The basics for vision are:
+ve vision allows you to fully use the non linear part of the transmitter characteristics for sync pulses. So you get better TX efficiency. Deosn't help so much for colour where vision information goes below black. +ve vision means that impulse interference doesn't affect syncs. The white spots can be reduced to some extent by an interference limiter circuit. With -ve vision impulse interference is hardly visible but can affect sync. This is largely sorted by flywheel sync. Good AGC is easier with -ve. Good AGC for +ve needs more complex gating circuits to measure the black level.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
29-01-2020, 06:27 PM
(29-01-2020, 03:21 PM)ppppenguin Wrote: You can modulate any combination of vision polarity and AM/FM sound using intercarrier methods. It's simple. Just modulate the sound on to the difference frequency between S and V carriers, then add it to the vision signal ready to be modulated on the RF carrier. It places more stringent linearity requirements on everything after the signals are added. Agreed! As long as the amplitude of the composite signal never fell below zero (AM peak on the sound, at the same time as minimum-amplitude V signal). Because if it did, demodulating would be a bit more of a challenge, no peak-detector diode.
29-01-2020, 11:46 PM
Talking of positive modulation for colour: If we had used it would some cheapskate British makers have used mean level AGC like they did on 405? Were there any French SECAM colour sets with mean level AGC? I'm presuming it would look dreadful...
30-01-2020, 07:38 AM
It would be truly appalling in SECAM since the luminance would be affected by the AGC while the FM chroma wouldn't be. I also assume that all colour TVs had some form of black level clmaping or DC restoration. Again it would be truly horrible if they didn't.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
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