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.
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.
