#1 - Analogue Television Broadcast Modulators - Operation and Basic Theory 

In the last half of the 1970's, I worked in the lab at NRK (the Norwegian broadcaster) while at the university. We had a problem covering football games where the red shorts did not follow the players around the green pitch. To remedy this a 'trouser puller' ('buksepådrager') was created. This was a passive filter that had the opposite group delay and amplitude characteristics as a 500 meter coax cable. One man worked for months selecting capacitors and tuning coils, all mounted on a 50 cm long board with solder tabs. It worked perfectly, and versions were made for different lengths of coax.

@19:21 Sync stretching is used to compensate for sync compression in the linear amplifier chain of your typical broadcast transmitter. The sync tips get compressed somewhat depending on how much headroom your amplifier has. In tube amplifiers, for example, as the tube ages, its peak output capability will drop. Stretching the sync back out can compensate for some of that. Usually this is done in a processing amplifier (proc amp) ahead of the modulator.

Thank you for an excellent video which was most interesting. I used to work on microwave links for TV distribution to the transmitters. One reason for the vestigial sideband is that an SSB-plus-carrier signal suffers from quadrature distortion. Transmitted waveforms having a high modulation index are made to be somewhat pointed at the top. The large amplitude waveforms for TV occur at low frequencies, so it is convenient to retain DSB at low video frequencies. The black level clamping occurs during the back porch, which is used as a reference level. A simpler system just clamps with a diode to the sync tip. The sync pulse stretching is useful if the transmit power amplifier is non linear. Most RF amplifiers tend to compress as the drive voltage goes up, and it is desirable to keep picture/sync ratio at 70/30. Not sure if you mentioned gamma correction, but it happens that a full colour signal might exceed 1V PP on some occasions, and I believe the allowable max is a little greater. Maybe the white clipper is to stop this happening to protect the RF amplifier. In addition to group delay we also used to test for differential gain and differential phase, which are diseases of FM microwave links.

back in the 80's colour televisions had a Chroma delay pot to bring the colours in time with the luma. the Baird 9000 series was the first I recall with SAW filters, it was literally a glass panel with a led input and a photo detector on the output.

Really interesting. Thanks for putting so much effort into making this. Always fascinating to read comments from folks who used to work in the industry adding context and extra info, so thanks to them too!

The white clip was set to 103% IRE here in Denmark, if the video modulation was to high, you would get video am noise in sound, some older transmitters could not handle more than 105%, so they shut down if the videosignal was to high. Here in DK the transmiters was operated by the Danish Post and Telegraph in the older times, we just delivered the signal to them with correct dc level, and correct sync/videolevels. They must have had the IF modulator locally placed at our MCR area, because when we added NICAM stereo the NICAM encoder also was in the MCR area.

Thanks for making this. I learnt the theory of analogue broadcasting back at university eons ago and worked in broadcast engineering for a time. Brought back a few memories, though most of the analogue tx equipment was already earmarked for the dumpster and I didn’t really get involved with it. It might be old tech but the RF engineering is still as relevant as it ever was.

So glad I’ve discovered this channel!

You just got a new subscriber, as a kid from the 70s, analogue TV is so nostalgic for me... I remember (UK based) NICAM 728 digital stereo ... (Near Instantaneous Companded Audio Multiplex, 728 KBPS)..., analogue satellite tv..., it was soooo much fun! I was a subscriber to Television Magazine here in the UK, sadly in the dying days of TV repair shops.

im from a tv repair background so this is far to complex for me but still interesting. tvs had a "black level clamp" or "dc correction" or "black restorer" never heard the term pedistal clamp

I don’t recall any group delay compensation at the transmitters in the uk

I was reading through video demystified by keith jack and on figure 8.21 it shoes rf modulation implementation. It looks like AM, so why are there different frequencies present/prominent during v sync? blanking and field/lines?

Group Delay? Some of these audiophool's worry about group delay in their speaker cables! I kid you not! Yes, it does occur with audio frequencies on paper. But really?

What is the standard video system IF frequency in Europe? In the USA. Canada, Mexico it's 70mhz FM. That goes for point to point microwave and satellite systems.

Maybe one day in future when current digital delivery network gets the bandwidth and efficiency of analog, can reintroduce this engineerin

analog FPV/crappy crappy transmitters in the 5ghz realm is still a thing