Vertical interval signals on 405

[valvekits]

I had assumed that if the burst amplitude varied, then so did the chroma so that ACC would take care of that. But I hadn't considered the possibility of the chroma varying independently of the burst.

Anyway, I did a bit more digging and for anyone interested, found a measuring differential gain and phase PDF document from TI.

Your right, it is academic, but also scary to realise that I was oblivious to all the attention to detail that was being given to so many things on the broadcasting side back then.


Eddie

[ppppenguin]

Thanks for digging out that document. I've done it that way, or in a very similar way, when designing equipment. For video processing and switching kit I was trying to measure dP and dG down to well below 0.1 degrees/0.1%. This isn't easy. The usual video test waveform with a vectorscope (many could do dP and dG measurements) was increasingly awkward and inaccurate much below 1%. I have a vector network analyser (HP4195A) and with care I could approach 0.01degrees/0.01%.

The reason for going very low in video processing is because a lot can be cascaded in a typical broadcast system.

Analogue video was hard. Maintaining accuracy through a lot of kit took a huge amount of engineering effort. Design, maintenance and operation.

[valvekits]

Another curious thing is moving out of a 75 ohm to 50 ohm environment. I noticed that this isn’t trivial when using your HP4195A in this context. TI have another application report (snoa370B) where this is discussed for your network analyser.

[ppppenguin]

I have the 75R kit for my 4195A. It's the 41952B transmission/reflection test set. That goes on one set of ports. For the other end there's a 75>50 minimum loss pad. All gets calibrated out when you do the calibration routines with open/short/terminated for 1 port measurements or open/thru for 2 port.

Need to take great care with 75R and 50R N connectors. 75 plug into 50 socket gives no contact. Other way round gives wrecked socket.

The things I have to be aware of in my lab are 50R BNC terminations. This is a video based lab and 50R stuff is forbidden. If I do have any 50R cables, terminations etc they are marked clearly with green and black tape or sleeving. Despite some belief to the contrary 75R and 50R BNC fittings are fully interconnectable without harm. There might be a slight mismatch but at frequencies I'm using the connectors are electrically very short and it doesn't matter.