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In Reply to: RE: Here's A More Direct Source posted by Bromo33333 on December 12, 2016 at 14:02:31
It reminds me a little of my early days working on PA's for TV transmitters - the modulation for color TV was a very complicated one with AM, FM and all of that mixed up at once.
I got into it when they were working on the HDTV standards, and back when they were thinking of doing it analog. Quickly they started using QAM signals in Europe (where I was working at the time) when the US went their own way with 8VSB (developed by Zenith before being consumed by, I think, LG or Samsung - can't remember).
Good times.
Glad to see an old RF guy like myself here. Too often people do "engineering by google" so you never know where the other fellow is standing.
Didn't know about the commutating time domain sampling method. SOunds like a clever way to re-use circuity or save some costs. Wouldn't occur to me to do it that way. But as a high power RF PA guy, I am all about KISS - and reliability. Since when you are making a 30kW RF system, you really don't want anything to fail!
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"We have met the enemy and he is us" - Pogo
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"the modulation for color TV was a very complicated one with AM, FM and all of that mixed up at once."
Not exactly, unless you are including the sound carrier.
The chroma signal is like the FM L-R channel in that it is carrier suppressed. Also, the detectors in an FM tuner for L-R are actually synchronous detectors. The detectors in the NTSC system had two synchronous detectors, one running 90 degrees out of phase thus yielding two discrete signals.
As with FM, it is not that different on the transmitting end. But it is a bit more complicated than that. The I signal had a bit more bandwidth but only a few TVs could use it. The RCA CTC111 I believe had what was called "wide I" demodulation but then they used a crap CRT. It was the early days of the inline gun CRTs and the picture would have been much better on a delta gun CRT. The pitch of those early inline gun jobs was so poor they shouldn't have bothered. Wide I demodulation also required another delay line. That chassis also had one of the earliest digital COMB filters on the market.
The I and Q signals were the difference signals for red and blue, I forget which is which. The green was derived from a matrix circuit, when the red and/or blue went down, it made the green go up and vice versa.
There is some complex math involved as they found that the green carried the most detail so it was fairly predominant in the monochrome signal, which underlies those difference signals.
But there was no FM in there, that was phase modulation and was not even intentional because when you mix two waveforms that are 90 degrees out of phase you get different phases which range from the instantaneous to the quadrature.
More useless knowledge, I have tons of it.
Getting back to the OP here, these ultra good digital formats will happen when not only the media is invented, but when the sources are available. I listen to some music from the 1950s and find a 128K MP3 to be quite adequate, though even on that material I have sometimes noticed a difference with a higher bitrate file. When I downloaded I would get several copies of everything and delete the inferior ones. It is actually somewhat surprising how good some of those old recordings are, considering the times. Others not so good.
There is now the holographic disk. It makes blu-ray obsolete. It has so much capacity nobody can use it. Perhaps that is the medium of choice. The ultra high quality formats would take downloading back to the 20th century, like it was on dialup. Imagine a five minute song being 800 MB.
Here's a wiki on those disks :
https://en.wikipedia.org/wiki/Holographic_Versatile_Disc
If you wanted to send someone a song, depending on your internet speed you might be better off just burning one of those and snail mailing it. However the technology has not been perfected. They've apparently done it but it is not quite market ready I guess. But they're talking capacities up in the terabytes.
They get that ready, THEN you can throw away the vinyl.
A bit of background info...
One method of implementing amplitude modulation is to multiply the input signal by a square wave at the carrier frequency, which can be implemented using diodes as switches. Because a square wave is just the superposition of an infinite series of cosines at the fundamental frequency of the square wave and its harmonics, the result of multiplying the square wave by the input signal is an infinite series of amplitude modulated signals. The fundamental and all of its harmonics are amplitude modulated by the input signal. Then you use a bandpass filter to select the one that matches the desired carrier frequency, usually the fundamental or third harmonic. This "switching" method of amplitude modulation is cheap and easy to implement in low level solid state circuits and is more linear than a square law modulator. So it's well suited for implementation on a chip and thus common.
When the FCC was soliciting proposals for FM stereo, two of the companies (sorry, I don't remember which) proposed transmitting sum (L+R) and difference (L-R) signals, with the sum transmitted in the 15 KHz baseband just like FM mono and the difference signal AM modulated into the channel space above that. If I remember correctly, one of the proposed implementations involved explicitly forming the difference signal and then using amplitude modulation, and the other proposal involved using a square wave to switch between the L signal and an inverted R signal. These two approaches are mathematically equivalent.
By the way, my background is originally RF as well (and computational electromagnetics). But I'm far from that now.
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