what does an analog TV demodulator really do?

Started by dtsao February 20, 2008
glen herrmannsfeldt wrote:
 > Jerry Avins wrote:
 > (snip)
 >> Separating the sound from the video went out well before color came 
on the scene. By taking the sound after the video detector, the 4.5 MHz 
frequency is not affected by any fine tuning of the LO. It used to be 
that the sound would be distorted if the picture was, but not with what 
is called intercarrier sound.
 >> The detector beats the sideband to baseband using the TV carrier as 
the beating signal. All rectifying (and some other) AM detectors do that.
 > Which one uses fewer tubes?

Most of my cousin's tube detectors used only one tube. There was one in 
my TV set that needed touching up, so I got the schematic out of the 
library. It was kind of weird. The sound carrier was fed to the control 
grid of a pentode. There was a passive tank circuit tied between the 
suppressor grid and ground, a bias arrangement that kept the tube near 
cutoff when quiescent. Audio was pulled off the plate, which was 
bypassed by a capacitor. How?

I eventually figured it out. The tank was (supposed to be) tuned to the 
4.5 MHz. The positive parts of the swing made pulses, the negative 
swings simply drove the tube further into cut-off. It was a plate 
detector with a twist. The tank circuit was the twist. Its Q was high 
enough so that it rang at the average carrier frequency. As the 
frequency of the input varied, the pulses got wider or narrower, 
changing the average DC in the pulses.

It turned out that ten years of component drift had thrown the tank too 
far out of tune. I was able to restore it by ear. I dubbed it a flywheel 
detector, the tank being the flywheel. I took the schematic to my 
cousin, the TV expert in the family (check it out: the discriminator 
patent is Avins and Seely) and asked him if he had ever heard of the 
weird thing, and did it work the way I thought. I suspect he was 
offended; after all, it was an RCA set. He told me the correct name 
(which I forget) and that it was one of his.

As for the video detector, all that needs is a diode.

Engineering is the art of making what you want from things you can get.