RadioBanter - View Single Post

Paul Keinanen · July 12th 10, 04:52 AM posted to rec.radio.amateur.homebrew

On Sun, 11 Jul 2010 19:25:06 -0700, (Dave Platt)
wrote:

If you're just interested in AM signals, then you could filter
the incoming signal, split it, amplify one half, and run this
through a *very* narrow-band filter (think "oscillator and PLL")
and use this as the LO input to the mixer. In effect, what
you'd be doing is creating an LO signal which is a copy of the
original signal's carrier, with the sidebands stripped away by
the ultra-narrow filter / PLL. This is a pretty basic way of
doing AM reception.

This is how some early video-IF chips performed the video detection
(instead of simple envelope detection). There was a high-Q resonance
circuit tuned to the nominal video-IF, which was excited by the visual
carrier. Due to the high Q, the resonator was "ringing" for a few
cycles, even if there was no carrier present for a few cycles.

This filtered carrier was then used by the product detector
(multiplier) to extract the video and intercarrier sound.

In countries using negative video modulation (sync 100 %, black 80-90
% and white about 10 %), the intercarrier sound system could be used.
After the video detector, the sound FM signal was at 4.5, 5.5, 6.0 or
6.5 MHz depending on system and was then processed with an ordinary FM
strip tuned to that frequency.

For proper operation, this required that both the visual as well as
audio carrier frequency was always present in order to produce the
frequency difference signal for the 4.5-6.5 MHz audio subcarrier.

For various reasons (bad filter group delay, overmodulated white
levels, especially subtitles), the video level dropped to 0 %, the
simple envelope detector did not work and hence the intercarrier audio
signal could not be formed and there was various forms "picture in
audio" buzz.

The ringing in high-Q section at the product detector made it possible
to "ride through" the short video signal interruption, intercarrier
audio-IF was produced constantly and thus the buzz in the audio was
reduced.

Paul OH3LWR