Quote:
Originally Posted by Tom Albrecht
Is the reason a slope detector wouldn't work with intercarrier sound because there is amplitude modulation present on the 4.5 MHz carrier from the video carrier?
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The passband on the video IF has a slope to it, where the picture carrier is located. This is the Nyquist slope. This slope introduces some fake FM in intercarrier sets. Only reason we can get away with this in intercarrier sets is the FM capture effect, which helps the FM detector ignore a weaker FM signal in favor of a stronger one. This weak fake FM comes from the video modulation sideband frequencies beating against the FM sound carrier, combined with the Nyquist slope.. Imagine if we tried to do intercarrier if the TV sound was still in AM. Buzz city...
When TV stereo sound was introduced, the sound IF, independent of the video IF, had its own pair of filters at 41-45MHz. One peak at the video carrier, without the above slope. And another peak on the sound carrier, before it's made into a 4.5MHz FM signal. This would also have worked if the sound was AM. And you wouldn't have to do fine tuning just to get the sound.