QSS and sound IF etc

Oct 08, 2003 5 Replies
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In a typical TV chassis, a single reference QSS mixer is used to synchronously demodulate the IF to give the sound and NICAM carriers. It uses a quadrature reference (shifted by 90 degrees), generated from the video demodulator. When the IF signal is multiplied by this reference, the video information is nulled leaving so that only the sound and NICAM carriers are demodulated.



Are there any papers on the net which explain this in more detail? The part I can't understand is if the sound carrier (FM) is added to the baseband video, before modulating a UHF carrier, why shouldn't this be nulled out as well! What am I missing?


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F

the

part

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I can't believe there isn't a knowledgeable TV engineer here!

Back to the drawing board.

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G

Whereas On Wed, 8 Oct 2003 19:00:41 +0100, "Fred" scribbled: , I thus relpy:

It's not modulated onto the video, just transmitted with it, essentially on another whole carrier.

Gary J. Tait . Email is at yahoo.com ; ID:classicsat
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F

the

part

as

Many thanks for your post.

But how does that make a difference? The colour subcarrier is essentially transmitted in the same way as the sound carrier. Given the information is in a vestigial sideband how can one discern the difference?

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Whereas On Fri, 10 Oct 2003 00:01:15 +0100, "Fred" scribbled: , I thus relpy:

The color subcarrier is on a different, lower frequency than the audio sub carrier.

Gary J. Tait . Email is at yahoo.com ; ID:classicsat
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F

essentially

is

Why should the frequency should make any difference. Otherwise a high pass filter would do the same thing. The QSS mixer uses a reference in quadrature to the carrier so the detector would only get an output if the sound and NICAM were phase modulated onto the vision carrier.

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