QSS and sound IF etc

F

Fred

Guest
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?
 
"Fred" <abuse@hotmail.com> wrote in message
news:3f84512f$0$10987$fa0fcedb@lovejoy.zen.co.uk...
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?
Click to expand...
I can't believe there isn't a knowledgeable TV engineer here!

Back to the drawing board.
 
Whereas On Wed, 8 Oct 2003 19:00:41 +0100, "Fred" <abuse@hotmail.com>
scribbled:
, I thus relpy:
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?

It's not modulated onto the video, just transmitted with it,
Click to expand...
essentially on another whole carrier.
--
Gary J. Tait . Email is at yahoo.com ; ID:classicsat
 
"Gary Tait" <seesig@xxx.yyy> wrote in message
news:a3dbovsl0s8789u63lb0kjpmf168g673uq@4ax.com...
Whereas On Wed, 8 Oct 2003 19:00:41 +0100, "Fred" <abuse@hotmail.com
scribbled:
, I thus relpy:
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?

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
Click to expand...
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?
 
Whereas On Fri, 10 Oct 2003 00:01:15 +0100, "Fred"
<abuse@hotmail.com> scribbled:
, I thus relpy:
"Gary Tait" <seesig@xxx.yyy> wrote in message
news:a3dbovsl0s8789u63lb0kjpmf168g673uq@4ax.com...
Whereas On Wed, 8 Oct 2003 19:00:41 +0100, "Fred" <abuse@hotmail.com
scribbled:
, I thus relpy:
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?

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

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?
Click to expand...
The color subcarrier is on a different, lower frequency than the audio
sub carrier.
--
Gary J. Tait . Email is at yahoo.com ; ID:classicsat
 
"Gary Tait" <seesig@xxx.yyy> wrote in message
news:kdjdov8bg1gvt5aut651eie7r3n4q147ao@4ax.com...
Whereas On Fri, 10 Oct 2003 00:01:15 +0100, "Fred"
abuse@hotmail.com> scribbled:
"Gary Tait" <seesig@xxx.yyy> wrote in message
news:a3dbovsl0s8789u63lb0kjpmf168g673uq@4ax.com...

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

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?


The color subcarrier is on a different, lower frequency than the audio
sub carrier.
--
Gary J. Tait . Email is at yahoo.com ; ID:classicsat
Click to expand...
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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