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PostPosted: Wed Nov 12, 2014 10:58 pm 
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Joined: Wed May 19, 2010 6:12 pm
Posts: 2295
lidnariq wrote:
psycopathicteen wrote:
Add a negative chroma signal, delayed by 90 degrees, to the composite signal.
Wut?

Composite = Luma + Chroma
Chroma = U·sin(t) + V·cos(t)
"Delayed Chroma by 90 degrees" → U·cos(t) - V·sin(t)
Composte + "Delayed Chroma by 90 degrees" = Luma + U(sin(t)+cos(t)) + V(cos(t)-sin(t)) = Luma + U·sin(t+π÷4)÷√2 + V·cos(t+π÷4)÷√2

That would just increase the saturation of the chroma signal.


It would roll off the lower sideband of the color carrier, while keeping the upper sideband intact.


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PostPosted: Thu Nov 13, 2014 1:38 am 
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Location: Seattle
Can you combine upper sideband modulation with input that's already QAM (i.e. complex)? I can't see any reason why not, but it feels funny.

Over the air, the signal is clamped to 4.2MHz bandwidth anyway, so it's already effectively vestigial side band modulated, retaining the lower side band.


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PostPosted: Thu Nov 13, 2014 9:52 am 
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Joined: Sun Sep 19, 2004 11:12 pm
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Location: NE Indiana, USA (NTSC)
Single sideband on a QAM signal would allow rainbows in one direction along the scanline, but not solid colors or rainbows in the other direction. Vestigial sideband on QAM would allow finer rainbows in one direction than the other.

True, chroma is clamped at 4.2 MHz only in RF. This means the usable chroma bandwidth is about 3.0-4.2 MHz, centered around the color burst frequency of 3.58 MHz. But In composite it can be bigger, and in S-Video it can reach the full 6.75 MHz bandwidth of Rec. 601.


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