Quote:
Originally Posted by NewVista
...what is wrong with SSB for HF chroma components? as HF Luminance is SSB.
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Basic color analog signal theory:
A single sideband part of the signal produces a quadrature signal component (all frequency components shifted by 90 degrees). A synchronous demodulator will ignore this component if a single SSB signal is transmitted.
However, the chroma signal has two signals transmitted at 90 degree phase difference. Each synchronous chroma demodulator then ignores any quadrature component of its desired chroma component (e.g., R-Y), but sees the quadrature component of the other chroma component (e.g., B-Y).
This is why the original NTSC specs extended only one component (I) into a vestigial sideband region. The I demodulator sees the lower I sideband, and no Q quadrature high frequencies are present because they aren't transmitted; the Q demodulator is narrowband and therefore does not see the quadrature components due to the wideband I signal.
Transmitting wideband on both chroma axes [edit: and then cutting off part of the upper sideband] introduces quadrature distortion of higher frequency chroma, but since receivers are commonly narrowband, they do not see this distorted color detail (or any color detail!). In PAL, some quadrature distortion is tolerable because of the cancellation of phase errors, so more detail can hypothetically be squeezed out of the lower sideband of non-symmetrical chroma sidebands.