DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Color-under recording was introduced
in U-matic VCRs, using
a low-frequency subcarrier of
approximately 688 kHz. A dub
connector carried separate luma
and chroma components; this
interface was known as Y’/C688.
The original VHS VCRs used
a low-frequency subcarrier at
about 629 kHz, and some industrial
equipment implements the
corresponding Y’/C629 interface.
(Chroma carrier frequency in Hi8
VCRs is approximately 743 kHz.)
the original video, so this mixing destroys the coherence
of subcarrier and line rate. Chroma, now on
a 3.58 MHz subcarrier, is summed with luma to
produce the composite playback output.
Although playback chroma from a color-under VCR is
remodulated onto a crystal-stable subcarrier, the luma
and chroma signals – and the sync component of both –
contain timebase errors. However, the horizontal scanning
of a television receiver has sufficient range to track
the timebase error in sync, and a crystal oscillator can
lock to the subcarrier. So a TBC is not necessary upon
playback to a monitor or television receiver. Some
residual timebase jitter will be visible if a TBC is not
used; the residual jitter limits effective resolution.
After color-under recording, the frequency interleaving
that may have once been present in the signal is lost
from the composite interface. There is no point in
attempting to extract more than about 3 MHz of luma
bandwidth. However, if an S-video interface is implemented
in a color-under VCR, luma and chroma can be
recorded without first having to be separated, and luma
and chroma can be played without being combined.
Much better performance can be obtained from the
S-video interface than from the composite interface.
SECAM cannot use the same heterodyne technique as
NTSC and PAL. Two different schemes have been
adopted to record SECAM on VHS VCRs.
• In SECAM VHS machines in France and Russia, luma
and SECAM frequency-modulated chroma are separated;
the chroma component is then subject to
frequency division by a factor of 4. The frequencydivided
chroma is recorded using the color-under
method. Upon playback, the chroma is frequency-multiplied
by 4.
• In the MESECAM VHS videotape format used in the
Middle East, SECAM (FM) chroma is downconverted
similarly to color-under PAL or NTSC. MESECAM refers
only to a VHS format; it is not a broadcast standard.
584 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES