DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Even if playback errors are so severe that correction is
defeated, the error locations are known. Concealment
uses correct data to interpolate sample values that are
unavailable owing to uncorrectable errors. Concealment
is ineffective for clustered error samples, so data is
shuffled between outer and inner coding to cause
neighboring samples to be widely dispersed on tape.
Compressed DVTRs can afford a much higher budget of
bits allocated to forward error correction. However, in
the face of uncorrectable errors, entire blocks or macroblocks
are lost, not individual samples as in uncompressed
DVTRs. It is necessary to conceal erred blocks
or macroblocks. This cannot be accomplished spatially:
Interfield or interframe interpolation is necessary.
Digital videotape formats
Many digital videotape formats – some would say too
many! – have been standardized. They are summarized
in Table 35.2 overleaf. SMPTE designates a digital
videotape standard by the letter D followed by a dash
and an integer. Several widely used formats have been
deployed without benefit of SMPTE standardization.
D-1 The D-1 format records uncompressed Rec. 601 (4:2:2)
SDTV data onto 19 mm tape, at a video bit rate of
about 172 Mb/s. (The designation D-1 properly applies
to the tape format, not to the Rec. 601 signal interface.)
D-2 In Composite digital SDTV (4f SC ), on page 108,
I outlined the circumstances that led to the introduction
of uncompressed composite 4f SC DVTRs. In 1988,
Ampex introduced the D-2 format, which records
uncompressed 4f SC video onto 19 mm metal particle
tape in a D-1-style cassette, at a bit rate of about
94 Mb/s. (Again, the designation D-2 properly applies
to the tape format, not the 4f SC signal interface.)
D-3 Several years later, Panasonic adapted D-2 technology
to 1 ⁄2-inch tape in a cassette almost the same size as
a VHS cassette; this became the D-3 standard. (In
addition to the VHS-size cassette, D-3 accommodates
a large cassette with a recording time of 4 hours.)
CHAPTER 35 VIDEOTAPE RECORDING 421