Transmission and Recording Standards: DV (tape and FireWire), Cable, and Satellite...
DV is short for the Digital Video standard.
It should not be confused with DV cassettes.
DV cassettes were original launched as “DVCs”, but in an inexplicable
marketing move the manufacturers decided to start calling them just “DV”.
DV cassettes are commonly used to store DV material, but the distinction
between the cassette and the video standard is important, since other media
(such as Digital8 tapes) are also used to store DV material.
DV is digitized video
sampled at the “D1” rate used for DVD material (720 pixels per scanline),
although the color information is sampled at half the D1 rate: 4:1:1 in 525-line
(NTSC), and 4:2:0 in 625-line (PAL) formats. DV is a coding standard that can be used to record a video
signal on tape or to transmit video between two devices such as a camcorder and
a PC.
Unlike MPEG-2, which
performs compression by comparing each frame with the next, DV uses only
intraframe compression. Each
compressed frame depends entirely on itself, and not on any data from preceding
or following frames. However, it
does use adaptive interfield compression for the two interlaced fields
that form a whole frame. If the
compressor detects little difference between the two interlaced fields of a
frame, it will compress them together, freeing up some of the "bit
budget" to allow for higher overall quality.
This means that static areas of images will be more accurately
represented than areas with a lot of motion, and this can sometimes be observed
as a slight degree of "blockiness" in the immediate vicinity of moving
objects.
The video frames are
compressed using a Discrete Cosine Transform (DCT), the same sort of compression
used by JPEG in still image compression. However, the DCT of DV allows for more local optimization of
quantizing tables within the frame than JPEG, giving higher quality at the
nominal 5:1 compression factor that a JPEG frame would show.
DV video information is
carried in a nominal 25 Mbps data stream. With
the addition of audio, subcode information (including timecode), Insert and
Track Information (ITI), and error correction, the total emerging bit stream
comes to about 36 Mbps, which is over 7 times the average read-rate of a DVD.
A DV digital bit stream can
be laid down on a Hi8-quality 8mm tape running at twice the old analog recording
speed, as in Sony’s Digital8 camcorders.
Alternatively, the digital
bit stream can be laid down on Ľ inch (6.35mm) metal evaporate tape, as in
“DV” cassettes (DVCs). The
speed at which the DV tape is run is determined by the required quality.
At the low end is “Consumer LP Mode”, where the helical scan track
width is 6.7 microns. Next up from
this is Consumer SP Mode which uses a 10 micron track.
Next comes Sony professional camcorders’ standard which uses 15 microns
tracks. And top-of-the-range is
Panasonic's DVCPRO with a track width of 18 microns as well as increased track
pitch. The Panasonic standard also
specifies use of a metal particle tape for better durability and adds a
longitudinal analog audio cue track and a control track to improve editing
performance and user-friendliness in linear editing operations.
A DV 36 Mbps bit stream can
be transmitted between two devices over an IEEE1394 connection (also called
FireWire or, by Sony, i.LINK). FireWire,
developed by Apple, co-evolved with the DV standard and the packet structure
used to store DV signals on tape is almost identical to that used for
transmission over FireWire. FireWire
has a theoretical maximum speed of 400 Mbps, so a DV-encoded video can be
transmitted between two devices at over 10 times the real-time duration of the
material, assuming that both devices store the material on a hard disk (not
tape) and that the disk’s read/write performance can keep up with this speed.
DV is never used for long-distance video transmission (cable TV, satellite) because it is so bandwidth-hungry.
Cable and Satellite
MPEG-2 is the most commonly
used VBR (variable bit-rate) algorithm for sending signals over cable and
satellite, probably due to pressure from the studios to use a format with
built-in encryption to protect movie material from copying.
The bit-rate used may be lower or higher than the 5Mbps average/9.8 Mbps
peak used on DVDs, depending on the expectations of the viewer and the type of
material. For channels like BBC America, a lower bit-rate tends to be
used. For movie channels, an
average rate at or above the DVD average of 5 Mbps will be used.
In some parts of the world rates significantly higher than 5 Mbps are
used. For example, in the
Netherlands, “Canal+” transmits in full CCIR 601 resolution with a peak
bit-rate of 15 Mbps.
In an ideal world, cable and satellite companies would already have started moving to MPEG-4 instead of adopting MPEG-2 but this is unlikely to happen because of studio paranoia about MPEG-4 and the lack of tied-in encryption standard.
More...
Notes on various Video and Audio Coding Standards