What Is DV? by Stephen Davega Digital Video
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What is DV? By Stephen DaVega Digital Video (DV) is an international standard consumer video format launched in 1996. DV, originally known as DVC (Digital Video Cassette), was the first digital recording format in the reach of consumer markets, it uses a 1/4 inch (6.35mm) metal evaporate tape to record very high quality digital video. The video is sampled at the same rate as D-1, D-5, or Digital Betacam video -- 720 pixels per scanline -- although the color information is sampled at half the D-1 rate: 4:1:1 in 525-line (NTSC), and 4:2:0 in 625-line (PAL) formats. The DV format has its roots in professional digital component video (D-1). Created by a consortium of 10 companies: Matsushita Electric Industrial Corp (Panasonic), Sony Corp, Victor Corporation of Japan (JVC), Philips Electronics, N.V., Sanyo Electric Co. Ltd, Hitachi, Ltd., Sharp Corporation, Thompson Multimedia, Mitsubishi Electric Corporation, and Toshiba Corporation. Since then others have joined up; there are now over 60 companies in the DVC consortium. Its smaller tape form factor MiniDV, has since become one of the standards for consumer and semiprofessional video production. The DV specification (originally known as the Blue Book, current official name IEC 61834) defines both the codec and the tape format. Features include intraframe compression for uncomplicated editing, a standard interface for transfer to non-linear editing systems (IEEE 1394/FireWire), and good video quality, especially compared to earlier consumer analog formats such as 8 mm, Hi-8 and VHS-C. DV now enables filmmakers to produce cheap movies, associated with no-budget cinema. DV begins as an 8-bit, 4:2:2 format just like D1. During the sampling process, all video samples are kept and one half the color samples are discarded. This means there are 720 video samples per horizontal line (excluding blanking time) and only 360 color samples. Therefore, DV becomes a 4:1:1 format providing an excellent quality consumer recording medium and a very capable professional news gathering tool; so much so that many consider DV equal to or better than professional Beta SP. There have been some variants on the DV standard, most notably the more professional DVCAM and DVCPRO standards by Sony and Panasonic, respectively. Also, there is a recent high-definition version called HDV, which is rather different on a technical level since it only uses the DV and MiniDV tape form factor, but MPEG-2 for compression. Technical Standards Video compression DV raw data rate undergoes MPEG II compression via DCT (discrete cosine transform) intraframe compression at a fixed bitrate of 25 megabits per second (hence the designation DV25), a compression ratio of about 5:1, which amounts to roughly 3.6 megabytes per second or 4 minutes per Gigabyte. Once you add in audio, subcode (including timecode), Insert and Track Information (ITI), and error correction, the total data stream come to about 36 Mbps. At equal bitrates, DV is somewhat better than MJPEG (motion-JPEG, which is very similar), and is comparable to intraframe MPEG-2. Note that many MPEG-2 encoders for acquisition applications do not use intraframe compression. DV uses intraframe compression: Each compressed frame depends entirely on itself, and not on any data from preceding or following frames. However, it also uses adaptive interfield compression; 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. In theory, this means that static areas of images will be more accurately represented than areas with a lot of motion; in practice, this can sometimes be observed as a slight degree of "blockiness" in the immediate vicinity of moving objects, as discussed below. Depending on the image contents, the encoder adaptively decides whether to compress picture fields separately or combine two fields into a single compression block. As such, DV coding can be thought of as something half-way between Motion JPEG and MPEG As a curiosity, the consumer version (DV) sports one of the densest recording techniques based on magnetic tape media - more than 0.4 megabits per square millimeter. Imagine the data from your 3.5" HD floppy recorded on a single-sided 5x6 mm piece of tape. New equipment from Sony will push this even further with the deployment of LP mode, which will reduce the track width to 6.67 um and multiply the recording time by 1.5. Video specifications will remain the same. Chroma subsampling The chroma subsampling is 4:1:1 for NTSC or 4:2:0 for PAL, which reduces the amount of color resolution stored. Therefore, not all analog formats are outperformed by DV. The Betacam SP format, for example, can still be desirable because it has similar color fidelity, no digital artifacts and good low-light performance. The lower sampling of the color space is also a reason why DV is sometimes avoided in applications where chroma-key will be used. However, a large contingent feel the benefits (no generation loss, small format, digital audio) are an acceptable tradeoff given the compromise in color sampling rate. Audio DV allows either 2 digital audio channels (usually stereo) at 16 bit resolution and 48 kHz sampling rate, or 4 digital audio channels at 12 bit resolution and 32 kHz sampling rate. For professional or broadcast applications, 48 kHz is used almost exclusively. In addition, the DV spec includes the ability to record audio at 44.1 kHz, the same sampling rate used for CD audio, but in practice, this option is rarely used. Connectivity The IEEE 1394 or Firewire serial data transfer bus is not a part of the DV specification, but co-evolved with it. Nearly all DV cameras have a IEEE 1394 interface and analog composite video and Y/C outputs. High end DV VCRs may have additional professional outputs such as SDI, SDTI or analog component video. All DV variants have a timecode, but some older or consumer computer applications fail to take advantage of it. On computers, DV streams are usually stored in container formats such as AVI or Quicktime, but sometimes raw DV data is recorded directly. Physical format The DV format uses "L-size" cassettes, while MiniDV cassettes are called "S-size". Both MiniDV and DV tapes can come with a low capacity embedded memory chip (a scant 4 Kbits for MiniDV cassettes). This embedded memory can be used to quickly sample stills from edit points (for example, each time the record button on the camcorder is pressed when filming, a new "scene" timecode is entered into memory). DVCPRO has no "S-size", but an additional "M-size" as well as an "XL-size" for use with DVCPRO HD VTRs. All DV variants use a tape that is 1/4 inch (6.35 mm) wide. The "L" cassette is about 120 x 90 x 12 mm and can record up to two hours of video. What's the difference between DV(miniDV), DVCAM, and DVCPRO? Not a lot! The basic video encoding algorithm is the same between all three formats. The VTR sections of the US$20,000 DVCAM DXC-D130 or DVCPRO AJ-D700 cameras will record no better an image than the lowly DV format DCR- VX1000 at US$4,000 (please note: I am not saying that the camera section and lens of the VX1000 are the equals of the high-end pro and broadcast cameras: there are significant quality differences! But the video data recorded in all three formats is essentially identical, though there may be minor differences in the actual codec implementations). A summary of differences (and similarities) is tabled in Technical Details. The consumer-oriented DV uses 10 micron tracks in SP recording mode. Newer camcorders offer an LP mode to increase recording times, but the 6.7 micron tracks make tape interchange problematic on DV machines, and prevents LP tapes from being played in DVCAM or DVCPRO VTRs. Sony's DVCAM professional format increases the track pitch to 15 microns (at the loss of recording time) to improve tape interchange and increase the robustness and reliability of insert editing. Panasonic's DVCPRO increases track pitch and width to 18 microns, and uses a metal particle tape for better durability. DVCPRO also adds a longitudinal analog audio cue track and a control track to improve editing performance and user-friendliness in linear editing operations. How good are the DV formats compared to other formats? DV formats are typically reckoned to be equal to or slightly better than Betacam SP and MII in terms of picture quality (however, DV holds up better over repeated play cycles, where BetaSP shows noticeable dropout). They are a notch below Digital-S and DVCPRO50, which are themselves a (largely imperceptible) notch below Digital Betacam, D-1, and D-5. They are quite a bit better than 3/4" U-matic, Hi8, and SVHS. On a scale of 1 to 10, where 1 is just barely video and 10 is as good as it gets, I would arrogantly rate assorted formats as follows: D-5 (10-bit uncompressed digital) 10 D-1 (8-bit uncompressed digital) 9.9 Digital Betacam, Ampex DCT 9.7 Digital-S, DVCPRO50 9.6 DV (miniDV), DVCAM, DVCPRO 9.2 MII, Betacam SP 9.1 D-3, D-2 (composite digital) 9 1" Type C 8.9 3/4" SP 6.5 3/4", Hi8, SVHS 5 Video 8, Betamax 4 VHS 3 EIAJ Type 1, Fisher-Price Pixelvision 1 [Note that I place D-2 and D-3 uncompressed composite digital formats lower than any of the component formats, even the analog and compressed ones.