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DV

From Academic Kids

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A MiniDV tape

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Digital Video (DV) is a video format launched in 1996, and, in 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.

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.

Contents

Technical Standards

Video compression

DV uses DCT intraframe compression at a fixed bitrate of 25 megabits per second, which amounts to roughly 3.6 megabytes per second or 4 minutes per Gigabyte. At equal bitrates, DV is somewhat better than MJPEG (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.

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

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DV cassettes
Left to right: DVCAM-L, DVCPRO-M, MiniDV

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 Mbits 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.

MiniDV / DV

The "L" cassette is about 120 x 90 x 12 mm and can record up to two hours of video. The better known MiniDV tapes are 65 x 48 x 12 mm and hold either an hour or an hour and a half of video depending on whether the video is recorded at Standard Play (SP) or Extended Play (EP). The tapes sell for less than USD 5 each as of 2003. DV on SP has a helical scan track width of 10 micrometres, EP uses a track width of only 6.7 micrometres.

Software is currently available for ordinary home computers which allows users to record any sort of computer data on MiniDV cassettes using common DV decks or camcorders. A 60-minute MiniDV tape will hold approximately 13 Gigabytes of data in this form of usage as the DV video format has a constant data rate of 3.6 Megabytes per second (3.6 MB/s x 60 seconds x 60 minutes = 12,960 MB per hour = 12.9 GB per hour).

DVCAM

Sony's DVCAM is a semiprofessional variant of the DV standard that uses the same cassettes as DV and MiniDV, but transports the tape 50% faster, leading to a higher track width of 15 micrometres. The data format is the same as DV, but because of the greater track width the tapes are much more robust, and the EP mode of DV is not supported. All DVCAM recorders and cameras can play back DV material, but DVCPRO support was only recently added to some models. DVCAM tapes (or DV tapes recorded in DVCAM mode) have their recording time reduced by one third.

DVCAM is not available in higher bitrates or HD modes. Sony reserves this market for their digital Betacam and HDCAM product lines.

DVCPRO

Panasonic's DVCPRO was specifically created for ENG use (NBC's newsgathering division was a major customer), with better linear editing capabilities and robustness. It has an even greater track width of 18 micrometres and uses another tape type (Metal Particle instead of Metal Evaporated). Additionally, the tape has a longitudinal analog audio cue track. Audio is only available in the 16 bit/48 kHz variant, there is no EP mode, and DVCPRO always uses 4:1:1 color subsampling (even in PAL mode). Apart from that, standard DVCPRO data (also known as DVCPRO25) is the same as DV. However, unlike Sony, Panasonic chose to promote its DV variant for professional high-end applications. With two DV codecs running in parallel for a data rate of 50 Mbit/s and 4:2:2 color sampling, the DVCPRO50 standard was created for ENG compatibility but with reserves for HDTV upscaling. The HD variant, DVCPRO HD (also known as DVCPRO100), uses four parallel codecs and even higher tape speed for a data rate of 100 Mbit/s, at a HDTV resolution of 720p progressive or 1080i interlaced. A camcorder using as special variable-framerate (from 4 to 60 fps) variant of DVCPRO HD called VariCam is also available. All these variants are backward compatible but not forward compatible.

DVCPRO cassettes are always labeled with a pair of run times, the smaller of the two being the capacity for DVCPRO50. A "M" tape can hold up to 66/33 minutes of video. The color of the lid indicates the format: DVCPRO tapes have a yellow lid, longer "L" tapes made specially for DVCPRO50 have a blue lid and DVCPRO HD tapes have a red lid.

The DVCPRO VCRs can play back DV and DVCAM tapes, but MiniDV tapes usually require an adaptor.

Other variants

Sony's XDCAM format allows recording of DVCAM streams on an optical medium similar to a Blu-Ray Disc, while Panasonic's DVCPRO P2 uses recording of DV/DVCPRO/DVCPRO50 streams on PCMCIA-compatible flash memory cards. Ikegami's Editcam System can record in DVCPRO or DVCPRO50 format on a removable hard disk. Note that most of these distinctions are for marketing purposes only - since DVCPRO and DVCAM only differ in the method in which they write the DV stream to tape, all these non-tape formats are virtually identical in regard to the video data.

JVC's D-9 format (also known as Digital-S) is very similar to DVCPRO50, but records on tapes in the S-VHS form factor. (NOTE: D-9 is not to be confused with D-VHS, which uses MPEG-2 compression at a significantly lower bitrate)

The Digital8 standard uses the DV codec, but uses Hi8 tapes to record onto. Because of Video8's and Hi8's success in the past, many precious recordings exist in these formats. The Digital8 format was designed as a consumer format for transition from analog to digital. The video and audio quality of Digital8 is comparable to DV.


See also

External links

fr:DV ja:DV nl:Digital Video pl:DV sk:DV sv:DV

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