DV
From Wikipedia, the free encyclopedia
DV is a format for recording and playing back digital video. It was launched in 1995 with joint efforts of leading producers of video camera recorders.
Original DV specification, known as Blue Book, has been standardized within IEC 61834 family of standards. These standards define common features such as cassettes, recording modulation method, magnetization and basic system data in part 1, and delve into the specifics of 525-60 and 625-50 systems in part 2.[1] The detailed documentation is not available for free, but can be purchased from IEC web store.
Contents |
[edit] Overview
DV uses discrete cosine transform (DCT) to compress every video frame individually. Before applying DCT compression, some color information is removed from original video using chroma subsampling to reduce the amount of data to be compressed. Baseline DV uses 4:1:1 in its 60 Hz variant and 4:2:0 in 50 Hz variant. Relatively low chroma resolution is a reason why DV is sometimes avoided in chroma keying applications, though advances in chroma keying techniques and software made producing quality keys from DV material possible.[2][3]
The sampling raster of the baseline DV video is the same as that of the ITU-R Rec.601 with 720 pixels per line for both 4:3 and 16:9 frame aspect ratios, which results in different pixel aspect ratios for fullscreen and widescreen video.[4][5] The 60 Hz system has 480 lines, while the 50 Hz system has 576 lines in a frame.
The video, the corresponding audio and metadata are packaged into 80-byte Digital Interface Format (DIF) blocks which are multiplexed in a 150-block sequence. DIF blocks are the basic units of DV streams and can be stored as files in raw form or wrapped in such file formats as AVI, QuickTime and MXF.[6][7] One video frame is formed from either 10 or 12 such sequences, depending on scanning rate, which results in the data rate of about 25 Mbit/s. When written to tape, each sequence correspond to one complete track.[4]
For audio, DV allows either two PCM channels (usually stereo) at 16-bit resolution and 48 kHz sampling rate, or four 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 specification includes the ability to record audio at 44.1 kHz, which is the same sampling rate used for CD audio, but this option is rarely used in practice.
Baseline DV employs unlocked audio. This means that the sound may be +/- ⅓ frame out of sync with the video. However, this is the maximum drift of the audio/video synchronization, it is not compounded throughout the recording.
[edit] Variants
Sony and Panasonic have created their proprietary versions of DV, which use the same compression scheme, but improve on robustness, linear editing capabilities, color rendition and raster size.
All DV variants except for DVCPRO Progressive are recorded to tape within interlaced video stream. Film-like frame rates are possible by using pulldown. DVCPRO HD supports native progressive format when recorded to P2 memory cards.
[edit] DVCPRO
DVCPRO, also known as DVCPRO25, is a variation of DV developed by Panasonic and introduced in 1995 for use in electronic news gathering (ENG).
Unlike baseline DV, DVCPRO uses locked audio and 4:1:1 chroma subsampling for both 50 Hz and 60 Hz variants to reduce generation loss.[8] Audio is available only in the 16-bit/48 kHz variant.
When recorded to tape, DVCPRO uses wider track pitch - 18 μm vs. 10 μm of baseline DV, which reduces the chances of dropout errors when video is recorded to tape. Two extra longitudinal tracks provide audio cue and for timecode control. Tape is transported 80% faster compared to baseline DV, resulting in shorter recording time. Long Play mode is not available.
[edit] DVCAM
In 1996 Sony responded with its own professional version of DV called DVCAM.
Like DVCPRO, DVCAM uses locked audio, which prevents audio synchronization drift that may happen on DV If several generations of copies are made.[9]
When recorded to tape, DVCAM uses 15 μm track pitch, which is 50% wider compared to baseline. Accordingly, tape is transported 50% faster, which reduces recording time by one third compared to DV. Because of the wider track and track pitch, DVCAM has the ability to do a frame accurate insert tape edit, while DV may vary by a few frames on each edit compared to the preview.
[edit] DVCPRO50
DVCPRO50 was introduced by Panasonic in 1997 for high-value ENG and digital cinema, and is often described as two DV-codecs working in parallel.
The DVCPRO50 doubles the coded video data rate to 50 Mbit/s, cutting recording time in half compared to base DVCPRO. Chroma resolution is improved by using 4:2:2 chroma sampling. The resulting picture quality is reputed to rival Digital Betacam.
DVCPRO50 is used in many productions where high definition is not required. For example, BBC used DVCPRO50 to record high-budget TV series, such as Space Race (2005) and Rome (2006).[10][11]
[edit] DVCPRO Progressive
DVCPRO Progressive was introduced by Panasonic for news gathering, sports journalism and digital cinema. Offering 480 or 576 lines of progressive scan recording with 4:2:0 chroma subsampling and four 16-bit 48 kHz PCM audio channels, it was meant as an intermediate format during the transition time from standard definition to high definition video.[12][13]
The format offered six modes for recording and playback: 16:9 progressive (50 Mbit/s), 4:3 progressive (50 Mbit/s), 16:9 interlaced (50 Mbit/s), 4:3 interlaced (50 Mbit/s), 16:9 interlaced (25 Mbit/s), 4:3 interlaced (25 Mbit/s).[14]
The format has been superseded with DVCPRO HD.
[edit] DVCPRO HD
DVCPRO HD, also known as DVCPRO100 is a high definition format that can be thought of as four DV codecs that work in parallel. Video data rate depends on frame rate and can be as low as 40 Mbit/s for 24 frame/s mode and as high as 100 Mbit/s for 50/60 frames/s modes. Like DVCPRO50, DVCPRO HD employs 4:2:2 color sampling.
DVCPRO HD uses smaller raster size than broadcast high definition television: 960x720 pixels for 720p, 1280x1080 for 1080/59.94i and 1440x1080 for 1080/50i. Similar horizontal downsampling is used in many other HD formats. To maintain compatibility with HDSDI, DVCPRO100 equipment upsamples video during playback.
Variable framerates (from 4 to 60 frame/s) are available on VariCam camcorders. DVCPRO HD equipment is backward compatible with older DV/DVCPRO formats.
When recorded to tape, DVCPRO HD uses the same 18 μm track pitch as other DVCPRO flavors. A long play variant, DVCPRO HD-LP, doubles the recording density by using 9 μm track pitch.
DVCPRO HD is codified as SMPTE 370M; the DVCPRO HD tape format is SMPTE 371M, and the MXF Op-Atom format used for DVCPRO HD on P2 cards is SMPTE 390M.
While technically DVCPRO HD is a direct descendant of DV, it is used almost exclusively by professionals. Tape-based DVCPRO HD cameras exist only in shoulder mount variant. The different camera models accept either M or L sized cassettes. Tapeless cameras record DVCPRO HD onto solid-state P2 memory cards. DVCPRO HD has never become an inexpensive format and lost consumer high definition market to HDV and AVCHD.
[edit] Progressive recording
None of tape-based DV variants except for DVCPRO Progressive supports native progressive recording, therefore progressively acquired video is recorded within interlaced video stream using pulldown. The same technique is used in television industry to broadcast movies. Progressive-scan DV camcorders for 60 Hz market record 24-frames/s video using 2-3 pulldown and 30-frames/s video using 2-2 pulldown. Progressive-scan DV camcorders for 50 Hz market record 25-frames/s video using 2-2 pulldown.
Progressive video can be recorded with interlaced delivery in mind, in which case high-frequency information between fields is blended to suppress interline twitter. If the goal is progressive-scan distribution like Web videos, progressive-scan DVD-video or filmout, then no filtering is applied. Video recorded with 2-2 pulldown and no vertical filtering is conceptually identical to progressive segmented frame.
Consumer-grade DV camcorders capable of progressive recording usually offer only 2-2 pulldown scheme because of its simplicity. Such a video can be edited as either interlaced or progressive and does not require additional processing aside of treating every pair of fields as one complete frame. Canon and Panasonic call this format Frame Mode, while Sony calls it Progressive Scan recording. 24 frame/s recording is available only on professional DV camcorders and requires pulldown removal if editing at native frame rate is required.
DVCPRO HD supports native progressive recording at 50 or 60 frames/s in 720p mode. To record video acquired at 24, 25 or 30 frames/s frame repeating is used. Frame repeating is similar to field repeating used in interlaced video, and is also called pulldown sometimes.
[edit] Recording media
[edit] Magnetic tape
DV was originally designed for recording onto magnetic tape. Tape is enclosed into cassettes of four different sizes: small, medium, large and extra-large. All DV cassettes use tape that is ¼ inch (6.35 mm) wide.
Small cassettes, also known as S-size or MiniDV cassettes, had been intended for amateur use, but have become accepted in professional productions as well. MiniDV cassettes are used for recording baseline DV, DVCAM as well as HDV.
Medium or M-size cassettes are used in professional Panasonic equipment and are often called DVCPRO tapes. Panasonic video recorders that accept medium cassette can play back from and record to medium cassette in different flavors of DVCPRO format; they will also play small cassette containing DV or DVCAM recording, via an adapter.
Large or L-size cassettes are accepted by most standalone DV tape recorders and are used in many shoulder mount camcorders. Despite that this cassette size can be used in both Sony and Panasonic equipment, they are often called DVCAM tapes. Older Sony decks would not play large cassettes with DVCPRO recordings, but newer models can do that.
Extra-large cassettes or XL-size have been designed for use in Panasonic equipment and are sometimes called DVCPRO XL. These cassettes is not widespread, only two models of standalone Panasonic tape recorders can accept them.
Technically, any DV cassette can record any variant of DV video. Nevertheless, manufacturers are often label cassettes with DV, DVCAM, DVCPRO, DVCPRO50 or DVCPRO HD mark and indicate recording time with regards to the label posted. Cassettes labeled as DV indicate recording time of baseline DV; another number can indicate recording time of Long Play DV. Cassettes labeled as DVCPRO have yellow lid and indicate recording time when DVCPRO25 is used; with DVCPRO50 the recording time will be twice shorter, with DVCPRO HD it will be four times shorter. Cassettes labeled as DVCPRO50 have blue lid and indicate recording time when DVCPRO50 is used. Cassettes labeled as DVCPRO HD have red lid and indicate recording time when DVCPRO HD-LP format is used; a second number may be used for DVCPRO HD recording, which will be twice shorter.
Panasonic stipulated use of particular magnetic tape formulation — Metal Particle (MP) — as inherent part of its DVCPRO family of formats. Regular DV tape uses Metal Evaporate (ME) formulation, which was pioneered for use in Hi8 camcorders. Early Hi8 ME tape was plagued with excessive dropouts, which forced many shooters to switch to more expensive MP tape.[15] After the technology improved, the dropout rate was greatly reduced, nevertheless Panasonic deemed ME formulation not robust enough for professional use. Tape-based Panasonic camcorders do not use small DV cassettes, effectively preventing use of ME tape.
DV cassettes can come with a memory-in-cassette (MIC) low capacity EEPROM memory chip. Using the I²C protocol, cameras and recording decks can record any data desired onto this chip like contents list, times and dates of recordings, camera settings or video thumbnails, taken each time the record button on the camcorder is pressed. MIC functionality is optional, it is not widely used in consumer equipment. Most tapes available to consumers do not include the MIC chip. Almost every recorder and camcorder includes the hardware to read and write to the MIC, but the firmware does not always have the code to actually perform the task.
[edit] File-based media
With proliferation of tapeless video recording, DV video can be recorded on optical discs, solid state memory cards and hard disk drives. Video can be stored either as native DIF bitstream or wrapped into an audio/video container such as as AVI, QuickTime and MXF.
- DV-DIF is the raw form of DV video. The files usually have extensions *.dv or *.dif.
- DV-AVI is Microsoft's implementation of DV video file, which is wrapped into an AVI container. Two variants of wrapping are available: with Type 1 the multiplexed audio and video is saved into the video section of a single AVI file, with Type 2 video and audio are saved as separate streams in an AVI file (one video stream and one to four audio streams). This container is used primarily on Windows-based computers, though Sony offers two tapeless recorders, the HDD-based HVR-DR60[16] and the CompactFlash-based HVR-MRC1K[17], for use with DV/HDV camcorders that can record in DV-AVI format either making a file-based copy of the tape or bypassing tape recording altogether.
- Quicktime-DV is DV video wrapped into Quicktime container. This container is used primarily on Apple computers.
- MXF-DV wraps DV video into MXF container, which is presently used on P2-based equipment and on XDCAM/XDCAM EX equipment.
[edit] Connectivity
Nearly all DV camcorders and decks have IEEE 1394 (FireWire, i.LINK) ports for digital video transfer. When video is captured onto a computer it is stored in a container file, which can be either raw DV stream, AVI, WMV or Quicktime. Whichever container is used, the video itself is not re-encoded and represents a complete digital copy of what has been recorded onto tape. If needed, the video can be recorded back to tape to obtain full and lossless copy of the original footage.
Some camcorders also feature a USB2 port for computer connection. This port is usually used for transferring still images, but not for video transfer. Camcorders that offer video transfer over USB, usually do not deliver full DV quality.
High end cameras and VTRs 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.
[edit] Usage
DV now enables filmmakers to produce movies inexpensively, and is strongly associated with independent film and citizen journalism[citation needed].
The high quality of DV images, especially when compared to Video8 and Hi8 which were vulnerable to an unacceptable amount of video dropouts and "hits", prompted the acceptance by mainstream broadcasters of material shot on DV. The low costs of DV equipment and their ease of use put such cameras in the hands of a new breed of videojournalists. Programs such as TLC's Trauma: Life in the E.R. and ABC News's Hopkins: 24/7 were shot on DV.
Professional television reporters such as Anderson Cooper and Katie Couric seem to shoot some of their stories themselves.[18]
DVCPRO HD has become the preferred high definition standard of the BBC Factual. This format will be replaced with AVC-intra when it becomes viable.[19]
[edit] Application software support
Most DV players, editors and encoders only support the basic DV format, but not its professional versions. DV Audio/Video data can be stored as raw DV data stream file (data is written to a file as the data is received over FireWire, file extensions are.dv and.dif) or the DV data can be packed into AVI container files. The DV meta-information is preserved in both file types.
Most Windows video software only supports DV packed into AVI containers, as they use Microsoft's avifile.dll, which only supports reading avi files. A few notable exceptions exist:
- Apple Inc.'s QuickTime Player: QuickTime by default only decodes DV to half of the resolution to preserve processing power for editing capabilities. However, in the "Pro" version the setting "High Quality" under "Show Movie Properties" enables full resolution playback.
- DVMP Basic & DVMP Pro: full decoding quality. Plays AVI (inc DVCPRO25 and DVCAM) and.dv files. Also displays the DV meta-information (e.g. timecode, date/time, f-stop, shutter speed, gain, white balance etc)
- The VLC media player (Free software): full decoding quality
- MPlayer (also with GUI under Windows and Mac OS X): full decoding quality
- muvee Technologies autoProducer 4.0: Allows editing using FireWire IEEE 1394
- Quasar DV codec (libdv) - open source DV codec for Linux
- DVswitch - open source linear editor for DV streams, with live streaming capabilities
[edit] Mixing tapes from different manufacturers
There is controversy over whether or not using tapes from different manufacturers can lead to dropouts.[20][21][22] A research undertaken by Sony revealed no hard evidence of the above statement. The only evidence found was that using ME tapes in equipment designed for MP tapes can cause tape damage and hence dropouts.[23]
The perpetuation of this controversy, based solely on hearsay, is misleading to end users. Sony has done a significant amount of internal testing to simulate head clogs as a result of mixing tape lubricants, and has been unable to recreate the problem. Sony recommends using cleaning cassettes once every 50 hours of recording or playback. For those who are still skeptical, Sony recommends cleaning video heads with a cleaning cassette before trying another brand of tape.
[edit] Related video formats
In 1999 Sony retrofitted its 8-mm camcorders with DV encoding scheme, creating Digital8. This allowed recording 40 minutes of DV video onto one-hour Video8/Hi8 cassette. Digital8 did not get as widespread acceptance as MiniDV and Sony discontinued Digital8 camcorders in 2007.
In 2003 a consortium of four companies - JVC, Sony, Canon and Sharp - presented HDV, a format for recording and playback of high-definition video on a DV cassette tape.[24] HDV quickly caught on with many professional users due to its low cost, portability and image quality acceptable for many professional productions. HDV is backwards compatible with DV, meaning that HDV equipment can play and record DV content. Many Sony HDV camcorders can play DVCAM recordings.
[edit] References
- ^ Recording – Helical-scan digital video cassette recording system using 6,35 mm magnetic tape for consumer use
- ^ fxguide.com
- ^ adamwilt.com
- ^ a b DVCAM format overview
- ^ Digital Video and Field Order
- ^ Sustainability of Digital Formats: DV-DIF
- ^ DVswitch - DV format
- ^ The DV, DVCAM & DVCPRO Formats - color sampling
- ^ "BBC Training: DV Tape Formats". http://www.bbctraining.com/onlineCourse.asp?tID=5173&cat=2782.
- ^ Rome wasn't shot in a day, it was shot in HDX!
- ^ DVC Pro 50 Camcorder SDX-900
- ^ Caporale Studios Shoots Feature Films with Panasonic 480p DVCPRO50 Camcorder
- ^ 480p production systems
- ^ AJ-PD900WP Operating instructions
- ^ How DV works
- ^ "HVR-DR60 HDV hard disk recorder". http://pro.sony.com/bbsc/ssr/product-HVRDR60.
- ^ "HVR-MRC1K Memory Recording Unit". http://pro.sony.com/bbsc/ssr/product-HVRMRC1K.
- ^ Katie Couric: anchor, reporter ... shooter?!?
- ^ HD at BBC
- ^ http://www.zenera.com/dvtape.html#mix_brands
- ^ http://www.taperesources.com/useful_info.html#why_clean_change
- ^ http://www.adamwilt.com/Tidbits.html#DVgunking
- ^ The Truth About Tape Lubricant
- ^ "HDV Press Release". http://www.hdv-info.org/page2.htm.
[edit] External links
- DVworkshops.com Free DV Video Handbook. Cameras, Lights and Interview techniques
- Adam Wilt's DV page with in-depth technical information
- Tips and training for working with DVCPROHD, HDV and DV.
- Digital Video Information Network messageboards
- Technical Documentation on the storage of DV data in AVI files: short and in detail
|
||||||||||||||||||||||||||||||||||||
