DV
From Wikipedia, the free encyclopedia
 |
This article needs additional citations for verification. Please help improve this article by adding reliable references (ideally, using inline citations). Unsourced material may be challenged and removed. (January 2009) |
|
Digital Video (DV) is a digital video format created by Sony, JVC, Panasonic and other video camera producers, and launched in 1995. In its smaller tape form factor MiniDV, has since become a standard for home and semi-professional video production; it is sometimes used for professional purposes as well, such as filmmaking and electronic news gathering (ENG). 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 and good video quality, especially compared to earlier consumer analog formats such as Video8, Hi8 and VHS-C. 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’ Hopkins: 24/7 were shot on DV. CNN’s Anderson Cooper is perhaps the best known of the generation of reporter/videographers who began their professional careers shooting their own stories.
There have been some variants on the DV standard, most notably Sony's DVCAM and Panasonic's DVCPRO formats targeted at professional use. Sony's consumer Digital8 format is another variant, which is similar to DV but recorded on Hi8 tape. Other formats such as DVCPRO50 utilize DV25 encoders running in parallel.
MiniDV tapes can also be used to record a high-definition format called HDV in cameras designed for this codec, which differs significantly from DV on a technical level as it uses MPEG-2 compression. MPEG-2 is more efficient than the compression used in DV, in large part due to inter-frame/temporal compression.[1] This allows for higher resolution at bitrates similar to DV. On the other hand, the use of inter-frame compression can cause motion artifacts and complications in editing.[2] Nonetheless, HDV is being widely adopted for both consumer and professional purposes and is supported by many editing applications using either the native HDV format or intermediary editing codecs.[3]
[edit] Technical standards
[edit] Before compression
To avoid aliasing, optical low pass filtering is necessary (although not necessarily implemented in all camera designs). Essentially, blurry glass is used to add a small blur to the image. This prevents high-frequency information from getting through and causing aliasing. Low-quality lenses can also be considered a form of optical low pass filtering.
Before arriving at the codec compression stage, light energy hitting the sensor is transduced into analog electrical signals. These signals are then converted into digital signal by an analog to digital converter (ADC or A/D). This signal is then processed by a digital signal processor (DSP) or custom ASIC and undergoes the following processes:
Processing of raw input into (linear) RGB signals: For Bayer pattern-based sensors (i.e. sensors utilizing a single CCD or CMOS and color filters), the raw input has to be demosaiced. For Foveon-based designs, the signal has to be processed to remove cross-talk between the color layers. In pixel-shifted 3CCD designs, a process somewhat similar to de-mosaicing is applied to extract full resolution from the sensor.
Matrix (for colorimetry purposes): The digital values are matrixed to tweak the camera's color response to improve color accuracy and to make the values appropriate for the target colorimetry (SMPTE C, Rec. 709, or EBU phosphor chromaticities). For performance reasons, this matrix may be applied after gamma correction and combined with the matrix that converts R'G'B' to Y'CbCr.
Electronic white balance may be applied in this matrix, or in the matrix operation applied after gamma correction.
Gamma correction: Gamma correction is applied to the linear digital signal, following the Rec. 601 transfer function (a power function of 1/0.45). Note that this increases the quantization error from before.
Matrix (R'G'B' to Y'CbCr conversion): This matrix converts the gamma-corrected R'G'B' values to Y'CbCr color space. Y'CbCr color space facilitates chroma subsampling. This operation introduces yet more quantization error, in large part due to a difference in the scale factors between the Y' and Cb and Cr components.
Chroma Subsampling: Since human vision has greater acuity for luminance than color, performance can be optimized by devoting greater bandwidth to luminance than color. Chroma subsampling approximates this by converting R'G'B' values into Y'CbCr color space. The Cb and Cr color difference components are stored at a lower resolution than the Y' (luma) component.
Sharpening is often used to counteract the effect of optical low pass filtering. Sharpening can be implemented via finite impulse response filters.
The data is now compressed using one of several algorithms including discrete cosine transform (DCT), adaptive quantization (AQ), and variable-length coding (VLC).
[edit] Video compression
DV uses DCT intraframe compression at a fixed bitrate of 25 megabits per second (25.146 Mbit/s), which, when added to the sound data (1.536 Mbit/s), the subcode data, error detection, and error correction (approx 8.7 Mbit/s) amounts in all to roughly 36 megabits per second (approx 35.382 Mbit/s). At equal bitrates, DV performs somewhat better than the older MJPEG codec, and is comparable to intraframe MPEG-2. (Note that many MPEG-2 encoders for real-time acquisition applications only use intraframe compression [I-frames only], but not interframe compression [P and B frames].) DCT compression is lossy, and sometimes suffers from artifacting around small or complex objects such as text. The DCT compression has been specially adapted for storage onto tape. The image is divided into macroblocks, each consisting of 4 luminance DCT blocks and 1 chrominance DCT block. Furthermore 6 macroblocks, selected at positions far away from each other in the image, are coded into a fixed amount of bits. Finally, the information of each compressed macroblock is stored as much as possible into one sync-block on tape. All this makes it possible to search video on tape at high speeds, both forward and reverse, as well as to correct faulty sync blocks very well.
[edit] Chroma subsampling
The chroma subsampling is 4:1:1 for NTSC, 4:1:1 for DVCPRO PAL, and 4:2:0 for other PAL. Not all analog formats are outperformed by DV. The Betacam SP format, for example, can still be desirable because it has slightly greater chroma fidelity and no digital artifacts.
Low chroma resolution is a reason why DV is sometimes avoided in applications where chroma-key will be used. Nevertheless, advances in keying software (i.e. the combination of chroma keying with different keying techniques,[4] chroma interpolation[5]) allows for reasonable quality keys from DV material.
[edit] 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), although in practice this option is rarely used. DVCAM and DVCPRO both use locked audio while standard DV does not. This means that at any one point on a DV tape the audio may be +/- ⅓ frame out of sync with the video. However, this is the maximum drift of the audio/video sync; it is not compounded throughout the recording. In DVCAM and DVCPRO recordings the audio sync is permanently linked to the video sync.
[edit] Connectivity
The FireWire (aka IEEE 1394) serial data transfer bus is not a part of the DV specification, but co-evolved with it. Nearly all DV cameras have an IEEE 1394 interface and analog composite video and Y/C outputs. High end DV 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. Some camcorders also feature a USB2 port for computer connection, but these are sometimes not capable of capturing the DV stream in full detail, and are instead used primarily for transferring certain digital data from the camcorder such as still pictures and computer-format video files (such as MPEG4-encoded video). This carries Audio Video and Control Signals.
On computers, DV streams are usually stored in container formats such as MOV, MXF, AVI or Matroska.
[edit] Physical format
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 (MIC) (most commonly, a scant 4 Kbit for MiniDV cassettes, but the system supports up to 16 Kbit). 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 ¼ inch (6.35 mm) wide.
[edit] MiniDV
The "L" cassette is about 120 × 90 × 12 mm and can record up to 4.6 hours of video (6.9 hours in EP/LP). The better known MiniDV "S" cassettes, 65 × 48 × 12 mm and hold either 60 or 90 minutes of video (13 or 19.5 GB[citation needed]) depending on whether the video is recorded at Standard Play (SP) or Extended Play (sometimes called Long Play) (EP/LP). 80 minute tapes that use thinner tape are also available and can record 120 minutes of video in EP/LP mode. The tapes sell for as little as US$2.50 each in quantity as of 2008. DV on SP has a helical scan track width of 10 micrometres, while EP uses a track width of only 6.7 micrometres. Since the tolerances are much tighter, the recorded tape may not play back properly or at all on other devices.
 Canon ZR850 with three Sony MiniDV tapes |
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. Though originally intended for the consumer market as a high-quality replacement for VHS, L-size DV cassettes are largely nonexistent in the consumer market, and are generally used only in professional settings. Even in professional markets, most DV camcorders support only MiniDV, though many professional DV VTRs support both sizes of tape.[citation needed]
[edit] DVCAM
Sony's DVCAM is a professional variant of the DV standard that uses the same cassettes as DV and MiniDV, but transports the tape 50% faster having 50% wider track, 15 micrometres instead of 10 micrometres. This variant uses the same codec as regular DV, however, the wider track lowers the chances of dropout errors. The LP mode of consumer DV is not supported. All DVCAM recorders and cameras can play back DV material, but DVCPRO support was only recently added to some models like DSR-1800, DSR-2000, DSR-1600. DVCAM tapes (or DV tapes recorded in DVCAM mode) have their recording time reduced by one third.
Because of wider track DVCAM has the ability to do a frame accurate insert tape edit. DV will vary by a few frames on each edit compared to the preview. Another feature of DVCAM is locked audio. If several generations of copies are made on DV, the audio sync may drift. On DVCam this does not happen.[6]
[edit] Digital 8
After MiniDV and DV specifications have been introduced, Sony backported DV encoding scheme to 8-mm camcorders, creating Digital8. This allowed recording 40 minutes of DV video onto one-hour Video8/Hi8 cassette. The 18-μm track on Digital8 tape was even wider than on DVCAM cassette. Digital8 did not get as widespread acceptance as MiniDV and was Digital8 camcorders were discontinued by Sony in 2006.
[edit] DVCPRO
Panasonic specifically created the DVCPRO family for electronic news gathering (ENG) use, 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 (also known as DVCPRO25) is otherwise identical to DV at a bitstream level. However, unlike Sony, Panasonic chose to promote its DV variant for professional high-end applications.
DVCPRO50 is often described as two DV-codecs in parallel. The DVCPRO50 standard doubles the coded video bitrate from 25 Mbit/s to 50 Mbit/s, and uses 4:2:2 chroma subsampling instead of 4:1:1. DVCPRO50 was created for high-value ENG compatibility. The higher datarate cuts recording time in half (compared to DVCPRO25), but the resulting picture quality is reputed to rival Digital Betacam. BBC preferred DVCPRO50 camcorders over HDCAM cameras to film popular TV series, such as Space Race (2005) and Rome (2006).[7][8]
DVCPRO HD, also known as DVCPRO100, uses four parallel codecs and a recorded video bitrate of 40-100 Mbit/s, depending on the format flavour. DVCPRO HD encodes using 4:2:2 color sampling, compared to 4:2:0 or 4:1:1 for lower-bitrate video formats. DVCPRO HD horizontally compresses recorded images to 960x720 pixels for 720p output, 1280x1080 for 1080/59.94i or 1440x1080 for 1080/50i. This horizontal compression is similar to but more significant than that of other HD formats such as HDCam, HDV, AVCHD and AVCCAM. The final DCT compression ratio of DVCPRO HD is approximately 6.7:1. To maintain compatibility with HDSDI, DVCPRO100 equipment upsamples video during playback. A camcorder using a special variable-framerate (from 4 to 60 frame/s) variant of DVCPRO HD called VariCam is also available. All these variants are backward compatible but not forward compatible. 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.
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 formulation of the tape is the same, and the tapes are interchangeable between formats. The running time of each tape is 1x for DVCPRO, ½x for DVCPRO 50, ½x for DVCPRO HD EX, and ¼x for DVCPRO HD, since the tape speed changes between formats. Thus a tape made 126 minutes for DVCPRO will last approximately 32 minutes in DVCPRO HD.
[edit] Memory in cassette
Some MiniDV cassettes have a small memory chip referred to as memory in cassette (MIC). Cameras and recording decks can record any data desired onto this chip including a contents list, times and dates of recordings, and camera settings used. It is EEPROM memory using the I²C protocol. The members of the MIC range are available in two forms:
- The MIC-R family
- The MIC-R family works with serial EEPROM capacities between 1 Kbit and 8 Kbit (the I²C-compatible EEPROM devices: M24C01, M24C02, M24C04 and M24C08).
- The MIC-S family
- The MIC-S family works with serial EEPROM capacities greater or equal to 16 Kbit (the XI2C-compatible EEPROM devices: M24C16, M24C32 and M24C64).
Both families are compliant with the DV standard. For detailed information see datasheet: Serial I²C bus EEPROM (STMicroelectronics).
MIC functionality is not widely used on the consumer level; most tapes available to consumers do not even include the chip, which adds substantially to the price of each cassette. Most consumer equipment includes the circuitry to read and write to the chip even though it is rarely used.
[edit] Other digital formats
Digital video dates back to 1986, with the creation of the uncompressed D-1 format for professional use (although several professional video manufacturers such as Sony, Ampex, RCA, and Bosch had experimentally developed prototype digital video recorders dating back to the mid-to-late 1970s).
Sony has several digital specifications for professional use, the most common for standard definition use being Digital Betacam, a distant descendant of the Betamax products of the 1970s thru 1990s from which it received only some mechanical aspects, notably the form of the cassette. (Betamax itself descended from Sony's U-Matic ¾ inch videocassette system.)
JVC's D-9 format (also known as Digital-S) is very similar to DVCPRO50, but records on videocassettes 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 replaces the recording medium with the older Hi8 videocassette. Digital8 theoretically offers DV's digital quality, without sacrificing playback of existing analog Video8/Hi8 recordings. However, in practice the maximum quality of the format is unlikely to be achieved, since Digital8 is largely confined to low-end consumer camcorders. It is also a semi-proprietary format, being manufactured exclusively by Sony (although Hitachi also made Digital8 camcorders briefly).
DVD was originally created as a distribution format, but recordable DVDs quickly became available. Camcorders using miniDVD media are fairly common on the consumer level, but due to difficulties with editing the MPEG-2 data stream, they are not widely used in professional settings.
Sony also made a format called MicroMV, which stored MPEG-2 video on a matchbox-sized tape. Due to lack of platform support and the difficulties of editing MPEG-2 video, MicroMV never became popular and was discontinued by 2005.
Ikegami's Editcam System can record in DVCPRO or DVCPRO50 format on a removable hard disk.
Panasonic's P2 system uses recording of DV/ DVCPRO/ DVCPRO50/ DVCPROHD streams in an MXF wrapper on PC card-compatible flash memory cards.
Sony's XDCAM format allows recording of MPEG IMX, DVCAM and low resolution streams in an MXF wrapper on a Sony ProDATA disc, an optical medium similar to a Blu-ray Disc. Sony has also, in cooperation with Panasonic, created AVCHD, a medium-independent codec for consumer high definition video; it is currently used on camcorders containing hard disks, SD cards and DVD-R drives for storage.
[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
Type 1 and Type 2 DV AVI files
There are two types of DV-AVI files:
- Type 1: The multiplexed Audio-Video is kept in its original multiplexing and saved together into the Video section of the AVI file
- Type 2: Like type 1, but audio is also saved as an additional audio stream into the file.
- Supported by VfW applications, at the price of little increased file size.
Type 1 is actually the newer of the two types. Microsoft made the "type" designations, and decided to name their older VfW-compatible version "Type 2", which only furthered confusion about the two types. In the late 1990s through early 2000s, most professional-level DV software, including non-linear editing programs, only supported Type 1. One notable exception was Adobe Premiere, which only supported Type 2. High-end FireWire controllers usually captured to Type 1 only, while "consumer" level controllers usually captured to Type 2 only. Software is and was available for converting Type 1 AVIs to Type 2, and vice-versa, but this is a time-consuming process.
Many current FireWire controllers still only capture to one or the other type. However, almost all current DV software supports both Type 1 and Type 2 editing and rendering, including Adobe Premiere. Thus, many of today's users are unaware of the fact that there are two types of DV AVI files. In any event, the debate continues as to which – Type 1 or Type 2 – if either, is better.
[edit] Mixing tapes from different manufacturers
There is controversy over whether or not using tapes from different manufacturers can lead to dropouts.[9][10][11] The problem theoretically occurs when incompatible lubricants on tapes of different types combine to become tacky and deposit on tape heads. This problem was supposedly fixed in 1997 when manufacturers reformulated their lubricants, but users still report problems several years later. Much of the evidence relating to this issue is anecdotal or hearsay. In one case, a representative of a manufacturer (unintentionally) provided incorrect information about their tape products, stating that one of their tape lines used "wet" lubricant instead of "dry" lubricant.[12] The issue is complicated by OEM arrangements: a single manufacturer may make tape for several different brands, and a brand may switch manufacturers.
It is unclear whether or not this issue is still relevant, but as a general rule many DV experts recommend sticking with one brand of tape[citation needed].
[edit] See also
[edit] Formats
[edit] References
- ^ dv.com
- ^ dv.com
- ^ dv.com
- ^ fxguide.com
- ^ adamwilt.com
- ^ "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
- ^ 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
- ^ http://www.2-popforums.com/forums/showthread.php?t=53122
[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
|
|||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||
|
||||||||||||||||||||||||||
|
||||||||||||||||||||
