http://adamwilt.com/ http://www.adamwilt.com/DV-FAQ-tech.html#DVformatsWhat is DV?
DV is an international standard created by a consortium of 10 companies for a consumer digital video format. The companies involved were 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 DV consortium.
DV, originally known as DVC (Digital Video Cassette), 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. (See below for a discussion of color sampling.)
The sampled video is compressed using a Discrete Cosine Transform (DCT), the same sort of compression used in motion-JPEG. However, DV's DCT allows for more local optimization (of quantizing tables) within the frame than do JPEG compressors, allowing for higher quality at the nominal 5:1 compression factor than a JPEG frame would show. See Guy Bonneau's discussion of DV vs MJPEG compression for more details, or download the DVCAM Overview documents (PDFs) from http://www.sony.ca/dvcam/brochures.htm for a nice tutorial on 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.
DV video information is carried in a nominal 25 megabit per second (Mbits/sec) data stream. Once you add in audio, subcode (including timecode), Insert and Track Information (ITI), and error correction, the total data stream comes to about 29 Mbits/sec or 3.6 MBytes/sec. Roger Jennings' papers run through the detailed numbers.
What's the difference between DV, DVCAM, and DVCPRO?
Not a lot! The basic video encoding algorithm is the same between all three formats. The VTR sections of the US$16,500 DSR450 (DVCAM) or AJ-SDX900 (in DVCPRO25 mode) cameras will record no better an image than the VTR section of the cheapest DV consumer camcorder (please note: I am not saying that the camera section and lens of chead DV camcorder 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. 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.
Newer DV 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 most DVCAM or DVCPRO VTRs.
Digital8?
Sony's Digital8 uses DV compression atop the existing Video8/Hi8 technological base. Digital8 records on Video8 or Hi8 tapes, but these run at twice their normal speed (in the NTSC world; 1.5x in PAL land) and thus hold half the time listed on the label (2/3rds the time in PAL).
Digital8 also plays back existing Video8 and Hi8 tapes, even over 1394/i.Link/FireWire, allowing such tapes to be read into NLEs (at least, those for which the lack of timecode is not an issue -- batch capture utilities won't work, since Video8/Hi8 timecodes are not sent across the 1394 connection). Digital8 tapes themselves use the same timecode as DV.
Digital8 is largely a camcorder-only format, though two "Video Walkman" portable player/recorders are available. It appears to be the 8mm division's way of keeping its customer base from defecting to DV. By leveraging the massive investments of 15 years in 8mm analog camcorders and transports, the unit cost of Digital8 gear is kept very low, roughly half of what a comparable DV camcorder would cost, and its ability to play back legacy analog tapes is worthwhile for those with large libraries of 8mm.
Hitachi also produced Digital8 camcorders although these seem to be hard to come by (thanks to James M. DeLuca for bringing these "stealth" camcorders to my attention).
All Digital8 camcorders can record from the analog inputs (at least outside the EU), and all are equipped with i.Link ports for digital dubbing and NLE connections.
What are the DV artifacts I keep hearing about?
DV artifacts [Pix: Artifacts] come in three flavors: mosquito noise, quilting, and motion blocking. Other picture defects [Pix: Defects] encountered are dropouts and banding (a sign of tape damage or head clogging).
The most noticeable spatial artifact is mosquito noise around any sharp, contrasty edges. These are compression-induced errors usually seen around sharp-edged fine text, dense clusters of leaves, and the like; they show up as pixel noise within 8 pixels of the fine detail or edge causing them. The best place to look for them is in fine text superimposed on a non-black background. White on blue seems to show it off best. The magnitude of these errors and their location tends to be such that if you monitor the tape using a composite video connection, the artifacts will often be masked by dot-crawl and other composite artifacts.
A spatial quilting artifact can sometimes appear at the boundaries between 8x8 pixel blocks, most noticeable on shallow diagonals or on slightly-defocused backgrounds, typically when there is some motion in the scene to make the fixed "grid pattern" a bit more obvious. Some DV codecs seem to be much more prone to this than others, and with a few the quilting really starts to appear only after a few generations of rendering.
Motion blocking occurs when the two fields in a frame (or portions of the two fields) are too different for the DVC codec to compress them together. "Bit budget" must be expended on compressing them separately, and as a result some fine detail is lost, showing up as a slight blockiness or coarseness of the image when compared to the same scene with no motion. Motion blocking is best observed in a lockdown shot of a static scene through which objects are moving: in the immediate vicinity of the moving object (say, a car driving through the scene), some loss of detail may be seen. This loss of detail travels with the object, always bounded by DCT block boundaries. However, motion blur in the scene usually masks most of this artifact, making this sort of blocking almost impossible to see in most circumstances.
Finally, banding or striping of the image occurs when one head of the two on the scanner is clogged or otherwise unable to recover data. The image will show 10 horizontal bands (12 in PAL countries), with every other band showing a "live" picture and the alternate bands showing a freeze frame of a previous image or of no image at all (or, at least in the case of the JVC GR-DV1u, a black-and-white checkerboard, which the frame buffers appear to be initialized with). Most often this is due to a head clog, and cleaning the heads using a standard manufacturer's head cleaning tape is all that's required. It can also be caused by tape damage, or by a defective tape. If head cleaning and changing the tape used don't solve it, you may have a dead head or head preamp; service will be required.
This sort of banding dropout occurs fairly often; about once per DV tape in my experience. Usually it isn't even noticeable -- a single frame of banding due to a momentarily clogged head won't be visible unless there's motion in the scene to show off the frozen stripes. Have a look through your old tapes frame by frame (on a slow day, of course!) and you might be surprised how often you'll be able to find a single, subtly banded frame. For what it's worth, I've only rarely found such a banded frame on any DVCAM footage I've shot, which indicates to me that DV is right on the edge of reliability. DVCAM, with its 15 micron track width, or DVCPRO with its 18 micron track, are sufficiently on the safe side of the bleeding edge so that this sort of droput is much less likely to occur.
Bear in mind that analog BetaSP typically has several dropouts per minute; the last time I measured visible dropout rates on Hi8 and S-VHS I got numbers in the range of a dropout every 3-5 seconds (Hi8) and every 7-20 seconds (S-VHS). One visible dropout per hour-long tape, on average, is not something to get flustered about. But if it does bother you, shoot DVCAM or DVCPRO instead.
...
1394/FireWire
What is 1394 and/or "FireWire"?
IEEE-1394 is a standard communications protocol for high-speed, short-distance data transfer. It has been developed from Apple Computer's original "FireWire" proposal (FireWire is a trademark of Apple Computer). Check out the 1394 Trade Association, white papers on Adaptec's website, and DVCentral's links for pointers to additional 1394 sites for detailed information.
Sony calls their implementation of 1394 "i.LINK".
Why are DV and 1394 always discussed together?
They appear to have been developed together. The data stored on DV tape appear to reflect the packet structure sent across a 1394 link to a frightening degree of exactness. Certainly the DV format and 1394 High Performance Data Bus co-evolved, such that the first consumer DV camcorder in the USA (the Sony DCR-VX1000 and its single-chip brother the VX700) was also the first 1394-equipped consumer product available.
What does a 1394 connection do for me?
Plenty of good things:
You can make digital dubs between two camcorders or VTRs using 1394 I/O, and the copy will be identical to the original.
You can do cuts-only linear editing over 1394, with no generation loss.
You can stick a 1394 board into your computer (PC or Mac), and transfer DV to and from your hard disk. If your system can support 3.6 MBytes/sec sustained data rate -- simple enough with many A/V rated SCSI-2 drives and with most ATA/EIDE drives these days -- the world of computer-based nonlinear editing is open to you without paying the quality price of heavy JPEG compression and its associated artifacts, or the monetary price of buying heavy-duty NLE hardware and banks of RAID-striped hard drives.
Some time ago I edited a friend's wedding, going from Hi8 camera originals to a DV edit master. The 20-minute ceremony was covered by two cameras; we sync-rolled the VTRs and mixed the show in real time as if it were live. At the end, we weren't sure we liked it. So we dubbed it off via 1394 to another DV cassette, inserted a fresh DV cassette, and had another bash at the edit. This time, we liked it. We put the tape into the VX1000 and set up the DHR-1000 VTR as the recorder, using the built-in editor to drop the second attempt in frame-accurately atop the first across the 1394 wire. No generation loss. And we still had the first edit on the backup tape, should we have changed our minds.
Is 1394 that much better than Y/C or component analog?
Yes. A 1394 dub is a digital copy. It's identical to the original. That's really nice.
Yes, you can do almost the same thing with a SMPTE 259M SDI (serial digital interface) transfer. But VTRs with SDI cost big money. 1394 is built into many low-end cameras and VTRs, and the connecting cable -- even at Sony prices -- is only US$50; you can find it for US$20 if you shop around.
Also, transferring via 1394 is a digital copy, a data dump (as it is over the expensive SDTI interface on high-end DVCAM and DVCPRO VTRs). No decompression or recompression occurs. Transferring DV around as baseband video, even digitally over SDI, subjects it to the small but definite degradation of repeated decompression/recompression.
What's the deal with DVCPRO gear and 1394?
DVCPRO, or D-7, is a DV-based format with a few subtle differences in its datastream. These changes were made by Panasonic's engineers to improve the robustness and reliability of the DVCPRO system when compared to DV, but they do mean that certain data header bits do not conform to Blue Book standards. Thus a direct data interchange between DVCPRO gear and DV/DVCAM gear is not possible in the same way that DV and DVCAM gear can interchange data; furthermore some nonlinear editor systems are not capable of accepting or generating a DVCPRO-compatible signal.
As a result, DVCPRO gear with 1394 connections can only exchange data with other DVCPRO systems, not with DV or DVCAM gear. Since a 1394 transfer is a direct data dump, this is understandable; if a cross-format transfer were to be possible it would require that one deck or the other "translate" the signal to or from the DVCPRO data format to the Blue Book format.
As far as incompatibility with 1394 transfers to and from NLEs, this limitation is expected to diminish (and eventually vanish) as developers get a chance to work with DVCPRO over 1394, and to provide switches inside their programs to supply a Blue Book or DVCPRO datastream as required. Matrox, Canopus, and Apple, for example, have DVCPRO compatible NLEs.
Remember, DVCPRO was designed first and foremost as an ENG format; robustness of the signal was paramount, and interconnection of gear in the ENG world is done via analog or via SDI (1394 is too limited an interface for the broadcast world, where the ability to switch and route video over thousand-meter runs is both necessary and taken for granted; 1394 has a length limit of 4.5 meters and requires a point-to-point session-level communication instead of a switchable open-ended transmission). 1394 was added to the DVCPRO lineup as an afterthought, at the prompting of customers, and as it becomes more prevalent (and if the marketplace demands it) you'll see more NLEs capable of dealing with DVCPRO data as readily as with Blue Book data, and possibly even realtime DV/DVCPRO format translators.
All materials on this page copyright © 2000-2006 by Adam J. Wilt.
Topic: Digital8, DV, DVCAM, DVCPRO and FireWire (Read 5121 times)