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DV: Digital Video Format for the Masses (3/2000)
by Douglas Dixon
"The DV format is great," says Brian Connor, President of RSVP, Inc., a video production services, rental, and post-production house in Broomall, PA. "DV is wonderful for consumers; now you can edit video at home."
The Promise of DV
Up to recently, trying to edit video on consumer equipment has been fraught with limitations and compromises. Professional videographers do not have these problems; they have expensive high-quality camera systems like Sony BetaSP and Digital Betacam to shoot video, and high-end non-linear editing systems from companies like Avid for video editing. This equipment is designed to handle the data transfer and storage requirements of full-resolution full-rate video, and permit frame-accurate editing without losing video quality.
But if you try to edit deck to deck with consumer video equipment like VHS recorders and 8mm camcorders, you quickly find that they do not provide the precise control to make frame-accurate cuts, and suffer a major loss in quality with each generation of copying.
If you try to edit on a home PC, you discover that PC's really cannot handle full-size full-rate video. To edit video on a PC, you need some way of getting the video in and out of your system, and some form of video compression to try to fit into the available transfer rate and storage size. And even with a bulked-up PC, you still end up with so much data that you have no way to save your productions.
DV actually offers a solution to all these problems. The DV format stores video frames digitally, which means you can access them with frame accuracy, and there is no quality loss when they are copied (it's just an exact copy of the digital data). DV camcorders include a compression chip, which means that you do not need a separate compression board or software step, and the video is already reduced to a size which is feasible for a home PC to handle.
DV camcorders use the FireWire (IEEE-1394) high-speed digital interface, which means you can copy between units without loss, and also feed your camcorder video into your PC (with a FireWire board) at full resolution and full frame rate. Finally, DV camcorders store the compressed digital video on DV cassette tapes, which means you automatically have a digital backup of your original video, and you can also save your final edited video by transferring it to DV tape.
DV In Action
DV almost sounds like magic. You can get yourself a DV camcorder for around $800. At first glance, it seems like any other old 8mm camcorder; though maybe it's a little smaller, and it's also more expensive. But look at all it does for you!
It's a video camera (of course), with extensive features like exposure control and zoom and stabilization. It's a video recorder and playback device. It's an analog to digital converter, capturing and digitizing both video and audio input. It's a video and audio compression system that runs in real time at video rate. It's also a real-time decompression system, so you can play back and view the recorded material. And it's also a high-speed digital data-transfer device with the FireWire interface. Not bad!
Using a DV camcorder still can make sense even if you do not want to edit it on a computer; DV is still a great format for saving and copying your video. To begin with, DV provides higher video resolution: around 500 horizontal lines, compared to around 425 for S-Video and Hi-8, and 300 lines or less for 8mm and VHS. And since DV camcorders have analog video and audio output jacks, and some also have analog input jacks, a DV camcorder can serve as a general video converter and digitizer.
You can plug your existing equipment into a DV camcorder is just like any other analog camcorder. You simply plug in to the analog output jacks to view your digital DV tapes on your television, or to record them on analog VHS or 8mm tapes. You can also save analog material in DV. You can feed in material from your old tapes, and digitize and archive them in digital DV format. And with DV, your material is saved in digital format, so you can make copies of your tapes for a simple edit or to share with others, without suffering the quality loss that you get with analog tapes.
Of course, even though you can use DV simply as a digital camcorder, you would be missing out on the really fun part until you hook up to your computer to edit the video. Once you install a FireWire board in your PC (or get a PC with FireWire built in), you can connect your camcorder to your PC and transfer your the video and audio between the DV tape and your PC's hard disk. Since the FireWire DV interface protocol includes both data and control information, you actually can control your camcorder from the PC software to skip through the tape and access the clips that you want.
The real win with editing DV video on a home PC is that it is real video: full video resolution and full frame rate. This is not the compromised low-resolution low-rate jumpy and smeary video that you are used to seeing on PC's. This is the real thing; when you finish the production, and write it out to DV tape again, it is still real video, at the same quality as the original. If you have been wishing for real video on your PC, then DV finally provides the goods.
"The DV format gets gear into more people's hands, and to the mass consumer," says Barry Byrne, Creative Director with New York Digital Design (NYD2), a digital production and communications agency in Lebanon, NJ. "It gives the producer more options to acquire images, and it's all usable, and immediately available."
So, what's the magic behind the DV format? It's a careful trade-off of using just enough compression to reduce the amount of data to manageable proportions, without requiring too much processing expense or damaging the video quality. It's a consumer-market design that supports the small Mini-DV cassette format and the resulting further miniaturization of camcorders. And it's also a nice coupling with the FireWire interface standard to provide efficient data transfer.
The DV format was designed in 1994 by an industry consortium as a digital video standard for consumer equipment. The design starts with the fundamental principal that the data will be stored on tape, which means that it is designed for a single fixed data rate. The data also must be compressed with a "symmetric" compression algorithm, which means that it must be able to be both compressed (to tape) and uncompressed (from tape) in real time, and with relatively inexpensive consumer hardware.
By comparison, the DVD video format for feature-length movies uses the "asymmetric" MPEG-2 compression algorithm. DVD video still needs to be uncompressed in real time when it is played, but can take significantly longer to be compressed when the movie is being authored to DVD. Since the MPEG compression can be done off-line, it can use much more sophisticated algorithms to squeeze down the video.
The DVD format was designed to store movies of two hours or longer, plus audio and auxiliary material. Therefore, DVD uses the MPEG-2 compression algorithm to compress the video to rates of around 3 1/2 Mbits/sec. In comparison, the DV data rate is 25 Mbits/sec, so it does not require as much compression, and can use much simpler processing. Even at this rate, a tiny standard Mini-DV cassette tape holds one hour of material
Since the DV format is designed for this fixed data rate, it can use simpler "intraframe" compression, which means that each frame is compressed independently, like the Motion JPEG format. More sophisticated algorithms like MPEG use "interframe" compression, in which the compression takes advantage of information between neighboring frames (the algorithm actually tracks motion from frame to frame and compresses only the differences between frames). This advanced compression results in much better compression, at the cost of more processing and more resources for both compression and decompression. The DV "intraframe" format also has the advantage of being simpler to use to access and edit the individual frames.
For audio, the DV format supports several different options, including high-quality 16-bit stereo at 48 KHz and two 12-bit stereo pairs at 32 KHz. These use PCM (Pulse Code Modulation), similar to CD audio format and quality. The DV audio is also synchronized with the video; although the audio is not strictly locked exactly to each video frame, it can drift only slightly relative to the video frame rate, so it does not drift out of sync over time.
Once the video and audio are compressed and stored on tape, the DV camcorder
still needs a way to transfer the data to other cams and to PC's. The FireWire interface
turns out to be just perfect for this application, even though it was developed
independently by Apple as a general high-speed data transfer interface. This interface was
standardized as IEEE-1394, and provides a
DV Video Compression
DV video compression begins with full-resolution video frames, 720 x 480 resolution for NTSC or 720 x 576 for European PAL. (OK, this is actually 6 pixels short of full 720 x 486 CCIR 601, but who's counting?) The original video is therefore 720 x 480 x 24 bits of color, or around 1000 KBytes per frame, or 30 MB/sec at 30 frames per second.
The first step in video compression is to reduce the amount of color data. Since the human eye is less sensitive to color detail than to intensity, some of the color data can be discarded without an observable difference in the video quality. DV pushes the envelope a bit by reducing the color to 1/4 resolution. While this can introduce color smearing artifacts along sharp edges between high-contrast colors, it is typically not a problem in practice. This trade-off was made in order to quickly and easily chop the amount of video data in half, from an average of 24 bits per pixel to only 12 bits per pixel. This reduces the video data rate from around 30 MB/sec to 15 MB/sec.
Next, the DV format uses "intraframe" compression based on the DCT (Discrete Cosine Transform) algorithm, much like the M-JPEG format. However, M-JPEG only reduces the color to half resolution, and uses a less flexible coding scheme, so DV can produce better quality at a similar data rate. DV achieves approximately 5:1 compression, down to the fixed data rate of around 3.1 MBytes/sec, or 25 Mbits/sec. This is the video data rate; with audio and control information, the full DV data rate is actually 3.75 MB/sec, or 30 Mb/sec.
DV can produce artifacts like color smearing and blocky compression when shooting video under difficult conditions. "Sometimes you can see artifacts, especially at low light levels or with a busy background," says Brian Conner of RSVP, "but you're hard pressed to tell it's DV when you're shooting a person against a blue sky on a bright sunny day." For professional use, Barry Byrne of New York Digital Design suggests "DV is best used for shooting narrative forms and interactive features, where you can minimize the camera movement and reduce the contrast."
DV for Consumers
By mid 1999, DV camcorders for consumers were available from a wide variety of manufacturers. While higher-quality units still cost over $1000, prices on some units have dropped below $800. Consumer DV camcorders use the one-hour Mini-DV tape cassette, which uses a 1/4 tape in a cassette that is only 2 x 2.2 x 1/2 inch, or about half the size (and thinner) than the 8mm format. This means that Mini DV cams can be shrunk down to fit in your hand, down to smaller than 5 by 4 by 3 inches, and weighing around 1 1/2 pounds.
DV camcorders typically include features such as both a viewfinder and pop-out LCD screen, optical and digital zoom control, stabilization, analog output, and FireWire (IEEE 1394) output. Also look for additional features including 3 CCD chips (for better color quality), high-resolution still capture, analog video and audio input, and other digital effects.
In early 1999, Sony confused the consumer DV market by introducing a new DV format called Digital 8. The idea behind Digital 8 was to provide compatibility with the older 8mm format; Digital 8 uses 8mm cassette tapes, so it can play existing 8mm (analog) video. It also records DV digital video on 8mm tapes, but runs the tape twice as fast, so a standard 120-minute 8mm tape provides 60 minutes of Digital 8 recording time, the same as Mini DV. Sony recommends using the more expensive Hi8 format tapes for digital recordings.
The Digital 8 format was designed by Sony for consumers who already have an extensive library of 8mm tapes. The 8mm tape format is also currently less expansive and more available than DV tapes (DV cassettes cost around $7.50 each in bulk, while Hi8 tapes are around $4.50). However, since Mini-DV uses a smaller tape, the Mini DV cams can be smaller and lighter. They are also available from a wide range of manufacturers, providing a wider selection of options and prices.
Sony actually also offers more options exclusively on its Mini DV cams, including Memory Stick storage for data transfer, 3 CCD technology, mega-pixel CCD imagers, higher quality lenses, and 4K Memory in Cassette (MIC) for indexing and titling. Anyway, if you're going DV, the first thing you should do is convert all your existing tapes to DV format, instead of playing the 8mm originals over and over again.
DV for Professionals
The DV format has also been adopted for professional use. Manufacturers like Sony, Panasonic, and Canon offer more ruggedized Mini-DV cameras with 3 CCD image sensors, interchangable lenses, and professional microphones for prices around $4000.
The DV format has also been extended for professional use by Sony and Panasonic in competing professional formats, Sony DVCAM and Panasonic DVCPRO. These use a wider track pitch and faster tape speed to provide better robustness and reliability, and permit frame-accurate editing. These cameras cost around $5000, depending on the quality of the CCD image sensors and lenses.
The DVCAM format introduced by Sony has higher-quality digital audio and includes a 2 Mbit MIC data store. The DVCPRO format introduced by Panasonic uses higher-quality metal-particle tapes. It is an open standard that is supported by multiple manufacturers, and also can play DVCAM and Mini-DV formats. Panasonic has extended DVCPRO to DVCPRO50 at double the data rate (50 Mbits/sec), with lower compression and higher quality half-resolution 4:2:2 color format.
"Our experience with digital is all positive," says Brian Connor of RSVP. "Digital BetaCam is in lots of edit suites, corporations use it as the master format. The DV format is good, and finally starting to hit its stride. With DVCAM, DVCPRO, and Mini-DV you can get a $4000 camera that is about as good as BetaCam."
Barry Byrne of NYD2 is pleased with their experience with DVCPRO digital cameras: "It's pretty neat, and all self contained. It's been a dream for a long time to be able to take the video right from the camera."
"All these formats provide the advantage to acquire video from so many rich sources," says Byrne. "With DV, the small cameras, reduced rigging, and wireless mikes means that one person can do the shoot. But DV is not used as a primary production vehicle. The small size is hard to move; you can't mount it on a steadycam. It's difficult to compose the shot, especially with a moving camera."
"We haven't even reached a crescendo," says Connor. "There's still a shake-out coming in the digital world, and we're waiting for it to straighten out."
So, how do you get into DV? First, get a Mini-DV camcorder so you can start shooting digital video. You'll probably want to get a cam with analog inputs so you also can convert your old tapes to digital DV format. Now you're doing digital video!
Once you have a collection of digital material, you'll want to get it into your computer so you can edit it. This requires that your machine has a FireWire (IEEE 1394) interface. A few computers ship with built-in support for interface to DV cams through FireWire, including the new Apple iMac DV, and the Sony VAIO PC's (although Sony calls the FireWire interface "i.LINK" in their products).
Otherwise, you need to install a FireWire board. These are available from companies such as Digital Origin, DPS, Pinnacle, and Truevision for around $500, and typically include the FireWire card, device control software for interfacing to your DV cam, and DV editing software which supports the full DV format. Since not every DV cam works with every DV board, make sure to check the compatibility list before you buy a FireWire board.
If you already have a favorite video editing application, you also may need to update to the latest version to get support for the DV format, and support for interfacing directly to a DV cam. This may also involve updating the Microsoft video compression drivers and / or the Apple QuickTime version on your system to get the latest DV support in your system software.
With a DV cam and a FireWire board you can edit full-res full-rate video productions on your own PC. Of course, this is probably a lot more video data than you are used to processing (remember, it's 3.75 MB/sec), so you may soon be tempted to upgrade your system with more disk space, and even a faster processor for software DV decompression. But even without a bulked-up machine, you can still do short video productions, or use DV simply to quickly import video clips before reducing them down to Web size. Plus, you get the convenience of the cool miniature DV cams, and the security of saving your videos in digital format.
Adam Wilt's DV site http://www.adamwilt.com/DV.html
DV and FireWire Central, DV-List http://www.dvcentral.org/
Sony Consumer: Digital 8, DV Handycams http://www.sony.com/consumer
Sony Professional: DVCAM http://www.sony.com/professional
Panasonic: DVCPRO http://www.panasonic.com/PBDS/index.html
1394 Trade Association: FireWire http://www.1394ta.org/