DV Questions & Answers

Digital video is arguably the most exciting development in videography in the last decade, and this holiday season DV camcorders are on everyone’s wish list. We all know that DV looks better, but some of our readers want more details on why it looks better. In this Q&A session, we’re going to answer some of your questions about the technology behind the tiny-but-powerful digital formats and answer a few other questions to help you gain a better understanding of the capability and promise of this innovative video format.

Q. What’s the difference between Mini DV and just DV?

A. DV is a recording and playback standard for digital video, regardless of the format. Mini DV is a tape format that’s intended for consumer camcorders.

The full-size DV tapes, which can hold up to 4 1/2-hours of digital video and audio, are used primarily by broadcast professionals. Interestingly, you don’t need special adapters to play Mini DV tapes in full-size DV VCRs since the two tape formats are compatible (see comparison in Figure 1).

DV Compression

Q. Since there is so much compression in DV, doesn’t that cause a considerable loss of video quality? And aren’t those tiny tracks recorded on that little tape going to have far too many problems with dropouts?

A. As far as the absolute quality of DV, that’s something you’ll have to judge for yourself. It’s far better than any previous consumer video format and has found wide acceptance in the professional video community as well. The image quality speaks for itself.

As for the potential for dropouts causing problems, designers of the DV format did not ignore this issue. Manufacturers store additional information on the tape that corrects errors and verifies data that is read on the tape. If both data and the codes needed to recover it accurately are lost, DV provides methods for generating "best guess" information based on surrounding frames. Figure 2 shows how DV corrects errors, but in practice it seems that error correction and interpolation are rarely needed. Generally speaking, dropouts and data loss are extremely rare.



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Q. Our videos are a treasure and I’m concerned about the long-term stability of magnetic tape. How well is DV suited for storage?

A. Whether digital or analog, any recording made on magnetic tape is subject to degradation over time. Wear of the tape, dust, damage and print-through are always a threat. Fortunately, digital recordings are much more immune to the effects of all these problems. Analog recordings attempt to track the recorded signal directly, and any change in the recorded signal results in a corresponding loss of information. Digital systems aren’t so touchy. DV records information as ones and zeros and small changes in the value written to the tape have no effect on the data that will be read from the tape [See Error Correction Sidebar for clarification on why this is so].

This fundamental digital advantage, combined with the error correction and interpolation discussed above, makes DV the most reliable form of video recording ever placed in the hands of the consumer. However, like all other recordings on magnetic tape, they are subject to long term degradation since nothing lasts forever.

Apples and Apples

Q. What’s the difference between DV, FireWire, IEEE 1394, and iLINK?

A. DV is a videotape format, while the others refer to an interface standard. DV standards define the way video and audio are compressed and recorded onto magnetic tape. IEEE 1394 defines one way digital data communicates between digital devices. While you can use FireWire to transmit digital video and audio, you can also use it to transmit any other digital data. Apple Computer originally developed the interface standard under the trade name of FireWire. Sony later adopted the trade name iLINK, but they are all really the same thing. In summary then, DV is a recording format, and FireWire/IEEE 1394/iLINK is a digital serial protocol.

With a DV camcorder, the FireWire protocol holds much more than just the audio and video. DV tape transmits all stored information through the interface, including exposure information, time-code, etc. And the FireWire cable carries command and control information to and from the camcorder as well. In general, all you need to edit DV on your PC is a fast hard drive, a FireWire card, a DV camcorder with a FireWire jack and appropriate editing software (See Cable Compatibility Concerns sidebar).

A Question of Compatability

Q. I want to copy video to my computer’s hard drive and edit it, but I can’t find a connector on my PC that fits the FireWire cable that I got with my camcorder. Do I need an adapter or something to allow me to copy my video to my hard drive?

A. The FireWire interface isn’t a standard feature of every computer. If you don’t find the proper connector on the front or back of your PC, you’ll probably have to add a FireWire card to be able to copy your video to your hard drive. Before installing such a card though, you need to check a number of compatibility issues carefully. To avoid problems, your hard drive must be able to support a sustained (not burst) data rate of nearly 5 megabytes per second. You must insure that the card you choose is compatible with your PC and the other cards installed therein (e.g. anticipate IRQ conflicts), and you must insure that the card and the software you choose is compatible with your camcorder. If you’re comfortable tinkering with and tuning your PC, you can probably resolve any issues that arise. But you may find you’ll need expert technical advice when trying to upgrade an older PC to deal with video. Fortunately for those who want to edit digital video without endless tinkering with a PC, there are a couple of alternatives to make life easier.

Consider upgrading to a PC pre-configured for editing DV. I use Sony camcorders and Sony PCs, which has the advantage of tech support from a single source. If you’ve been around PCs for long, you know that occasional problems are a fact of life, and it’s nice not to have to worry about whom to call when problems arise. To avoid problems you must still investigate compatibility issues carefully. Verify that the computer you select is compatible with the camcorder you’re using before you buy.

Even the PC pre-configured for DV editing, and compatible with your camcorder, can cause problems over time. You must manage your disk space, and keep the drives defragmented for top performance. Any software or hardware you add on may create conflicts and/or other problems. If possible, dedicate a PC to your video editing tasks, and you’ll avoid unnecessary headaches.

An economical and efficient solution is a turnkey appliance like those offered by Draco and Applied Magic. These boxes do only one thing, edit video, and they make it easier than ever. They free you entirely from configuration issues, and let you focus on producing quality video. Until recently this type of dedicated video editing appliance was beyond the reach of most amateurs, but new models by both companies are opening this option to more and more of us. The turnkey editors make it all easy enough for a child. Please note however that even these boxes may not be compatible with every DV camcorder or VCR on the market, so ask about compatibility before you buy.

Still Shot Mode

Q. What is still shot mode?

A. Most DV camcorders have a still shot mode that allows you to record a still image directly onto the DV tape. Basically, your camcorder records a single image from the CCD repeatedly for a period of time, usually around 8 seconds. The compression and recording method is identical to that used for motion video, so the image quality is similar. Most DV camcorders use one or more methods to improve image quality in the still shot mode, ranging from inter-field interpolation (interlaced scan) to progressive scan CCDs.

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