DV artifacts come in three flavors: mosquito noise, quilting, and motion blocking. Other picture 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.

Last update: 08:23 PM Tuesday, February 28, 2006