Dropouts

[jasarsenault]

The more I learn about different formats the more I hear discussion about "drop-out". Could someone explain what this means?

[Laurent Andrieux]

This concerns video on tape only. It is a problem that occures sometimes. The symptom is the missing of some lines on the image. It occures if you have dust on the video heads, basically. It can be either on the recording tape, and in this case you have lost the information for ever, or on the player deck. So when you see that, you have to check it's on the original tape or just a play back problem.

 

The missing lines look white, basically, but most of the player decks now handle this pretty well, they can display grey lines instead or even calculate and replace the missing lines by a mean of the upper and lower lines.

 

This is why it is absolutly forbidden to smoke in an editing room, and why video tapes and recorder fear dust.

 

The tape actually doesn't touch the heads and is supposed to stay at a very close distance from it (a few microns). The particules that are in the smoke are big enough to make this distance too long, and if the tape is too far from the head, it doesn't read the tape anymore and that causes a drop out. I'm not even talking about hash, just the smoke can do that.

[jasarsenault]

Thanks!

[Phil Rhodes]

Hi,

 

That's accurate for analogue tape formats, but the symptoms of dropout can be very different on digital formats. Most commonly, you'll lose a couple of macroblocks (eight pixel squares on DV or DVCAM) which are generally copied from the last frame. Depending on the frame content, this may be visible, but miniDV drops out a lot and it's not often a problem.

 

Phil

[John Sprung]

Digital tape formats have some redundancy in the data they record, and they can reconstruct what is lost to small dropouts. Standard definition digital betacam had what they called "error correction and concealment", I'm not sure if some of the earlier formats may have had it, too.

 

One of the things they keep an eye on for you in the machine rooms at post facilities is the amount of error concealment that's happening in the playback of a tape. If there's a lot of it, it might be a good idea to clone the tape.

 

 

 

-- J.S.

[Alvin Pingol]

Some more information and images regarding dropouts on Adam Wilt's site here.

[Michael Nash]

Analogue tape formats can have dropouts and "correction" as well. In Betacam SP for example, a tape dropout (loss of video signal on a portion of the tape) will usually apear as a small horizontal black line for the duration of one frame, or sometimes a few horizontal lines with loss of color for one frame. The odd dropout can usually be attributed to the tape, but multiple or frequent dropouts are a sign of something else going wrong, like dirty heads in the recorder or playback machine.

 

Professional Betacam decks have "dropout compensators" that do a pretty good job of covering these small tape errors. I'm reminded of this every time I play back footage in the camera, where the recorder doesn't have dropout compensators. There are tons of little tiny "hits" that never appear upon playback in a studio deck.