HDV error correction

I’ve been helping out on a shoot using our XL-H1s over the past few weeks, and one thing I’ve noticed is that using Sony PR tape stock, we’re seeing roughly one dropout per tape. This isn’t so bad, and we’re recording to harddisk as well, but it has gotten me thinking about HDV dropouts.

In DV, a dropout might mean you lost a few macroblocks scattered throughout the frame (the macroblocks are even laid to tape in a non-sequential order so that you don’t lose a whole chunk of image to a dropout – clever!). In HDV, you lose 15 entire frames. I’m very curious to know whether this is due to technical necessity, or a choice on the part of engineers. It seems to me that if you handled an HDV dropout the same way you handle a DV dropout, the effect would somewhat similar. However, since HDV is interframe within the GOP, you’d end up with a dropout artifact that potentially travelled around the screen for 15 frames. Have they chosen to just not display the whole chunk of frames because it would look worse to display the artifact?

A dropout on an HDV recording will also knock out more picture information (since there’s more picture data per byte) than a similar dropout in DV, but I still can’t grasp why we need to lose a whole 15 frames. Even if the dropout hit on the I-Frame of a GOP, you can still often do a passable job reconstruction a frame using P-frames, especially if you could look back to the previous GOP.

So, I want better error correction in HDV. Make it so.

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