Software Dropout Compensation?

[teeg]

I understand that many expensive professional TBCs and Betacam VTRs have this dropout compensation where the last good line is looped to replace the damaged section of video.

However, as a cheaper alternative, is there a way of going frame by frame in a captured DV-AVI file and cutting and pasting a small streak of video from the last good frame and inserting it into the frame with the dropout?

[redwudz]

You should be able to do that with VirtualDub. Make your selection, copy and paste. However, frame by frame is a little more difficult as you can only cleanly cut on key frames. And if your key frames are far apart, it might get a little choppy. And audio sync may become an issue. But try it.

[teeg]

Oh, I thought all frames were equal in a dv-avi file.

Thanks.

[edDV]

I understand that many expensive professional TBCs and Betacam VTRs have this dropout compensation where the last good line is looped to replace the damaged section of video.

However, as a cheaper alternative, is there a way of going frame by frame in a captured DV-AVI file and cutting and pasting a small streak of video from the last good frame and inserting it into the frame with the dropout?

The manual frame by frame process is called "Rotoscoping". The trick is to find a way to automate detection and pixel correction.

Classic hardware TBC dropout compensators detect dropouts by monitoring the raw modulated RF signal off the tape playback head. One or more previous lines are saved in memory allowing dropped pixels to be replaced with those from the previous line. Frame synchronizers allow pixels from the previous frame to be used.

What is needed for software dropout compensation is a reliable detection algorithm that won't give false positives.

[teeg]

Thanks Ed.

If I was to use an Edirol VMC for A/D conversion (for my PAL VHS tapes) would this help masking any dropouts as it has a frame sync, or is a DOC needed as well?

[redwudz]

I had a very short video with a lot of single frames corrupted. I resaved it by making every frame a key frame. Then I was able to cut any frame out. I'm really not familiar with how the key frame interval is set up, but this from the glossary explains it a little. AFAIK, this also applies to AVI:

I Frame
An I frame is encoded as a single image, with no reference to any past or future frames. Often video editing programs can only cut MPEG-1 or MPEG-2 encoded video on an I frame since B frames and P frames depend on other frames for encoding information.

Or this:

P Frame
A P-frame is a video frame encoded relative to the past reference frame. A reference frame is a P- or I-frame. The past reference frame is the closest preceding reference frame.

Each macroblock in a P-frame can be encoded either as an I-macroblock or as a P-macroblock. An I-macroblock is encoded just like a macroblock in an I-frame. A P-macroblock is encoded as a 16x16 area of the past reference frame, plus an error term. To specify the 16x16 area of the reference frame, a motion vector is included. A motion vector (0, 0) means that the 16x16 area is in the same position as the macroblock we are encoding. Other motion vectors are relative to that position. Motion vectors may include half-pixel values, in which case pixels are averaged. The error term is encoded using the DCT, quantization, and run-length encoding. A macroblock may also be skipped which is equivalent to a (0, 0) vector and an all-zero error term. The search for good motion vector (the one that gives small error term and good compression) is the heart of any MPEG-1 video encoder and it is the primary reason why encoders are slow. MPEG FAQ

I know there's a lot more to it than that, but the frame makeup does effect how you can edit a video.

[teeg]

Because I'll be using 'raw' DV at some 13-14GB/hour, every frame is a key frame as it uses spatial compression rather than temporal which I think compares information in successive frames to further reduce bitrate.

This means that editing should be far simpler than dealing with MPEG files, I hope.