VirtualDub, Avidemux changes bitrate?
 

I have done a lot of searching but can't seem to find the answer.

I simply want to trim the beginning and end of an AVI file captured with an ATI USB 600 and the huffyuv codec through VirtualDub. I am using Direct Stream Copy in VirtualDub (1.9.11). I am using Copy in AviDemux.

With both applications, the overall bitrate went from 47.7 Mb/s to 84.6 Mb/s

I cannot see any other options to resolve this. Is this expected behavior trimming a huffyuv file? I can trim it when converting to h.264 but just trying to save 1hr50min of "dead space" in the mean time. Also, if I ever have some files I want to put away in the lossless format, this would be wasteful. Thank you.

Original File

Avidemux File

VirtualDub File

I cannot speak for Avidemux. I use Virtualdub to trim unwanted frames from a lossless file such as a Huffyuv AVI. I use Direct Stream Copy option in the Video menu and do a File -> Save as AVI (F7) once I have either selected a range using Edit -> Set selection start (Home) and Set selection end (End). Alternatively, I also use those same Set selection start and end and then cut those frames from the copy loaded in Virtualdub, then save. If a range is not selected, then it will save all the frames remaining.

There should not be anything else you have to do with Virtualdub setup to make an exact copy when in the Direct stream copy Video mode, and it should save very quickly (the rate of frames per second it processes should be very high), and not look like it is trying to re-encode the file.

However, I don't know how Virtualdub operates under the hood. I would guess it doesn't even use a codec (i.e. Huffyuv) when it's doing a Direct stream copy. I say that, because when Direct stream copy is selected in the Video menu, the Compression option in the Video menu is disabled, so you are not able to select a destination codec.

If you are certain the edited file is still Huffyuv encoded, then my guess as to why you are seeing such a different bitrate is that this is an average over the entire video. Huffyuv is a variable rate encoder. If you look at the metrics in Virtualdub as it is encoding a capture, you'll see the rate changes depending on what is being captured. Some tapes are nice and clean and so it can encode much more efficiently, while others have a lot more noise and so the bit rate is higher because it is trying to preserve all that noise without any loss of information. If most of what you cut from the original file is just a blank screen, then the average bit rate over the 2 hour file is going to be greatly reduced because it can encode that part of the video much more efficiently than the few minutes of video you are trimming out of the 2 hour file. If it can encode almost 2 hours of blank screen at a very low bitrate, but the few minutes of video you are saving is encoded at a bitrate several times higher than that, then the average of the 2 hours of blank + your few minutes of video is going to be much lower than when you only save the few minutes of video.

Hope this helps.

The specs for all of the files are above.

You make a good point. A lot of "blank" video in the original file. Almost 110 minutes. That long section may have a much lower bitrate and the section with actual video. I was expecting my file size to go down in a linear fashion.

I will do a test with a file that only needs a little bit trimmed on each end and see what happens.