What
container format are you taling about here?
AAC streams are usually stored in
MP4 container. And, as far as MP4 is concerned, the
ObjectTypeIndication for "MPEG-4 Audio" is
0x40, whereas
0xFF would mean "no object type specified":
https://web.archive.org/web/20180312...rg/object.html
But be aware that "MPEG-4 Audio" could mean
any audio format from the MPEG-4 standard! So
ObjectTypeIndication alone doesn't tell you the exact audio format. You have to look at
AudioObjectType too. That would be
2 for LC-AAC:
https://stackoverflow.com/a/4644066
Having said that: The "classic" VirtualDub (
not VirtualDub
2) did
not even support MP4 output, only WAV or AVI. However, storing AAC streams in WAV or AVI container is at least "unusual" and non-standard.
Furthermore, the WAV/AVI container uses the "wFormatTag" field of the WAVEFORMATEX structure to indicate the audio format – which almost certainly is
not related to the
ObjectTypeIndication of the MP4 container
And, because WAV/AVI was never really intended to store AAC streams, there probably is
no "officially correct"
wFormatTag value for AAC – maybe some "de facto" standard has been established though.
To make a long story short, you may be comparing apples and oranges
(BTW: VirtualDub
2 most likely reads your input file using its FFmpeg-based input driver. This means that the audio gets decoded to PCM
before it even enters the VirtualDub2 core application. That's why "Direct Stream Copy" doesn't work)