Doom9's Forum - View Single Post - AAC_ACM v1.9 (fcchandler) Tag ID FF issue

LoRd_MuldeR · 1st July 2021, 02:10

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 VirtualDub2) 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: VirtualDub2 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)

1st July 2021, 02:10	#2 \| Link
LoRd_MuldeR Software Developer Join Date: Jun 2005 Location: Last House on Slunk Street Posts: 13,248	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* VirtualDub2) 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: VirtualDub2 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) __________________ Go to https://standforukraine.com/ to find legitimate Ukrainian Charities 🇺🇦✊ Last edited by LoRd_MuldeR; 1st July 2021 at 02:35.