[Libav-user] AAC decoding delay/take 2

[Info || Non-Lethal Applications]

Hi there, 

I already posted that a couple of weeks ago but I didn’t get a reply.
I thought I’d try my luck again.

I am currently working on my own video player.
I just tried it with a bunch of files and I found out that the audio is always about one frame after the video whenever the video contains AAC audio.
I tried a bunch of test files with various video codecs (mostly Pro Res and H.264), resolutions etc … whenever AAC audio is present, the audio is delayed.
The test files have all been in the QuickTime container.

To verify, I created two test files from the same Final Cut project. One Pro Res with PCM audio and one H.264 with AAC audio.
While decoding, I wrote the raw decoded samples to a file and checked them in Audacity. Technically, they should be equal.
See the attached screenshot.

The upper track is perfectly equal to the source audio track.
The bottom audio track is the captured AAC output. 

As you can see, the AAC output is more than 2000 samples delayed.

Can anyone explain where this delay is coming from?




Best regards!

Flo
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