[FFmpeg-devel] Deterministic Noise Generation option for audio codecs that involve noise generation (RNG)

Jonathan Campbell jonathan at castus.tv
Sat Sep 7 10:21:16 CEST 2013

I'm writing a program to provide "random access" decoding for various 
file formats including those involving MP3 or AC-3 and I want to bring 
up something that might help improve the FFMPEG ABI.

I found that I can provide sample-level random access for MP3 if, when 
non-sequential access is needed, I just step back about 8-16 MP3 frames 
(compensating for the bit reservoir and MDCT window) and decode up to 
the one I need. The problem is AC-3 and it's noise coding. If I try the 
same trick with AC-3, stepping back 2-3 frames so that the MDCT window 
can build up, no matter how precisely I do it there are discontinuities 
between the previous decode and the decode I started again from the same 

I had written a hack for FFMPEG 1.0.1 where by setting a flag, if noise 
bands are present, the codec seeds the RNG with a checksum of the AC-3 
or AAC frame and then proceeds to decode as normal. Then, decoding the 
same AC-3 frames always results in the same PCM output, and the same 
trick I used with MP3 then works correctly. It can be set for programs 
that want this behavior while existing code can leave it unset and 
maintain existing decode behavior.

Open source video editing projects could benefit from that feature when 
allowing the user to edit from formats and containers involving these 

If I rewrite that patch for FFMPEG 1.2.2 and FFMPEG 2.0, would any of 
you be interested in incorporating it upstream?

On a related note, can anyone tell me if a similar hack can be made for 
video codecs that use noise coding (are there any)?

-- Jonathan C.

More information about the ffmpeg-devel mailing list