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androidandroid-mediacodecmediaextractor

Trim aac-mp4 audio in android (mediaCodec/extractor)

I want to trim an existing aac-mp4 audio file. For the first time I want to "trim" 0 bytes, basically just to copy the file using MediaCodec/MediaExtractor.

Questions:

  1. The header is fixed size and I can just copy it from the old file? Or it has some infos about the track duration and I need to update it? If it has fixed size which is that (in order to know how many bytes should I copy from the old file)?
  2. Should I only use the extractor's getSampleData(ByteBuffer, offset) and advance() or I should also use the MediaCodec and extract the samples(decode) and then encode them again with an encoder - and write the encoded values?
Solution

    1. If you use MediaExtractor, you probably aren't going to read the raw file yourself, so I don't see what header you're proposing to copy. This is probably easiest to do with MediaExtractor + MediaMuxer; just copy the MediaFormat and the packets you get from MediaExtractor to MediaMuxer.

    2. This depends on how you want to do the trimming. It's absolutely simplest to not involve MediaCodec at all, but just copy packets from MediaExtractor to MediaMuxer, and skip the packets at the start that you want to omit (or use seekTo() for seeking to the right start position).

    But keep in mind that audio frames have a certain length; for AAC-LC it's usually 1024 samples, which for 48 kHz audio is 21 milliseconds. So if you only copy individual packets, you can't get any closer trimming granularity than 21 milliseconds, for 48 kHz. This probably is fine for most cases, but if the audio has a lower sample rate, say 8 kHZ, the granularity ends up as high as 128 ms.

    If you want to trim to a more exact position than the individual packets allow you, you need to decode using MediaCodec, skip the right amount of samples, repackage output frames from the decoder into new full frames for the encoder, and encode this.