使用mediacodec的Aac编码器用一个通道初始化，但输出为两个通道

Question

The aac decoder is initialized as below:

MediaFormat outfmt = new MediaFormat();
outfmt.setString(MediaFormat.KEY_MIME, "audio/mp4a-latm");
outfmt.setInteger(MediaFormat.KEY_AAC_PROFILE, mAudioProfile);
mSampleRate = format.getInteger(MediaFormat.KEY_SAMPLE_RATE);
outfmt.setInteger(MediaFormat.KEY_SAMPLE_RATE, mSampleRate);
mChannels = format.getInteger(MediaFormat.KEY_CHANNEL_COUNT);
outfmt.setInteger(MediaFormat.KEY_CHANNEL_COUNT, mChannels);
outfmt.setInteger(MediaFormat.KEY_BIT_RATE, 64000);
audioEncoder.configure(outfmt, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
            audioEncoder.start();

But the encoder behaviors different on two devices.

One outputs normal presentation: 64000 128000 192000 256000 320000

Another outputs as two channels: 64000 64000 128000 128000 192000 192000 256000 256000 320000 320000

And the format extracted using MediaExtractor is different on two devices:

the normal one is {max-input-size=1572864, aac-profile=2, csd-0=java.nio.ByteArrayBuffer[position=0,limit=2,capacity=2], sample-rate=16000, durationUs=8640000, channel-count=1, mime=audio/mp4a-latm, isDMCMMExtractor=1}

The other is {max-input-size=798, durationUs=8640000, channel-count=1, mime=audio/mp4a-latm, csd-0=java.nio.ByteArrayBuffer[position=0,limit=2,capacity=2], sample-rate=16000}

So the original audio has one channel and the encoder is configured with one channel too.But the encoder outputs as in two channel way.

Does it matter with isDMCMMExtractor flag?

Help!Help! @fadden

Answer 1

First off, the question is very hard to understand - both of the listed MediaFormat contents show channel-count=1 , so there's very little actual explanation of the issue itself, only an explanation of other surrounding details.

However - the software AAC decoder in some android versions (4.1 if I remember correctly, possibly 4.2 as well) will decode mono AAC into stereo - not sure if some of the hardware AAC decoders do the same. You can argue whether this is a bug or just unexpected behaviour, but it's something you have to live with. In the case that the decoder returns stereo data even though the input was mono, both stereo channels will have the same (mono) content.

So basically, you have to be prepared to handle this - either pass the actual format information from the decoder (not from MediaExtractor) to whoever is using the data (eg reconfigure the audio output to stereo), or be prepared to mix down stereo back into mono if you really need to have the output in mono format.