[Libav-user] FMP4 -> ADTS/AAC using libavformat/libavcodec

[Ronak]

Hi,

I’m writing a C++ program to validate the integrity of a Fragmented MP4 file containing AAC audio.
This program would parse the FMP4 file, read each audio packet, attachment ADTS headers, and then try to decode the AAC using libfdk_aac.

I am using libavformat, and I am able to parse the MP4 atoms correctly, however I’m having issues figuring out the best way to attach the ADTS headers.

My current idea is to use a libavformat mp4 demuxed and send that to an adts muxer. I’ve written all the code to do it now; but I’m now getting errors like so:

[adts @ 0x7fae51001600] MPEG-4 AOT 0 is not allowed in ADTS

I’m not sure what to do now.

My code is as below:

AudioAssetReader::AudioAssetReader(const AudioAssetReaderInput &input) throw (invalid_argument):
				input(input), segmentsIter(input.getBeginSegmentsIter()), audioBufferSize(409600), adtsPacketBufferSize(409600) {
	if (input.getSegments()->empty()) {
		throw invalid_argument("A reader must read at least 1 segment of audio!");
	}

	this->audioBuffer = new uint8_t[audioBufferSize];

	// create the main context used to parse the audio asset, segment by segment
	bool isBufferWritable = false;
	this->assetSegmentReaderContext = avio_alloc_context(this->audioBuffer, this->audioBufferSize, isBufferWritable, this, AudioAssetReader::readSegment, NULL, NULL);
	this->assetParsingContext = avformat_alloc_context();
	this->assetParsingContext->pb = this->assetSegmentReaderContext;
	this->assetParsingContext->flags = AVFMT_FLAG_CUSTOM_IO;

	// create another context used to append ADTS headers to the parsed segments, if we need them
	this->initializeADTSMuxerContext(input);
}

void AudioAssetReader::initializeADTSMuxerContext(const AudioAssetReaderInput &input) {

	this->adtsPacketBuffer = new uint8_t[adtsPacketBufferSize];
	this->adtsPacket = av_packet_alloc();

	// create another context used to append ADTS headers to the parsed segments, if we need them
	bool isBufferWritable = true;
	avformat_alloc_output_context2(&this->adtsMuxerContext, NULL, "adts", NULL);
	this->adtsMuxerWriterContext = avio_alloc_context(this->adtsPacketBuffer, this->adtsPacketBufferSize, isBufferWritable, this, NULL, AudioAssetReader::writeADTSPacket, NULL);
	this->adtsMuxerContext->pb = this->adtsMuxerWriterContext;

	// initialize an ADTS stream
	AVCodecID aacCodec = AVCodecID::AV_CODEC_ID_AAC;
	AVCodec *codec = avcodec_find_encoder(aacCodec);
	AVStream *stream = avformat_new_stream(this->adtsMuxerContext, codec);
	stream->id = this->adtsMuxerContext->nb_streams - 1;
	stream->time_base.den = input.getSampleRateHz();
	stream->time_base.num = 1;

	// configure the stream with the details of the AAC packets
	AVCodecParameters *codecParameters = stream->codecpar;
	codecParameters->codec_id = aacCodec;
	codecParameters->bit_rate = input.getBitrateBps();
	codecParameters->profile = this->getAACProfileForCodec(input.getCodec());
	codecParameters->sample_rate = input.getSampleRateHz();
	codecParameters->channels = input.getChannelCount();
	codecParameters->codec_type = AVMEDIA_TYPE_AUDIO;
	codecParameters->channel_layout = av_get_default_channel_layout(input.getChannelCount());

	uint8_t* header = new uint8_t[7];
	memset(header, 1, 7);
	//header[3] = 2 << 6;

	codecParameters->extradata = header;
	codecParameters->extradata_size = 7;

	// write out a header to the muxer; this is a no-op for our purposes
	const int result = avformat_write_header(this->adtsMuxerContext, NULL);
	if (result < 0) {
		char *message = new char[AV_ERROR_MAX_STRING_SIZE];
		av_make_error_string(message, AV_ERROR_MAX_STRING_SIZE, result);
		cerr<<endl<<"Failed to write the ADTS muxer header due to: "<<result<<": "<<message<<endl;
		delete[] message;
	}
}

Ronak
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