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

[Ronak Patel]

> On Oct 3, 2019, at 6:22 PM, Ronak via Libav-user <libav-user at ffmpeg.org> wrote:
> 
> I am in a time crunch and would appreciate a prompt reply for this…I’m amazed that it is this complicated to append an ADTS header to MPEG4 Audio.

So it looks like I need to make an MPEG4 AudioSpecificConfig.

However I wonder what’s the point of using this muxer if I have to do that. I already specified all the parameters in the CodecParameters struct.

I’m thinking of abandoning FFmpeg to write the adts header and just do this myself.

This is just too complicated of a public api with too little documentation which provides almost no value.

There isn’t even a public helper function I can call to make that AudioSpecificConfig.

I would suggest these apis be added to Ffmpeg to make this a lot easier. If these apis already exist; I would suggest they be documented MUCH better.

> 
> 
>> On Oct 3, 2019, at 5:44 PM, Ronak via Libav-user <libav-user at ffmpeg.org> wrote:
>> 
>> 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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