Decoding Opus audio data

Question

I am trying to decode an Opus file back to raw 48 kHz. However I am unable to find any sample code to do that.

My current code is this:

void COpusCodec::Decode(unsigned char* encoded, short* decoded, unsigned int len)
{
     int max_size=960*6;//not sure about this one

     int error;
     dec = opus_decoder_create(48000, 1, &error);//decode to 48kHz mono

     int frame_size=opus_decode(dec, encoded, len, decoded, max_size, 0);
}

The argument "encoded" might be larger amounts of data, so I think I have to split it into frames. I am not sure how I could do that.

And with being a beginner with Opus, I am really afraid to mess something up.

Could anybody perhaps help?

Answer 1

I think the opus_demo.c program from the source tarball has what you want.

It's pretty complicated though, because of all the unrelated code pertaining to

• encoding, parsing encoder parameters from command line arguments
• artificial packet loss injection
• random framesize selection/changing on-the-fly
• inband FEC (meaning decoding into two buffers, toggling between the two)
• debug and verification
• bit-rate statistics reporting

Removing all these bits is a very tedious job, as it turns out. But once you do, you end up with pretty clean, understandable code, see below.

Note that I

• kept the 'packet-loss' protocol code (even though packet loss won't happen reading from a file) for reference
• kept the code that verifies the final range after decoding each frame

Mostly because it doesn't seem to complicate the code, and you might be interested in it.

I tested this program in two ways:

• aurally (by verifying that a mono wav previously encoded using opus_demo was correctly decoded using this stripped decoder). The test wav was ~23Mb, 2.9Mb compressed.
• regression tested alongside the vanilla opus_demo when called with ./opus_demo -d 48000 1 <opus-file> <pcm-file>. The resultant file had the same md5sum checksum as the one decoded using the stripped decoder here.

MAJOR UPDATE I C++-ified the code. This should get you somewhere using iostreams.

• Note the loop on fin.readsome now; this loop could be made 'asynchronous' (i.e. it could be made to return, and continue reading when new data arrives (on the next invocation of your Decode function?)^[1]
• I have cut the dependencies on opus.h from the header file
• I have replaced "all" manual memory management by standard library (vector, unique_ptr) for exception safety and robustness.
• I have implemented an OpusErrorException class deriving from std::exception which is used to propagate errors from libopus

See all the code + Makefile here: https://github.com/sehe/opus/tree/master/contrib

^[1] for true async IO (e.g. network or serial communinication) consider using Boost Asio, see e.g. http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/overview/networking/iostreams.html

Header File

// (c) Seth Heeren 2013
//
// Based on src/opus_demo.c in opus-1.0.2
// License see http://www.opus-codec.org/license/
#include <stdexcept>
#include <memory>
#include <iosfwd>

struct OpusErrorException : public virtual std::exception
{
    OpusErrorException(int code) : code(code) {}
    const char* what() const noexcept;
private:
    const int code;
};

struct COpusCodec
{
    COpusCodec(int32_t sampling_rate, int channels);
    ~COpusCodec();

    bool decode_frame(std::istream& fin, std::ostream& fout);
private:
    struct Impl;
    std::unique_ptr<Impl> _pimpl;
};

Implementation File

// (c) Seth Heeren 2013
//
// Based on src/opus_demo.c in opus-1.0.2
// License see http://www.opus-codec.org/license/
#include "COpusCodec.hpp"
#include <vector>
#include <iomanip>
#include <memory>
#include <sstream>

#include "opus.h"

#define MAX_PACKET 1500

const char* OpusErrorException::what() const noexcept
{
    return opus_strerror(code);
}

// I'd suggest reading with boost::spirit::big_dword or similar
static uint32_t char_to_int(char ch[4])
{
    return static_cast<uint32_t>(static_cast<unsigned char>(ch[0])<<24) |
        static_cast<uint32_t>(static_cast<unsigned char>(ch[1])<<16) |
        static_cast<uint32_t>(static_cast<unsigned char>(ch[2])<< 8) |
        static_cast<uint32_t>(static_cast<unsigned char>(ch[3])<< 0);
}

struct COpusCodec::Impl
{
    Impl(int32_t sampling_rate = 48000, int channels = 1)
    : 
        _channels(channels),
        _decoder(nullptr, &opus_decoder_destroy),
        _state(_max_frame_size, MAX_PACKET, channels)
    {
        int err = OPUS_OK;
        auto raw = opus_decoder_create(sampling_rate, _channels, &err);
        _decoder.reset(err == OPUS_OK? raw : throw OpusErrorException(err) );
    }

    bool decode_frame(std::istream& fin, std::ostream& fout)
    {
        char ch[4] = {0};

        if (!fin.read(ch, 4) && fin.eof())
            return false;

        uint32_t len = char_to_int(ch);

        if(len>_state.data.size())
            throw std::runtime_error("Invalid payload length");

        fin.read(ch, 4);
        const uint32_t enc_final_range = char_to_int(ch);
        const auto data = reinterpret_cast<char*>(&_state.data.front());

        size_t read = 0ul;
        for (auto append_position = data; fin && read<len; append_position += read)
        {
            read += fin.readsome(append_position, len-read);
        }

        if(read<len)
        {
            std::ostringstream oss;
            oss << "Ran out of input, expecting " << len << " bytes got " << read << " at " << fin.tellg();
            throw std::runtime_error(oss.str());
        }

        int output_samples;
        const bool lost = (len==0);
        if(lost)
        {
            opus_decoder_ctl(_decoder.get(), OPUS_GET_LAST_PACKET_DURATION(&output_samples));
        }
        else
        {
            output_samples = _max_frame_size;
        }

        output_samples = opus_decode(
                _decoder.get(), 
                lost ? NULL : _state.data.data(),
                len,
                _state.out.data(),
                output_samples,
                0);

        if(output_samples>0)
        {
            for(int i=0; i<(output_samples)*_channels; i++)
            {
                short s;
                s=_state.out[i];
                _state.fbytes[2*i]   = s&0xFF;
                _state.fbytes[2*i+1] = (s>>8)&0xFF;
            }
            if(!fout.write(reinterpret_cast<char*>(_state.fbytes.data()), sizeof(short)* _channels * output_samples))
                throw std::runtime_error("Error writing");
        }
        else
        {
            throw OpusErrorException(output_samples); // negative return is error code
        }

        uint32_t dec_final_range;
        opus_decoder_ctl(_decoder.get(), OPUS_GET_FINAL_RANGE(&dec_final_range));

        /* compare final range encoder rng values of encoder and decoder */
        if(enc_final_range!=0
                && !lost && !_state.lost_prev
                && dec_final_range != enc_final_range)
        {
            std::ostringstream oss;
            oss << "Error: Range coder state mismatch between encoder and decoder in frame " << _state.frameno << ": " <<
                    "0x" << std::setw(8) << std::setfill('0') << std::hex << (unsigned long)enc_final_range <<
                    "0x" << std::setw(8) << std::setfill('0') << std::hex << (unsigned long)dec_final_range;

            throw std::runtime_error(oss.str());
        }

        _state.lost_prev = lost;
        _state.frameno++;

        return true;
    }
private:
    const int _channels;
    const int _max_frame_size = 960*6;
    std::unique_ptr<OpusDecoder, void(*)(OpusDecoder*)> _decoder;

    struct State
    {
        State(int max_frame_size, int max_payload_bytes, int channels) :
            out   (max_frame_size*channels),
            fbytes(max_frame_size*channels*sizeof(decltype(out)::value_type)),
            data  (max_payload_bytes)
        { }

        std::vector<short>         out;
        std::vector<unsigned char> fbytes, data;
        int32_t frameno   = 0;
        bool    lost_prev = true;
    };
    State _state;
};

COpusCodec::COpusCodec(int32_t sampling_rate, int channels)
    : _pimpl(std::unique_ptr<Impl>(new Impl(sampling_rate, channels)))
{
    //
}

COpusCodec::~COpusCodec()
{
    // this instantiates the pimpl deletor code on the, now-complete, pimpl class
}

bool COpusCodec::decode_frame(
        std::istream& fin,
        std::ostream& fout)
{
    return _pimpl->decode_frame(fin, fout);
}

test.cpp

// (c) Seth Heeren 2013
//
// Based on src/opus_demo.c in opus-1.0.2
// License see http://www.opus-codec.org/license/
#include <fstream>
#include <iostream>

#include "COpusCodec.hpp"

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        std::cerr << "Usage: " << argv[0] << " <input> <output>\n";
        return 255;
    }

    std::basic_ifstream<char> fin (argv[1], std::ios::binary);
    std::basic_ofstream<char> fout(argv[2], std::ios::binary);

    if(!fin)  throw std::runtime_error("Could not open input file");
    if(!fout) throw std::runtime_error("Could not open output file");

    try
    {
        COpusCodec codec(48000, 1);

        size_t frames = 0;
        while(codec.decode_frame(fin, fout))
        {
            frames++;
        }

        std::cout << "Successfully decoded " << frames << " frames\n";
    }
    catch(OpusErrorException const& e)
    {
        std::cerr << "OpusErrorException: " << e.what() << "\n";
        return 255;
    }
}