Opus encode & decode no error but not the same value

Question

I have to use Opus Codec to encode & decode audio datas in C++ and I have to encapsulate the functions. So I try to send a floats array to try to encode it and I decode the result of the Opus encoding function. Unfortunately, the result is not the same and I get a table that contains no value from the initial table.

Here is my code.

Encapsulation:

std::vector<float> codec::OpusPlugin::decode(packet_t &packet) {
    std::vector<float> out(BUFFER_SIZE * NB_CHANNELS);
    int ret = 0;

    if (!this->decoder)
        throw Exception("Can't decode since there is no decoder.");
    ret = opus_decode_float(this->decoder, packet.data.data(), packet.size, reinterpret_cast<float*>(out.data()), FRAME_SIZE, 0);
    if (ret < 0)
        throw Exception("Error while decoding compressed data.");
    return out;
}

// ENCODER

packet_t codec::OpusPlugin::encode(std::vector<float> to_encode) {
    std::vector<unsigned char> data(BUFFER_SIZE * NB_CHANNELS * 2);
    packet_t packet;
    int ret = 0;

    if (!this->encoder)
        throw Exception("Can't encode since there is no decoder.");
    ret = opus_encode_float(this->encoder, reinterpret_cast<float const*>(to_encode.data()), FRAME_SIZE, data.data(), data.size());
    if (ret < 0)
        throw Exception("Error while encoding data.");
    packet.size = ret;
    packet.data = data;
    return packet;
}

And there is the call of the functions:

 packet_t packet;
    std::vector<float> floats = {0.23, 0, -0.312, 0.401230, 0.1234, -0.1543};
    packet = CodecPlugin->encode(floats);
    std::cout << "packet size: " << packet.size << std::endl;
    std::vector<float> output = CodecPlugin->decode(packet);
    for (int i = 0; i < 10; i++) {
        std::cout << output.data()[i] << " ";
    }

Here is the packet_t structure, where I stock the return value of encode and the unsigned char array (encoded value)

typedef struct packet_s {
    int size;
    std::vector<unsigned char> data;
} packet_t;

The output of the program is *-1.44487e-15 9.3872e-16 -1.42993e-14 7.31834e-15 -5.09662e-14 1.53629e-14 -8.36825e-14 3.9531e-14 -8.72754e-14 1.0791e-13 which is not the array I initialize at the beginning.

I read a lot of times the documentation and code examples but I don't know where I did a mistake. I hope you will be able to help me.

Thanks :)

Community · Accepted Answer · 2021-10-07 11:32:16Z

We don't see how you initialize your encoder and decoder so we don't know what their sample rate, complexity or number of channels is. No matter how you have initialized them you are still going to have the following problems:

First Opus encoding doesn't support arbitrary frame sizes but instead 2.5ms, 5ms, 10ms, 20, 40ms or 60ms RFC 6716 - Definition of the Opus Audio Codec relevant section 2.1.4. Moreover opus supports only 8kHz, 12kHz, 16kHz, 24kHz or 48kHz sample rates. No matter which of those you have chosen your array of 10 elements doesn't correspond to any of the supported frame sizes.

Secondly Opus codec is a lossy audio codec. This means that after you encode any signal you will never (probably except some edge cases) be able to reconstruct the original signal after decoding the encoded opus frame. The best way to test if your encoder and decoder work is with a real audio sample. Opus encoding preserves the perceptual quality of the audio files. Therefore if you try to test it with arbitrary data you might not get the expected results back even if you implemented the encoding and decoding functions correctly.

What you can easily do is to make a sine function of 2000Hz(there are multiple examples on the internet) for 20ms. This means 160 array elements at a sample rate of 8000Hz if you wish to use 8kHz. A sine wave of 2kHz is within the human hearing range so the encoder is going to preserve it. Then decode it back and see whether the elements of the input and output array are similar as we've already established that it is unlikely that they are the same.

I am not good in C++ so I can't help you with code examples but the problems above hold true no matter what language is used.

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