Change tempo in non realtime context without time offset

[Regis Houde]

I try to use Soundtouch to change the tempo of a recording without introducing a time offset. As a matter of fact, I noticed that the output is always shift left (sounds starts before what is expected). For example, if a recording is at tempo 120 Bpm and the sounds starts at sample 1000, I would expect that the sound would start at sample 2000 if I change the tempo to 60 Bpm but it´s not the case, the sound starts before that.

Is it possible to avoid that? If not, is it possible to know the exact value of the offset?

thank you

[oparviai]

Hello. Given that you leave a message here, please verify that you have the latest version of SoundTouch available nowadays in here: https://codeberg.org/soundtouch/soundtouch

[Regis Houde]

I have the latest version. I am using the files from the tar release (https://www.surina.net/soundtouch/soundtouch-2.3.2.tar.gz)

I did more tests and here are the result.

Initial data : 120 BPM
Guitar starting at 7,894034 bar

Converting to following BPM, the Guitar start is offset as follow :

Tempo (bpm) Event (bar) Delta
60 7,8188 -0,0752
75 7,8521 -0,0419
90 7,8414 -0,0526
105 7,8588 -0,0353
120 7,8940 0,0000
135 7,8880 -0,0060
150 7,8801 -0,0139
165 7,9195 0,0254
180 7,9086 0,0146

Maybe, I am doing something wrong. Here is my code:

HANDLE stProcessor = soundtouch_createInstance();
try {

        soundtouch_setChannels(stProcessor, nbChannels);
        soundtouch_setSampleRate(stProcessor, asioDriverInfo.sampleRate);
        soundtouch_setTempo(stProcessor, newTempo/oldTempo);


        soundtouch_putSamples(stProcessor, oldData, oldNumSamples);

        soundtouch_flush(stProcessor); // To make sure we have everything!

        int index  = 0;
        int nReceived;

        do
        {
            nReceived = soundtouch_receiveSamples(stProcessor, newData + nbChannels*index, bufs.numSamples - index);
            index += nReceived;
            qInfo("nReceived = %d nReceived/nbChannels = %d", nReceived, nReceived/nbChannels);
        }
        while((nReceived > 0) && (index < bufs.numSamples));

        soundtouch_clear(stProcessor);

        }

}
catch (const std::exception& e)
{
    qInfo("ERROR Catch! %s", e.what());
}

soundtouch_destroyInstance(stProcessor);

std::free(oldData);
std::free(newData);