RtAudio + Qt : duplex not working with RME Fireface on Linux

This is my first post on Stackoverflow, I hope I'm doing this right.

I'm new to C++.

I've been playing with RtAudio and Qt (on linux, desktop and raspberry pi).

Backend is ALSA.

Audio out went fine both on my desktop computer (RME Fireface UCX in ClassCompilant mode and on the Raspberry Pi 3 with HifiBerry and PiSound)

Lately, I tried to add audio input support to my program. I read the duplex tutorial on RtAudio website, and tried to implement it inside my code.

As soon as I added the input StreamParameters to openStream I got a very cracky sound. Although, StreamStatus is ok in the callback...

I tried to create an empty C++ project, and simply copy the RtAudio tutorial. Sadly, the problem remains...

I added this to my project file in Qt Creator

LIBS += -lpthread -lasound

I think my issue is similar to this one, but I couldn't find how (or if) it was solved

I tried different buffer sizes (from 64 to 4096 and more), the cracks are less audible, but still present when buffer size increases

Do you know anything that should be done regarding RtAudio in duplex mode that might solve this ? It seems that buffer size is not the same when in duplex mode.

edit :

Out of curiosity (and despair), I tried even lower buffer sizes with the canonical example from RtAudio help : it turns out using buffer size 1, 2, 4 and 8 frames removes the cracks...

As soon as I use 16 frames, sound is awful

Even 15 frames works, I really don't get what's going on

Code Sample :

RtAudio::StreamOptions options;
options.flags |= RTAUDIO_SCHEDULE_REALTIME;

RtAudio::StreamParameters params_in, params_out;
params_in.deviceId = 3;
params_in.nChannels = 2;
params_out.deviceId = 3;
params_out.nChannels = 2;

When output only, it works :

try {
    audio.openStream(
        &params_out,
        NULL,
        RTAUDIO_SINT16,
        48000,
        &buffer_frames,
        &inout,
        (void *) &buffer_bytes,
        &options
    );
}
catch (RtAudioError& e) {
    std::cout << "Error while opening stream" << std::endl;
    e.printMessage();
    exit(0);
}

Cracks appear when changing NULL to &params_in :

try {
    audio.openStream(
        &params_out,
        &params_in,
        RTAUDIO_SINT16,
        48000,
        &buffer_frames,
        &inout,
        (void *) &buffer_bytes,
        &options
    );
}
catch (RtAudioError& e) {
    std::cout << "Error while opening stream" << std::endl;
    e.printMessage();
    exit(0);
}

Thank you for your help

Answering my own question.

I re did my tests from scratch on the Raspberry Pi 3 / PiSound.

It turns out I must have done something wrong the first time. The canonical example from RtAudio (and the input implementation I did for my program) work well at 64, 128, etc buffer sizes.

The desktop build still have cracky sound, but works with weird buffer sizes (like 25, 30 or 27). The problem most likely comes from the Fireface UCX which is not well supported on Linux (even in ClassCompilant mode).

Thank you for your help, and sorry if I wasted your time.