Error when recording multi-channel audio with loopback

I am trying to record multi-channel audio where one channel records audio from a microphone and two channels record "internal" sound (i.e., loopback) that is played via MATLAB to headphones. I am working with a digital audio interface (Focusrite Scarlett), ASIO drivers, Windows 10 and MATLAB R2022b.

With the code below I get the following error message:

Error using audioDeviceWriter/setup

The buffer size parameter of the audio input device must match the buffer size parameter of the audio output device.

Error in audioDeviceWriter/setupImpl

I have checked the buffer size parameter using asiosettings and also the settings of the Focusrite and they are set to the same value. I would assume that this applies to both the outputs and inputs of the Focusrite so I cannot explain the error message.

%% define properties of digital audio interface and recording set-up
device = 'Focusrite USB ASIO';
driver = 'ASIO'; 
Fs = 44100; % sampling frequency

%% set up device reader (for recording)
deviceReader = audioDeviceReader('SampleRate',Fs, ...
'Device',device, ...
'BitDepth','24-bit integer', ...
'Driver',driver, ...
'NumChannels',6, ...
'SamplesPerFrame', Fs/4);

setup(deviceReader); 

%% load audio file and set up device writer (for audio output)
fileReader = dsp.AudioFileReader('metronome90bpm.wav');
fileInfo = audioinfo('metronome90bpm.wav');

deviceWriter = audioDeviceWriter('SampleRate',fileInfo.SampleRate, ...
    'Device',device, ...
    'BitDepth','16-bit integer', ...
    'Driver',driver);

setup(deviceWriter, zeros(fileReader.SamplesPerFrame, ...
    fileInfo.NumChannels));

Each section on its own does its job, i.e., setting up the device reader and then recording something OR setting up the device writer and playing the audio.

I have tried the multichannel recording (with microphone + loopback) when playing an audiofile in the background (so not in MATLAB) with windows media player and then starting a recording using the device reader and that works fine.

Answer 1

[Jimmy Lapierre]

Hi Charlotte,

When using the reader and writer at the same time, some settings like frame size must match.

Here, fileReader.SamplesPerFrame (1024) does not match the deviceReader SamplesPerFrame (Fs/4).

Maybe you intend to set fileReader SamplesPerFrame to Fs/4 as well.

P.S. might be easier to use audioPlayerRecorder in this case.

[Charlotte Mock]

Thanks, Jimmy! It solved the problem.

Regarding using audioPlayerRecorder, as far as I know it doesn't allow loopback...?

[Jimmy Lapierre]

You might need to specify channel mapping properties to specify which channels you need, but I'm not sure I see what the limitation is.