Question

So I wrote this, basically to update the spectrogram in real time.

function spec(Fs, n_bits, n_channels, update_rate)
%# Initialise default parameters if not supplied
if (~exist('Fs', 'var'))
    Fs          = 44000;
end
if (~exist('n_bits', 'var'))
    n_bits      = 16;
end
if (~exist('n_channels', 'var'))
    n_channels  = 2;
end
if (~exist('update_rate', 'var'))
    update_rate = 5;
end
plot_colors = hsv(n_channels);

%# Initialise plots, one above each other in a single figure window
figure;

%# Time Domain plot
hold on

%# Setup the audiorecorder which will acquire data off default soundcard
audio_recorder = audiorecorder(Fs, n_bits, n_channels);

set(audio_recorder, 'TimerFcn', {@audioRecorderTimerCallback, ...
                                  audio_recorder});
set(audio_recorder, 'TimerPeriod', 1/update_rate);
set(audio_recorder, 'BufferLength', 1/update_rate);

%# Start the recorder
record(audio_recorder);

end

function audioRecorderTimerCallback(obj, event, audio_recorder)

Fs           = get(obj, 'SampleRate');
num_channels = get(obj, 'NumberOfChannels');
num_bits     = get(obj, 'BitsPerSample');

try
    if (num_bits == 8)
        data_format = 'int8';
    elseif (num_bits == 16)
        data_format = 'int16';
    elseif (num_bits == 32)
        data_format = 'double';
    else
        error('Unsupported sample size of %d bits', num_bits);
    end

    %# stop the recorder, grab the data, restart the recorder. May miss some data
    stop(obj);
    data = getaudiodata(obj, data_format);

    record(obj);

    if (size(data, 2) ~= num_channels)
        error('Soundcard does not support acquisition of %d channels', ...
              length(num_channels))
    end
    data_fft    = fft(double(data));        
    specgram(data_fft,512);                
catch
    %# Stop the recorder and exit
    stop(obj)
    rethrow(lasterror)
end    
drawnow;
end

I always end up with an empty recorder.

I don’t understand why there is this problem, I did record first.

Answer 1

The problem is that the spec function doesn’t return anything, so the audio that you’ve recorded is discarded when the function exits.

Change the function signature to

function audio_recorder = spec(Fs, n_bits, n_channels, update_rate)

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One minor style point is that you can set default values for functions much more cleanly with (cue shameless self-promotion) SetDefaultValue.