Parallel calculation and record matlab - matlab

Question

Hello I want to calculate realtime FFT plot of my input. With the following code i create a record and calculation. The point is that the calculation is takes to much time to get i nice plot update.
Fs = 44100; % sampling frequency in Hz
T = 1/5; % length of one interval signal in sec
t = 0:1/Fs:T-1/Fs; % time vector
nfft = 2^nextpow2(Fs); % n-point DFT
f = (0:nfft/2)'*Fs/nfft; % Frequency vector
%# prepare plots
figure
hrec(1) = subplot(211);
tplot(1) = plot(t, nan(size(t)), 'Color','b', 'Parent',hrec(1));
xlabel('Time [s]'), ylabel('Amplitude')
hrec(2) = subplot(212);
spec(2) = semilogx(f,nan(size(f)),'Color','b', 'Parent',hrec(2));
xlabel('Frequency [Hz]'), ylabel('Magnitude [dB]'), %XScale('log');
set(hrec, 'Box','on', 'XGrid','on', 'YGrid','on');set(hrec(2), 'XScale','log', 'Xlim',[20 20000]);
% specgram(sig, nfft, Fs);
% prepare audio recording
recObj = audiorecorder(Fs,16,1,0);
% Record
disp('Start Recording...')
for i=1:20
recordblocking(recObj, T);
%# get data and compute FFT
sig = getaudiodata(recObj);
fftMag = 20*log10( abs(fft(sig,nfft)) ); fftMag = fftMag(1:ceil((nfft+1)/2));
% update plots
set(tplot(1),'YData',sig);
set(spec(2), 'YData', fftMag);
title(hrec(1), num2str(i,'Interval = %d'))
drawnow % force MATLAB to flush any queued displays
end
disp('Done.')
So I create a parallel computed calculation:
% Record for 10 intervals of 1sec each
disp('Start speaking...')
for i =1:20
parfor ii=1:2
if ii == 1
recordblocking(recObj, T);
elseif ii == 2
%# get data and compute FFT
sig = getaudiodata(recObj);
fftMag = 20*log10( abs(fft(sig,nfft)) ); fftMag = fftMag(1:ceil((nfft+1)/2));recordblocking(recObj, T);
% update plots
set(tplot(1),'YData',sig);
set(spec(2), 'YData', fftMag);
title(hrec(1), num2str(i,'Interval = %d'))
drawnow %# force MATLAB to flush any queued displays
end
end
end
disp('Done.')
i get now Errors:
Error using audiorecorder/getaudiodata (line 742) Recorder is empty.
Error in parallel_function (line 466)
F(base, limit);
Error in par_plotupdate (line 25) parfor ii=1:2
How do I fix this, because i don't get this error when i don't use the parallel for loop. The last error comes i think from lack of information the first iteration. So i get an a-synchronized loop? or do i make it to difficult…
Thanks

Answer 1

Here is a implementation using asynchron recording. This way recording the audio data runs in the background and the execution of the code is only paused when no data is available. With nearly 100% of the cpu time available to process the data, it came out that processing was possible faster than real-time. This is why I did not use the parallel computing toolbox to further speed-up the process.
Fs = 44100; % sampling frequency in Hz
T = 1/5; % length of one interval signal in sec
t = 0:1/Fs:T-1/Fs; % time vector
nfft = 2^nextpow2(Fs); % n-point DFT
f = (0:nfft/2)'*Fs/nfft; % Frequency vector
%# prepare plots
figure
hrec(1) = subplot(211);
tplot(1) = plot(t, nan(size(t)), 'Color','b', 'Parent',hrec(1));
xlabel('Time [s]'), ylabel('Amplitude')
hrec(2) = subplot(212);
spec(2) = semilogx(f,nan(size(f)),'Color','b', 'Parent',hrec(2));
xlabel('Frequency [Hz]'), ylabel('Magnitude [dB]'), %XScale('log');
set(hrec, 'Box','on', 'XGrid','on', 'YGrid','on');set(hrec(2), 'XScale','log', 'Xlim',[20 20000]);
% specgram(sig, nfft, Fs);
% prepare audio recording
recObj = audiorecorder(Fs,16,1,0);
N=20;
% Record
disp('Start Recording...')
%set up one continuous recording for all N loops. Each loop processes T seconds of data
recObj.record(N*T);
%avoid empty recorder;
pause(.1);
for idx=1:N
%we are continuously recording and this iteration of the loop should process the data from startindex to endindex
startindex=1+(idx-1)*Fs*T;
endindex=(idx)*Fs*T;
audioData=recObj.getaudiodata();
%if not enough data is available, wait.
while size(audioData,1)<endindex
%amount of missing data can be caluclated, wait that time
pause((endindex-size(audioData,1))/Fs);
audioData=recObj.getaudiodata();
end
fprintf('processing index %d to %d\n',startindex,endindex);
%# get data and compute FFT
sig =audioData(startindex:endindex,:);
%If you want to use parallel computing, submit the next line to a worker (http://www.mathworks.com/help/distcomp/createtask.html). Keep in mind that workers can not update your UI, you have to get the results back from the workers.
fftMag = 20*log10( abs(fft(sig,nfft)) ); fftMag = fftMag(1:ceil((nfft+1)/2));
% update plots
set(tplot(1),'YData',sig);
set(spec(2), 'YData', fftMag);
title(hrec(1), num2str(idx,'Interval = %d'))
drawnow % force MATLAB to flush any queued displays
end
disp('Done.')