How to use DSP to silence commercials on the radio broadcast.

1. Radio ad blocker
Or how I learned to stop worrying and use DSP cargo
cult to mute the broadcast commercials

2. Analysing radio broadcast
Starting
jingle
Ad 1 Ad 2 Ad 3
Ending
jingle
Volume controlSample analyser
Broadcast

3. Research process: finding a
sample in the sound stream

4. Redefining the problem
• Let’s make it a simpler problem, by taking away the part
that we can figure out ourselves
• We can split the incoming stream into sound samples of
some length (moving window)
• So let’s try to find a way to compare two sound samples

5. Waveforms
Recorded jingle
Broadcast sample

6. Frequency domain
FFT(Broadcast sample)
FFT(Recorded jingle)

7. Cross correlation
Matching samples
Not matching samples

8. Prototyping with Octave
pkg load signal;
stream = audioread('stream.wav');
jingle1 = audioread('commercial-start.wav');
jingle2 = audioread('commercial-end.wav');
function retval = get_sample(data, from, to)
retval = data(from*32000 : to*32000-1);
endfunction
sample = get_sample(stream, 134, 135.5);
plot(sample);
plot(jingle2);
plot(real(fft(sample)));
plot(real(fft(jingle2)));
plot(xcorr(sample, jingle2));

9. Java implementation
Analyser
Broadcast
PCM
stdout
jingle 1
stdin
stderrrtl_fm -M wbfm
-f 89.50M
-g 0
-r 48000
ffmpeg -i
http://41.dktr.pl:8000/trojka.ogg
-f s16le
-acodec pcm_s16le
-
play -r 48000
-b 16
-c 2
-e signed
-t raw -

10. Reading PCM data
byte[] rawBuffer = new byte[4096];
final int SAMPLE_SIZE = 4; // 2 bytes * 2 channels
ByteBuffer sample = ByteBuffer.allocate(SAMPLE_SIZE);
sample.order(ByteOrder.LITTLE_ENDIAN);
while (true) {
int length = input.read(rawBuffer);
// consume the rawBuffer
for (int i = 0; i < length; i += SAMPLE_SIZE) {
sample.clear();
sample.put(rawBuffer, i, SAMPLE_SIZE);
sample.rewind();
while (sample.hasRemaining()) {
short left = sample.getShort();
short right = sample.getShort();
// ...
}
}
}

11. Analysing stream with
window
Writeposition

12. Round-robin buffer

13. Porting xcorr from Octave
N = max(length(X),length(Y));
maxlag = N - 1;
M = 2^nextpow2(N + maxlag);
pre = fft( postpad( prepad( X(:), length(X)+maxlag ), M) );
post = fft( postpad( Y(:), M ) );
cor = ifft( pre .* conj(post) );
R = cor(1:2*maxlag+1);
R = real(R);
public class Waveform {
private final float[] buffer;
private final int length;
// ...
}
Waveform x = currentWindow;
Waveform y = jingle
.setPadding(jingle.getSize() + windowSize - 1, next2Pow)
.doFft();
x.doFft();
x.doConjAndMultiply(y);
x.doIfft();
float[] result = y.getMaxReal(2 * windowSize + 1);

14. FFT usage in Java
// https://github.com/wendykierp/JTransforms
import org.jtransforms.fft.FloatFFT_1D;
short[] shortBuffer; // (-32678,32767)
float[] buffer = shortBuffer / 32768;
// fft()
FloatFFT_1D fft = new FloatFFT_1D(length);
fft.realForwardFull(buffer);
// ifft()
fft.complexInverse(buffer, true);
buffer[i]; // i%2 == 0; real part
buffer[i+1]; // imaginary part

15. Toolbox
• sox package for uncompressed files (wav, raw, etc.)
• sox - conversion
• play - playback
• ffmpeg - decoding compressed streams/files (ogg, mp3)
• Octave
• Audacity