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POWhat kind of output should I see from getFft?
 

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  1. POWhat kind of output should I see from getFft?
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    <p>Alright, so I am working on creating an Android audio visualization app. The problem is, what I get form the method getFft() doesn't jive with what google says it should produce. I traced the source code all the way back to C++, but I am not familiar enough with C++ or FFT to actually understand what is happening.</p> <p>I will try and include everything needed here:</p> <p><a href="http://www.google.com/codesearch/p?hl=en#uX1GffpyOZk/media/java/android/media/audiofx/Visualizer.java" rel="nofollow"><strong>(Java) Visualizer.getFft(byte[] fft)</strong></a></p> <pre><code> /** * Returns a frequency capture of currently playing audio content. The capture is a 8-bit * magnitude FFT. Note that the size of the FFT is half of the specified capture size but both * sides of the spectrum are returned yielding in a number of bytes equal to the capture size. * {@see #getCaptureSize()}. * &lt;p&gt;This method must be called when the Visualizer is enabled. * @param fft array of bytes where the FFT should be returned * @return {@link #SUCCESS} in case of success, * {@link #ERROR_NO_MEMORY}, {@link #ERROR_INVALID_OPERATION} or {@link #ERROR_DEAD_OBJECT} * in case of failure. * @throws IllegalStateException */ public int getFft(byte[] fft) throws IllegalStateException { synchronized (mStateLock) { if (mState != STATE_ENABLED) { throw(new IllegalStateException("getFft() called in wrong state: "+mState)); } return native_getFft(fft); } } </code></pre> <p><a href="http://www.google.com/codesearch/p?hl=en#uX1GffpyOZk/media/libmedia/Visualizer.cpp" rel="nofollow"><strong>(C++) Visualizer.getFft(uint8_t *fft)</strong></a></p> <pre><code>status_t Visualizer::getFft(uint8_t *fft) { if (fft == NULL) { return BAD_VALUE; } if (mCaptureSize == 0) { return NO_INIT; } status_t status = NO_ERROR; if (mEnabled) { uint8_t buf[mCaptureSize]; status = getWaveForm(buf); if (status == NO_ERROR) { status = doFft(fft, buf); } } else { memset(fft, 0, mCaptureSize); } return status; } </code></pre> <p><a href="http://www.google.com/codesearch/p?hl=en#uX1GffpyOZk/media/libmedia/Visualizer.cpp" rel="nofollow"><strong>(C++) Visualizer.doFft(uint8_t *fft, uint8_t *waveform)</strong></a></p> <pre><code>status_t Visualizer::doFft(uint8_t *fft, uint8_t *waveform) { int32_t workspace[mCaptureSize &gt;&gt; 1]; int32_t nonzero = 0; for (uint32_t i = 0; i &lt; mCaptureSize; i += 2) { workspace[i &gt;&gt; 1] = (waveform[i] ^ 0x80) &lt;&lt; 23; workspace[i &gt;&gt; 1] |= (waveform[i + 1] ^ 0x80) &lt;&lt; 7; nonzero |= workspace[i &gt;&gt; 1]; } if (nonzero) { fixed_fft_real(mCaptureSize &gt;&gt; 1, workspace); } for (uint32_t i = 0; i &lt; mCaptureSize; i += 2) { fft[i] = workspace[i &gt;&gt; 1] &gt;&gt; 23; fft[i + 1] = workspace[i &gt;&gt; 1] &gt;&gt; 7; } return NO_ERROR; } </code></pre> <p><a href="http://www.google.com/codesearch/p?hl=en#uX1GffpyOZk/media/libmedia/fixedfft.cpp" rel="nofollow"><strong>(C++) fixedfft.fixed_fft_real(int n, int32_t *v)</strong></a></p> <pre><code>void fixed_fft_real(int n, int32_t *v) { int scale = LOG_FFT_SIZE, m = n &gt;&gt; 1, i; fixed_fft(n, v); for (i = 1; i &lt;= n; i &lt;&lt;= 1, --scale); v[0] = mult(~v[0], 0x80008000); v[m] = half(v[m]); for (i = 1; i &lt; n &gt;&gt; 1; ++i) { int32_t x = half(v[i]); int32_t z = half(v[n - i]); int32_t y = z - (x ^ 0xFFFF); x = half(x + (z ^ 0xFFFF)); y = mult(y, twiddle[i &lt;&lt; scale]); v[i] = x - y; v[n - i] = (x + y) ^ 0xFFFF; } } </code></pre> <p><a href="http://www.google.com/codesearch/p?hl=en#uX1GffpyOZk/media/libmedia/fixedfft.cpp" rel="nofollow"><strong>(C++) fixedfft.fixed_fft(int n, int32_t *v)</strong></a></p> <pre><code>void fixed_fft(int n, int32_t *v) { int scale = LOG_FFT_SIZE, i, p, r; for (r = 0, i = 1; i &lt; n; ++i) { for (p = n; !(p &amp; r); p &gt;&gt;= 1, r ^= p); if (i &lt; r) { int32_t t = v[i]; v[i] = v[r]; v[r] = t; } } for (p = 1; p &lt; n; p &lt;&lt;= 1) { --scale; for (i = 0; i &lt; n; i += p &lt;&lt; 1) { int32_t x = half(v[i]); int32_t y = half(v[i + p]); v[i] = x + y; v[i + p] = x - y; } for (r = 1; r &lt; p; ++r) { int32_t w = MAX_FFT_SIZE / 4 - (r &lt;&lt; scale); i = w &gt;&gt; 31; w = twiddle[(w ^ i) - i] ^ (i &lt;&lt; 16); for (i = r; i &lt; n; i += p &lt;&lt; 1) { int32_t x = half(v[i]); int32_t y = mult(w, v[i + p]); v[i] = x - y; v[i + p] = x + y; } } } } </code></pre> <p>If you made it through all that, you are awesome! So my issue, is when I call the java method getFft() I end up with negative values, which shouldn't exist if the returned array is meant to represent magnitude. So my question is, what do I need to do to make the array represent magnitude?</p> <p><strong>EDIT:</strong> It appears my data may actually be the Fourier coefficients. I was poking around the web and found <a href="http://www.mathcs.org/java/programs/FFT/" rel="nofollow">this</a>. The applet "Start Function FFT" displays a graphed representation of coefficients and it is a spitting image of what happens when I graph the data from getFft(). So new question: <em>Is this what my data is? and if so, how do I go from the coefficients to a spectral analysis of it?</em></p>
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    1. CODidn't you already get an answer to this over here? http://stackoverflow.com/questions/4720512/android-2-3-visualizer-trouble-understanding-getfft
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    2. COThis is a different question. Original was this: First of all, what does "both sides of the spectrum" mean? How does this output differ from a standard FFT? This time I have relevant source code and a much more specific question.
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    3. CO@Evan, I am facing the same issue. When i plotted fft data on canvas it looks wired. Have you got any solution for this? Here is my question which i posted on SO. http://stackoverflow.com/questions/7024187/fft-data-displayed-on-line-graph-not-showing-smoothly
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      1. POWhat kind of output should I see from getFft?
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