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Secret Rabbit Code (aka libsamplerate)
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<META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
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<META NAME="Version" CONTENT="libsamplerate-0.1.0">
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<A HREF="index.html">Home</A><BR>
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<A HREF="license.html">License</A><BR>
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<A HREF="history.html">History</A><BR>
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<A HREF="download.html">Download</A><BR>
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<A HREF="quality.html">Quality</A><BR>
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<A HREF="api.html">API</A><BR>
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<A HREF="bugs.html">Bug Reporting</A><BR>
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<A HREF="win32.html">On Win32</A><BR>
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<A HREF="faq.html">FAQ</A><BR>
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<A HREF="ChangeLog">ChangeLog</A><BR>
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Author :<BR>Erik de Castro Lopo
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<H1><B>SRC Quality</B></H1>
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<B>This document not yet complete.</B><BR>
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When measuring the performance of a Sample Rate Converter, there are three
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<LI><B>Signal-to-Noise Ratio</B> - a measure of how much noise the sample
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rate conversion process adds to the signal.
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This is measured in decibels (dB) and the higher this value the
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For most sample rate converters, the SNR will vary depending on
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the input signal and the ratio between input and output sample
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The only valid comparison of SNR is between the worst case for
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<LI><B>Bandwidth</B> - most sample rate converters attenuate high
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frequencies as part of their operation.
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Bandwidth can be measured by finding the frequency where the
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attenuation is 3dB and expressing that as a percentage of the full
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bandwidth at that sampling rate.
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<LI><B>Speed</B> - the faster the better <B>:-)</B>.
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There are a number of sample rate converters available for downloading
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but I will limit the comparison ot Secret Rabbit Code to the following:
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<LI><A HREF="http://www.mega-nerd.com/SRC/download.html">sndfile-resample</A>
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which is a program (which uses libsamplerate) from the <B>examples/</B>
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directory of the Secret Rabbit Code source code distribution.
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<LI><A HREF="http://www-ccrma.stanford.edu/~jos/resample/Available_Software.html">
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by Julius O Smiths which seems to have been the first high quality converter
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available as source code.
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<LI><A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html">ResampAudio</A>
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<A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html">
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Audio File Programs and Routines</A>
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<LI><A HREF="http://home.sprynet.com/~cbagwell/sox.html">SoX</A> which is maintained
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SoX is also able to perfrom some low quality sample rate conversions but these
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will not be investigated.
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<LI><A HREF="http://shibatch.sourceforge.net/">Shibatch</A> which seems to be a
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frequency domain sample rate converter.
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Unfortunately, this converter does not handle arbitrary conversion ratios and
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hence could not be properly compared to the other converters.
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<LI><A HREF="http://sr-convert.sourceforge.net/">sr-convert</A> is another
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converter which does not handle arbitrary conversion ratios.
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It should be noted that the first three converters above are based on the algorithm
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by <A HREF="http://www-ccrma.stanford.edu/~jos/resample/"> Julius O. Smith</A>
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which emulates the conversion of the digital signal to an analogue one and then
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sampling the analogue signal at the new sample rate.
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<H3><B>Methodology</B></H3>
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Measuring the SNR of a converter is relatively straight forward.
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Generate an input signal consisting of a windowed sine wave, sample rate
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convert it and measure the signal-to-noise ratio of the output signal.
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A typical length for the original file is 30000 samples.
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The bandwidth of a sample rate converter is a little more difficult to measure.
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Currently this is done by generating two short files containing a windowed
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The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
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These file are then upsampled by a factor of 2 using the converter under test.
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If the attenutaion of the lower frquency is less than 3dB and higher frequency is
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more than 3dB, it is then possible to iteratively increase the lower frequency
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and decrease the upper frequency keeping the -3dB point bracketed.
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When the distance between the upper and lower frequency is sufficiently small,
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it is possible to obtain a very accurate estimate of the -3dB frequency.
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The speed of a sample rate converter is easy to measure; simply perform a
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conversion on a large file or a number of smaller files and time the conversion
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The above measurement techniques are built into a test program which is delivered
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with the Secret Rabbit Code source code distibution.
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This program is able to test the first four of the above converters.
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SoX provides three methods of resampling; a linear interpolator, a polyphase
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resampler and the Julius O. Smith simulated analogue filter method.
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<H3><B>Shibatch</B></H3>
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<B>More Coming Soon.</B>
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