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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)">
+ <META NAME="Version" CONTENT="libsamplerate-0.1.8">
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+Author :<BR>Erik de Castro Lopo
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+
+<H1><B>SRC Quality</B></H1>
+<CENTER><P>
+ <B>This document not yet complete.</B><BR>
+</P></CENTER>
+<P>
+ When measuring the performance of a Sample Rate Converter, there are three
+ factors to consider:
+</P>
+<UL>
+ <LI><B>Signal-to-Noise Ratio</B> - a measure of how much noise the sample
+ rate conversion process adds to the signal.
+ This is measured in decibels (dB) and the higher this value the
+ better.
+ For most sample rate converters, the SNR will vary depending on
+ the input signal and the ratio between input and output sample
+ rates.
+ The only valid comparison of SNR is between the worst case for
+ for each converter.
+ <LI><B>Bandwidth</B> - most sample rate converters attenuate high
+ frequencies as part of their operation.
+ Bandwidth can be measured by finding the frequency where the
+ attenuation is 3dB and expressing that as a percentage of the full
+ bandwidth at that sampling rate.
+ <LI><B>Speed</B> - the faster the better <B>:-)</B>.
+</UL>
+
+<P>
+ There are a number of sample rate converters available for downloading
+ but I will limit the comparison ot Secret Rabbit Code to the following:
+</P>
+<UL>
+ <LI><A HREF="http://www.mega-nerd.com/SRC/download.html">sndfile-resample</A>
+ which is a program (which uses libsamplerate) from the <B>examples/</B>
+ directory of the Secret Rabbit Code source code distribution.
+ <LI><A HREF="http://www-ccrma.stanford.edu/~jos/resample/Available_Software.html">
+ Resample</A>
+ by Julius O Smiths which seems to have been the first high quality converter
+ available as source code.
+ <LI><A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html">ResampAudio</A>
+ which is part of
+ <A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html">
+ Audio File Programs and Routines</A>
+ by Peter Kabal.
+ <LI><A HREF="http://home.sprynet.com/~cbagwell/sox.html">SoX</A> which is maintained
+ by Chris Bagwell.
+ SoX is also able to perform some low quality sample rate conversions but these
+ will not be investigated.
+ <LI><A HREF="http://shibatch.sourceforge.net/">Shibatch</A> which seems to be a
+ frequency domain sample rate converter.
+ Unfortunately, this converter does not handle arbitrary conversion ratios and
+ hence could not be properly compared to the other converters.
+ <LI><A HREF="http://sr-convert.sourceforge.net/">sr-convert</A> is another
+ converter which does not handle arbitrary conversion ratios.
+</UL>
+
+<P>
+It should be noted that the first three converters above are based on the algorithm
+by <A HREF="http://www-ccrma.stanford.edu/~jos/resample/"> Julius O. Smith</A>
+which emulates the conversion of the digital signal to an analogue one and then
+sampling the analogue signal at the new sample rate.
+</P>
+
+<!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
+
+<H3><B>Methodology</B></H3>
+<P>
+ Measuring the SNR of a converter is relatively straight forward.
+ Generate an input signal consisting of a windowed sine wave, sample rate
+ convert it and measure the signal-to-noise ratio of the output signal.
+ A typical length for the original file is 30000 samples.
+</P>
+<P>
+ The bandwidth of a sample rate converter is a little more difficult to measure.
+ Currently this is done by generating two short files containing a windowed
+ sine wave.
+ The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
+ These file are then upsampled by a factor of 2 using the converter under test.
+ If the attenutaion of the lower frquency is less than 3dB and higher frequency is
+ more than 3dB, it is then possible to iteratively increase the lower frequency
+ and decrease the upper frequency keeping the -3dB point bracketed.
+ When the distance between the upper and lower frequency is sufficiently small,
+ it is possible to obtain a very accurate estimate of the -3dB frequency.
+</P>
+<P>
+ The speed of a sample rate converter is easy to measure; simply perform a
+ conversion on a large file or a number of smaller files and time the conversion
+ process.
+</P>
+
+<P>
+ The above measurement techniques are built into a test program which is delivered
+ with the Secret Rabbit Code source code distibution.
+ This program is able to test the first four of the above converters.
+</P>
+<!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
+
+<H3><B>SoX</B></H3>
+<P>
+ SoX provides three methods of resampling; a linear interpolator, a polyphase
+ resampler and the Julius O. Smith simulated analogue filter method.
+</P>
+
+<H3><B>Shibatch</B></H3>
+<P>
+ Shibach
+</P>
+
+<P>
+ <B>More Coming Soon.</B>
+</P>
+
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projects / audio-libsamplerate.git / blobdiff