libsamplerate/doc/quality.html

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   6         Secret Rabbit Code (aka libsamplerate)
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  42 Author :<BR>Erik de Castro Lopo
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  60 <H1><B>SRC Quality</B></H1>
  61 <CENTER><P>
  62         <B>This document not yet complete.</B><BR>
  63 </P></CENTER>
  64 <P>
  65         When measuring the performance of a Sample Rate Converter, there are three
  66         factors to consider:
  67 </P>
  68 <UL>
  69         <LI><B>Signal-to-Noise Ratio</B> - a measure of how much noise the sample
  70                         rate conversion process adds to the signal.
  71                         This is measured in decibels (dB) and the higher this value the
  72                         better.
  73                         For most sample rate converters, the SNR will vary depending on
  74                         the input signal and the ratio between input and output sample
  75                         rates.
  76                         The only valid comparison of SNR is between the worst case for
  77                         for each converter.
  78         <LI><B>Bandwidth</B> - most sample rate converters attenuate high
  79                         frequencies as part of their operation.
  80                         Bandwidth can be measured by finding the frequency where the
  81                         attenuation is 3dB and expressing that as a percentage of the full
  82                         bandwidth at that sampling rate.
  83         <LI><B>Speed</B> - the faster the better <B>:-)</B>.
  84 </UL>
  85
  86 <P>
  87         There are a number of sample rate converters available for downloading
  88         but I will limit the comparison ot Secret Rabbit Code to the following:
  89 </P>
  90 <UL>
  91         <LI><A HREF="http://www.mega-nerd.com/SRC/download.html">sndfile-resample</A>
  92                 which is a program (which uses libsamplerate) from the <B>examples/</B>
  93                 directory of the Secret Rabbit Code source code distribution.
  94         <LI><A HREF="http://www-ccrma.stanford.edu/~jos/resample/Available_Software.html">
  95                         Resample</A>
  96                 by Julius O Smiths which seems to have been the first high quality converter
  97                 available as source code.
  98         <LI><A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html">ResampAudio</A>
  99                 which is part of
 100                 <A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html">
 101                         Audio File Programs and Routines</A>
 102                 by Peter Kabal.
 103         <LI><A HREF="http://home.sprynet.com/~cbagwell/sox.html">SoX</A> which is maintained
 104                 by Chris Bagwell.
 105                 SoX is also able to perform some low quality sample rate conversions but these
 106                 will not be investigated.
 107         <LI><A HREF="http://shibatch.sourceforge.net/">Shibatch</A> which seems to be a
 108                 frequency domain sample rate converter.
 109                 Unfortunately, this converter does not handle arbitrary conversion ratios and
 110                 hence could not be properly compared to the other converters.
 111         <LI><A HREF="http://sr-convert.sourceforge.net/">sr-convert</A> is another
 112                 converter which does not handle arbitrary conversion ratios.
 113 </UL>
 114
 115 <P>
 116 It should be noted that the first three converters above are based on the algorithm
 117 by <A HREF="http://www-ccrma.stanford.edu/~jos/resample/"> Julius O. Smith</A>
 118 which emulates the conversion of the digital signal to an analogue one and then
 119 sampling the analogue signal at the new sample rate.
 120 </P>
 121
 122 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
 123
 124 <H3><B>Methodology</B></H3>
 125 <P>
 126         Measuring the SNR of a converter is relatively straight forward.
 127         Generate an input signal consisting of a windowed sine wave, sample rate
 128         convert it and measure the signal-to-noise ratio of the output signal.
 129         A typical length for the original file is 30000 samples.
 130 </P>
 131 <P>
 132         The bandwidth of a sample rate converter is a little more difficult to measure.
 133         Currently this is done by generating two short files containing a windowed
 134         sine wave.
 135         The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
 136         These file are then upsampled by a factor of 2 using the converter under test.
 137         If the attenutaion of the lower frquency is less than 3dB and higher frequency is
 138         more than 3dB, it is then possible to iteratively increase the lower frequency
 139         and decrease the upper frequency keeping the -3dB point bracketed.
 140         When the distance between the upper and lower frequency is sufficiently small,
 141         it is possible to obtain a very accurate estimate of the -3dB frequency.
 142 </P>
 143 <P>
 144         The speed of a sample rate converter is easy to measure; simply perform a
 145         conversion on a large file or a number of smaller files and time the conversion
 146         process.
 147 </P>
 148
 149 <P>
 150         The above measurement techniques are built into a test program which is delivered
 151         with the Secret Rabbit Code source code distibution.
 152         This program is able to test the first four of the above converters.
 153 </P>
 154 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
 155
 156 <H3><B>SoX</B></H3>
 157 <P>
 158         SoX provides three methods of resampling; a linear interpolator, a polyphase
 159         resampler and the Julius O. Smith simulated analogue filter method.
 160 </P>
 161
 162 <H3><B>Shibatch</B></H3>
 163 <P>
 164         Shibach
 165 </P>
 166
 167 <P>
 168         <B>More Coming Soon.</B>
 169 </P>
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