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Current libsamplerate source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:24:45 +0100
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cannam@126 1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
cannam@126 2 <HTML>
cannam@126 3
cannam@126 4 <HEAD>
cannam@126 5 <TITLE>
cannam@126 6 Secret Rabbit Code (aka libsamplerate)
cannam@126 7 </TITLE>
cannam@126 8 <META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
cannam@126 9 <META NAME="Version" CONTENT="libsamplerate-0.1.8">
cannam@126 10 <META NAME="Description" CONTENT="The Secret Rabbit Code Home Page">
cannam@126 11 <META NAME="Keywords" CONTENT="libsamplerate sound resample audio dsp Linux">
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cannam@126 28 <BR>
cannam@126 29 <A HREF="index.html">Home</A><BR>
cannam@126 30 <A HREF="license.html">License</A><BR>
cannam@126 31 <A HREF="history.html">History</A><BR>
cannam@126 32 <A HREF="download.html">Download</A><BR>
cannam@126 33 <A HREF="quality.html">Quality</A><BR>
cannam@126 34 <A HREF="api.html">API</A><BR>
cannam@126 35 <A HREF="bugs.html">Bug Reporting</A><BR>
cannam@126 36 <A HREF="win32.html">On Win32</A><BR>
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cannam@126 38 <A HREF="lists.html">Mailing Lists</A><BR>
cannam@126 39 <A HREF="ChangeLog">ChangeLog</A><BR>
cannam@126 40 <BR>
cannam@126 41 <DIV CLASS="block">
cannam@126 42 Author :<BR>Erik de Castro Lopo
cannam@126 43 <!-- pepper -->
cannam@126 44 <BR><BR>
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cannam@126 46
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cannam@126 55 <!-- ######################################################################## -->
cannam@126 56 <!-- pepper -->
cannam@126 57 <TD VALIGN="top">
cannam@126 58 <DIV CLASS="block">
cannam@126 59
cannam@126 60 <H1><B>SRC Quality</B></H1>
cannam@126 61 <CENTER><P>
cannam@126 62 <B>This document not yet complete.</B><BR>
cannam@126 63 </P></CENTER>
cannam@126 64 <P>
cannam@126 65 When measuring the performance of a Sample Rate Converter, there are three
cannam@126 66 factors to consider:
cannam@126 67 </P>
cannam@126 68 <UL>
cannam@126 69 <LI><B>Signal-to-Noise Ratio</B> - a measure of how much noise the sample
cannam@126 70 rate conversion process adds to the signal.
cannam@126 71 This is measured in decibels (dB) and the higher this value the
cannam@126 72 better.
cannam@126 73 For most sample rate converters, the SNR will vary depending on
cannam@126 74 the input signal and the ratio between input and output sample
cannam@126 75 rates.
cannam@126 76 The only valid comparison of SNR is between the worst case for
cannam@126 77 for each converter.
cannam@126 78 <LI><B>Bandwidth</B> - most sample rate converters attenuate high
cannam@126 79 frequencies as part of their operation.
cannam@126 80 Bandwidth can be measured by finding the frequency where the
cannam@126 81 attenuation is 3dB and expressing that as a percentage of the full
cannam@126 82 bandwidth at that sampling rate.
cannam@126 83 <LI><B>Speed</B> - the faster the better <B>:-)</B>.
cannam@126 84 </UL>
cannam@126 85
cannam@126 86 <P>
cannam@126 87 There are a number of sample rate converters available for downloading
cannam@126 88 but I will limit the comparison ot Secret Rabbit Code to the following:
cannam@126 89 </P>
cannam@126 90 <UL>
cannam@126 91 <LI><A HREF="http://www.mega-nerd.com/SRC/download.html">sndfile-resample</A>
cannam@126 92 which is a program (which uses libsamplerate) from the <B>examples/</B>
cannam@126 93 directory of the Secret Rabbit Code source code distribution.
cannam@126 94 <LI><A HREF="http://www-ccrma.stanford.edu/~jos/resample/Available_Software.html">
cannam@126 95 Resample</A>
cannam@126 96 by Julius O Smiths which seems to have been the first high quality converter
cannam@126 97 available as source code.
cannam@126 98 <LI><A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html">ResampAudio</A>
cannam@126 99 which is part of
cannam@126 100 <A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html">
cannam@126 101 Audio File Programs and Routines</A>
cannam@126 102 by Peter Kabal.
cannam@126 103 <LI><A HREF="http://home.sprynet.com/~cbagwell/sox.html">SoX</A> which is maintained
cannam@126 104 by Chris Bagwell.
cannam@126 105 SoX is also able to perform some low quality sample rate conversions but these
cannam@126 106 will not be investigated.
cannam@126 107 <LI><A HREF="http://shibatch.sourceforge.net/">Shibatch</A> which seems to be a
cannam@126 108 frequency domain sample rate converter.
cannam@126 109 Unfortunately, this converter does not handle arbitrary conversion ratios and
cannam@126 110 hence could not be properly compared to the other converters.
cannam@126 111 <LI><A HREF="http://sr-convert.sourceforge.net/">sr-convert</A> is another
cannam@126 112 converter which does not handle arbitrary conversion ratios.
cannam@126 113 </UL>
cannam@126 114
cannam@126 115 <P>
cannam@126 116 It should be noted that the first three converters above are based on the algorithm
cannam@126 117 by <A HREF="http://www-ccrma.stanford.edu/~jos/resample/"> Julius O. Smith</A>
cannam@126 118 which emulates the conversion of the digital signal to an analogue one and then
cannam@126 119 sampling the analogue signal at the new sample rate.
cannam@126 120 </P>
cannam@126 121
cannam@126 122 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
cannam@126 123
cannam@126 124 <H3><B>Methodology</B></H3>
cannam@126 125 <P>
cannam@126 126 Measuring the SNR of a converter is relatively straight forward.
cannam@126 127 Generate an input signal consisting of a windowed sine wave, sample rate
cannam@126 128 convert it and measure the signal-to-noise ratio of the output signal.
cannam@126 129 A typical length for the original file is 30000 samples.
cannam@126 130 </P>
cannam@126 131 <P>
cannam@126 132 The bandwidth of a sample rate converter is a little more difficult to measure.
cannam@126 133 Currently this is done by generating two short files containing a windowed
cannam@126 134 sine wave.
cannam@126 135 The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
cannam@126 136 These file are then upsampled by a factor of 2 using the converter under test.
cannam@126 137 If the attenutaion of the lower frquency is less than 3dB and higher frequency is
cannam@126 138 more than 3dB, it is then possible to iteratively increase the lower frequency
cannam@126 139 and decrease the upper frequency keeping the -3dB point bracketed.
cannam@126 140 When the distance between the upper and lower frequency is sufficiently small,
cannam@126 141 it is possible to obtain a very accurate estimate of the -3dB frequency.
cannam@126 142 </P>
cannam@126 143 <P>
cannam@126 144 The speed of a sample rate converter is easy to measure; simply perform a
cannam@126 145 conversion on a large file or a number of smaller files and time the conversion
cannam@126 146 process.
cannam@126 147 </P>
cannam@126 148
cannam@126 149 <P>
cannam@126 150 The above measurement techniques are built into a test program which is delivered
cannam@126 151 with the Secret Rabbit Code source code distibution.
cannam@126 152 This program is able to test the first four of the above converters.
cannam@126 153 </P>
cannam@126 154 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
cannam@126 155
cannam@126 156 <H3><B>SoX</B></H3>
cannam@126 157 <P>
cannam@126 158 SoX provides three methods of resampling; a linear interpolator, a polyphase
cannam@126 159 resampler and the Julius O. Smith simulated analogue filter method.
cannam@126 160 </P>
cannam@126 161
cannam@126 162 <H3><B>Shibatch</B></H3>
cannam@126 163 <P>
cannam@126 164 Shibach
cannam@126 165 </P>
cannam@126 166
cannam@126 167 <P>
cannam@126 168 <B>More Coming Soon.</B>
cannam@126 169 </P>
cannam@126 170
cannam@126 171 <!-- <A HREF="mailto:aldel@mega-nerd.com">For the spam bots</A> -->
cannam@126 172
cannam@126 173 </DIV>
cannam@126 174 </TD></TR>
cannam@126 175 </TABLE>
cannam@126 176
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cannam@126 179