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Current libsamplerate source
author Chris Cannam
date Tue, 18 Oct 2016 13:24:45 +0100
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Chris@41 1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
Chris@41 2 <HTML>
Chris@41 3
Chris@41 4 <HEAD>
Chris@41 5 <TITLE>
Chris@41 6 Secret Rabbit Code (aka libsamplerate)
Chris@41 7 </TITLE>
Chris@41 8 <META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
Chris@41 9 <META NAME="Version" CONTENT="libsamplerate-0.1.8">
Chris@41 10 <META NAME="Description" CONTENT="The Secret Rabbit Code Home Page">
Chris@41 11 <META NAME="Keywords" CONTENT="libsamplerate sound resample audio dsp Linux">
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Chris@41 27 <DIV CLASS="nav">
Chris@41 28 <BR>
Chris@41 29 <A HREF="index.html">Home</A><BR>
Chris@41 30 <A HREF="license.html">License</A><BR>
Chris@41 31 <A HREF="history.html">History</A><BR>
Chris@41 32 <A HREF="download.html">Download</A><BR>
Chris@41 33 <A HREF="quality.html">Quality</A><BR>
Chris@41 34 <A HREF="api.html">API</A><BR>
Chris@41 35 <A HREF="bugs.html">Bug Reporting</A><BR>
Chris@41 36 <A HREF="win32.html">On Win32</A><BR>
Chris@41 37 <A HREF="faq.html">FAQ</A><BR>
Chris@41 38 <A HREF="lists.html">Mailing Lists</A><BR>
Chris@41 39 <A HREF="ChangeLog">ChangeLog</A><BR>
Chris@41 40 <BR>
Chris@41 41 <DIV CLASS="block">
Chris@41 42 Author :<BR>Erik de Castro Lopo
Chris@41 43 <!-- pepper -->
Chris@41 44 <BR><BR>
Chris@41 45 <!-- pepper -->
Chris@41 46
Chris@41 47 </DIV>
Chris@41 48 <IMG SRC=
Chris@41 49 "/cgi-bin/Count.cgi?ft=6|frgb=55;55;55|tr=0|md=6|dd=B|st=1|sh=1|df=src_qual.dat"
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Chris@41 55 <!-- ######################################################################## -->
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Chris@41 57 <TD VALIGN="top">
Chris@41 58 <DIV CLASS="block">
Chris@41 59
Chris@41 60 <H1><B>SRC Quality</B></H1>
Chris@41 61 <CENTER><P>
Chris@41 62 <B>This document not yet complete.</B><BR>
Chris@41 63 </P></CENTER>
Chris@41 64 <P>
Chris@41 65 When measuring the performance of a Sample Rate Converter, there are three
Chris@41 66 factors to consider:
Chris@41 67 </P>
Chris@41 68 <UL>
Chris@41 69 <LI><B>Signal-to-Noise Ratio</B> - a measure of how much noise the sample
Chris@41 70 rate conversion process adds to the signal.
Chris@41 71 This is measured in decibels (dB) and the higher this value the
Chris@41 72 better.
Chris@41 73 For most sample rate converters, the SNR will vary depending on
Chris@41 74 the input signal and the ratio between input and output sample
Chris@41 75 rates.
Chris@41 76 The only valid comparison of SNR is between the worst case for
Chris@41 77 for each converter.
Chris@41 78 <LI><B>Bandwidth</B> - most sample rate converters attenuate high
Chris@41 79 frequencies as part of their operation.
Chris@41 80 Bandwidth can be measured by finding the frequency where the
Chris@41 81 attenuation is 3dB and expressing that as a percentage of the full
Chris@41 82 bandwidth at that sampling rate.
Chris@41 83 <LI><B>Speed</B> - the faster the better <B>:-)</B>.
Chris@41 84 </UL>
Chris@41 85
Chris@41 86 <P>
Chris@41 87 There are a number of sample rate converters available for downloading
Chris@41 88 but I will limit the comparison ot Secret Rabbit Code to the following:
Chris@41 89 </P>
Chris@41 90 <UL>
Chris@41 91 <LI><A HREF="http://www.mega-nerd.com/SRC/download.html">sndfile-resample</A>
Chris@41 92 which is a program (which uses libsamplerate) from the <B>examples/</B>
Chris@41 93 directory of the Secret Rabbit Code source code distribution.
Chris@41 94 <LI><A HREF="http://www-ccrma.stanford.edu/~jos/resample/Available_Software.html">
Chris@41 95 Resample</A>
Chris@41 96 by Julius O Smiths which seems to have been the first high quality converter
Chris@41 97 available as source code.
Chris@41 98 <LI><A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html">ResampAudio</A>
Chris@41 99 which is part of
Chris@41 100 <A HREF="http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html">
Chris@41 101 Audio File Programs and Routines</A>
Chris@41 102 by Peter Kabal.
Chris@41 103 <LI><A HREF="http://home.sprynet.com/~cbagwell/sox.html">SoX</A> which is maintained
Chris@41 104 by Chris Bagwell.
Chris@41 105 SoX is also able to perform some low quality sample rate conversions but these
Chris@41 106 will not be investigated.
Chris@41 107 <LI><A HREF="http://shibatch.sourceforge.net/">Shibatch</A> which seems to be a
Chris@41 108 frequency domain sample rate converter.
Chris@41 109 Unfortunately, this converter does not handle arbitrary conversion ratios and
Chris@41 110 hence could not be properly compared to the other converters.
Chris@41 111 <LI><A HREF="http://sr-convert.sourceforge.net/">sr-convert</A> is another
Chris@41 112 converter which does not handle arbitrary conversion ratios.
Chris@41 113 </UL>
Chris@41 114
Chris@41 115 <P>
Chris@41 116 It should be noted that the first three converters above are based on the algorithm
Chris@41 117 by <A HREF="http://www-ccrma.stanford.edu/~jos/resample/"> Julius O. Smith</A>
Chris@41 118 which emulates the conversion of the digital signal to an analogue one and then
Chris@41 119 sampling the analogue signal at the new sample rate.
Chris@41 120 </P>
Chris@41 121
Chris@41 122 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
Chris@41 123
Chris@41 124 <H3><B>Methodology</B></H3>
Chris@41 125 <P>
Chris@41 126 Measuring the SNR of a converter is relatively straight forward.
Chris@41 127 Generate an input signal consisting of a windowed sine wave, sample rate
Chris@41 128 convert it and measure the signal-to-noise ratio of the output signal.
Chris@41 129 A typical length for the original file is 30000 samples.
Chris@41 130 </P>
Chris@41 131 <P>
Chris@41 132 The bandwidth of a sample rate converter is a little more difficult to measure.
Chris@41 133 Currently this is done by generating two short files containing a windowed
Chris@41 134 sine wave.
Chris@41 135 The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
Chris@41 136 These file are then upsampled by a factor of 2 using the converter under test.
Chris@41 137 If the attenutaion of the lower frquency is less than 3dB and higher frequency is
Chris@41 138 more than 3dB, it is then possible to iteratively increase the lower frequency
Chris@41 139 and decrease the upper frequency keeping the -3dB point bracketed.
Chris@41 140 When the distance between the upper and lower frequency is sufficiently small,
Chris@41 141 it is possible to obtain a very accurate estimate of the -3dB frequency.
Chris@41 142 </P>
Chris@41 143 <P>
Chris@41 144 The speed of a sample rate converter is easy to measure; simply perform a
Chris@41 145 conversion on a large file or a number of smaller files and time the conversion
Chris@41 146 process.
Chris@41 147 </P>
Chris@41 148
Chris@41 149 <P>
Chris@41 150 The above measurement techniques are built into a test program which is delivered
Chris@41 151 with the Secret Rabbit Code source code distibution.
Chris@41 152 This program is able to test the first four of the above converters.
Chris@41 153 </P>
Chris@41 154 <!--+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-->
Chris@41 155
Chris@41 156 <H3><B>SoX</B></H3>
Chris@41 157 <P>
Chris@41 158 SoX provides three methods of resampling; a linear interpolator, a polyphase
Chris@41 159 resampler and the Julius O. Smith simulated analogue filter method.
Chris@41 160 </P>
Chris@41 161
Chris@41 162 <H3><B>Shibatch</B></H3>
Chris@41 163 <P>
Chris@41 164 Shibach
Chris@41 165 </P>
Chris@41 166
Chris@41 167 <P>
Chris@41 168 <B>More Coming Soon.</B>
Chris@41 169 </P>
Chris@41 170
Chris@41 171 <!-- <A HREF="mailto:aldel@mega-nerd.com">For the spam bots</A> -->
Chris@41 172
Chris@41 173 </DIV>
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