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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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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
+<HTML>
+
+<HEAD>
+	<TITLE>
+	Secret Rabbit Code (aka libsamplerate)
+	</TITLE>
+	<META NAME="Author"      CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
+    <META NAME="Version"     CONTENT="libsamplerate-0.1.8">
+	<META NAME="Description" CONTENT="The Secret Rabbit Code Home Page">
+	<META NAME="Keywords"    CONTENT="libsamplerate sound resample audio dsp Linux">
+	<LINK REL=StyleSheet HREF="SRC.css" TYPE="text/css" MEDIA="all">
+</HEAD>
+
+<BODY TEXT="#FFFFFF" BGCOLOR="#000000" LINK="#FB1465" VLINK="#FB1465" ALINK="#FB1465">
+<!-- pepper -->
+<CENTER>
+	<IMG SRC="SRC.png" HEIGHT=100 WIDTH=760 ALT="SRC.png">
+</CENTER>
+<!-- pepper -->
+<BR>
+<!-- pepper -->
+<TABLE ALIGN="center" WIDTH="98%">
+<TR>
+<TD VALIGN="top">
+<BR>
+<DIV CLASS="nav">
+	<BR>
+	<A HREF="index.html">Home</A><BR>
+	<A HREF="license.html">License</A><BR>
+	<A HREF="history.html">History</A><BR>
+	<A HREF="download.html">Download</A><BR>
+	<A HREF="quality.html">Quality</A><BR>
+	<A HREF="api.html">API</A><BR>
+	<A HREF="bugs.html">Bug Reporting</A><BR>
+	<A HREF="win32.html">On Win32</A><BR>
+	<A HREF="faq.html">FAQ</A><BR>
+	<A HREF="lists.html">Mailing Lists</A><BR>
+	<A HREF="ChangeLog">ChangeLog</A><BR>
+<BR>
+<DIV CLASS="block">
+Author :<BR>Erik de Castro Lopo
+<!-- pepper -->
+<BR><BR>
+<!-- pepper -->
+
+</DIV>
+	<IMG SRC=
+	"/cgi-bin/Count.cgi?ft=6|frgb=55;55;55|tr=0|md=6|dd=B|st=1|sh=1|df=src_qual.dat" 
+	HEIGHT=30 WIDTH=100 ALT="counter.gif">
+</DIV>
+
+</TD>
+<!-- pepper -->
+<!-- ######################################################################## -->
+<!-- pepper -->
+<TD VALIGN="top">
+<DIV CLASS="block">
+
+<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>
+
+<!-- <A HREF="mailto:aldel@mega-nerd.com">For the spam bots</A> -->
+
+</DIV>
+</TD></TR>
+</TABLE>
+
+</BODY>
+</HTML>
+