| <!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> |
| |