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SRC Quality

+ This document not yet complete.
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+ When measuring the performance of a Sample Rate Converter, there are three + factors to consider: +

Signal-to-Noise Ratio - 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. +
Bandwidth - 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. +
Speed - the faster the better :-). +

+ +

+ There are a number of sample rate converters available for downloading + but I will limit the comparison ot Secret Rabbit Code to the following: +

sndfile-resample + which is a program (which uses libsamplerate) from the examples/ + directory of the Secret Rabbit Code source code distribution. +
+ Resample + by Julius O Smiths which seems to have been the first high quality converter + available as source code. +
ResampAudio + which is part of + + Audio File Programs and Routines + by Peter Kabal. +
SoX which is maintained + by Chris Bagwell. + SoX is also able to perform some low quality sample rate conversions but these + will not be investigated. +
Shibatch 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. +
sr-convert is another + converter which does not handle arbitrary conversion ratios. +

+ +

+It should be noted that the first three converters above are based on the algorithm +by Julius O. Smith +which emulates the conversion of the digital signal to an analogue one and then +sampling the analogue signal at the new sample rate. +

+ + + +

Methodology

+ 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. +

+ 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. +

+ 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. +

+ +

+ 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. +

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SoX

+ SoX provides three methods of resampling; a linear interpolator, a polyphase + resampler and the Julius O. Smith simulated analogue filter method. +

+ +

Shibatch

+ Shibach +

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+ More Coming Soon. +

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