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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
For most sample rate converters, the SNR will vary depending on
the input signal and the ratio between input and output sample
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:
which is a program (which uses libsamplerate) from the examples/
directory of the Secret Rabbit Code source code distribution.
by Julius O Smiths which seems to have been the first high quality converter
available as source code.
which is part of
Audio File Programs and Routines
by Peter Kabal.
- SoX which is maintained
by Chris Bagwell.
SoX is also able to perfrom 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.
It should be noted that all of the converters above other than Shibatch 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.
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
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
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.
SoX provides three methods of resampling; a linear interpolator, a polyphase
resampler and the Julius O. Smith simulated analogue filter method.
More Coming Soon.