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85 lines
4.0 KiB
85 lines
4.0 KiB
2 years ago
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layout: default
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# SRC Quality
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**This document not yet complete.**
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When measuring the performance of a Sample Rate Converter, there are three
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factors to consider:
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- **Signal-to-Noise Ratio** - a measure of how much noise the sample rate
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conversion process adds to the signal. This is measured in decibels (dB) and
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the higher this value the better. For most sample rate converters, the SNR
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will vary depending on the input signal and the ratio between input and output
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sample rates. The only valid comparison of SNR is between the worst case for
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each converter.
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- **Bandwidth** - most sample rate converters attenuate high frequencies as part
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of their operation. Bandwidth can be measured by finding the frequency where
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the attenuation is 3dB and expressing that as a percentage of the full
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bandwidth at that sampling rate.
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- **Speed** - the faster the better **:-)**.
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There are a number of sample rate converters available for downloading but I
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will limit the comparison of Secret Rabbit Code to the following:
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- [sndfile-resample](http://libsndfile.github.io/libsamplerate/download.html)
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which is a program (which uses libsamplerate) from the **examples/** directory
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of the Secret Rabbit Code source code distribution.
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- [Resample](https://ccrma.stanford.edu/~jos/resample/) by Julius O Smiths which
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seems to have been the first high quality converter available as source code.
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- [ResampAudio](http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html)
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which is part of [Audio File Programs and Routines](http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html)
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by Peter Kabal.
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- [SoX](http://home.sprynet.com/~cbagwell/sox.html) which is maintained by Chris
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Bagwell. SoX is also able to perform some low quality sample rate conversions
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but these will not be investigated.
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- [Shibatch](http://shibatch.sourceforge.net/) which seems to be a frequency
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domain sample rate converter. Unfortunately, this converter does not handle
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arbitrary conversion ratios and hence could not be properly compared to the
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other converters.
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- [sr-convert](http://sr-convert.sourceforge.net/) is another converter which
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does not handle arbitrary conversion ratios.
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It should be noted that the first three converters above are based on the
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algorithm by [Julius O. Smith](http://www-ccrma.stanford.edu/~jos/resample/)
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which emulates the conversion of the digital signal to an analogue one and then
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samples the analogue signal at the new sample rate.
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## Methodology
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Measuring the SNR of a converter is relatively straightforward. Generate an
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input signal consisting of a windowed sine wave, sample rate convert it and
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measure the signal-to-noise ratio of the output signal. A typical length for the
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original file is 30000 samples.
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The bandwidth of a sample rate converter is a little more difficult to measure.
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Currently this is done by generating two short files containing a windowed sine
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wave. The frequencies of the sine waves are 0.35 and 0.495 of the sample rate.
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These files are then upsampled by a factor of 2 using the converter under test.
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If the attenuation of the lower frequency is less than 3dB and higher frequency
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is more than 3dB, it is then possible to iteratively increase the lower
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frequency and decrease the upper frequency keeping the -3dB point bracketed.
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When the distance between the upper and lower frequency is sufficiently small,
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it is possible to obtain a very accurate estimate of the -3dB frequency.
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The speed of a sample rate converter is easy to measure; simply perform a
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conversion on a large file or a number of smaller files and time the conversion
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process.
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The above measurement techniques are built into a test program which is
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delivered with the Secret Rabbit Code source code distribution. This program is
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able to test the first four of the above converters.
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## SoX
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SoX provides three methods of resampling; a linear interpolator, a polyphase
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resampler and the Julius O. Smith simulated analogue filter method.
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## Shibatch
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Shibach
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**More Coming Soon.**
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