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diff --git a/lib/libsamplerate/libsamplerate-0.2.2/docs/quality.md b/lib/libsamplerate/libsamplerate-0.2.2/docs/quality.md new file mode 100755 index 00000000..5c69578e --- /dev/null +++ b/lib/libsamplerate/libsamplerate-0.2.2/docs/quality.md @@ -0,0 +1,84 @@ +--- +layout: default +--- + +# SRC Quality + +**This document not yet complete.** + +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 + 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 of Secret Rabbit Code to the following: + +- [sndfile-resample](http://libsndfile.github.io/libsamplerate/download.html) + which is a program (which uses libsamplerate) from the **examples/** directory + of the Secret Rabbit Code source code distribution. +- [Resample](https://ccrma.stanford.edu/~jos/resample/) by Julius O Smiths which + seems to have been the first high quality converter available as source code. +- [ResampAudio](http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/ResampAudio.html) + which is part of [Audio File Programs and Routines](http://www.tsp.ece.mcgill.ca/MMSP/Documents/Software/AFsp/AFsp.html) + by Peter Kabal. +- [SoX](http://home.sprynet.com/~cbagwell/sox.html) 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](http://shibatch.sourceforge.net/) 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](http://sr-convert.sourceforge.net/) 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](http://www-ccrma.stanford.edu/~jos/resample/) +which emulates the conversion of the digital signal to an analogue one and then +samples the analogue signal at the new sample rate. + +## Methodology + +Measuring the SNR of a converter is relatively straightforward. 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 files are then upsampled by a factor of 2 using the converter under test. +If the attenuation of the lower frequency 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 distribution. This program is +able to test the first four of the above converters. + +## SoX + +SoX provides three methods of resampling; a linear interpolator, a polyphase +resampler and the Julius O. Smith simulated analogue filter method. + +## Shibatch + +Shibach + +**More Coming Soon.** |