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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 deleted file mode 100755 index 5c69578e..00000000 --- a/lib/libsamplerate/libsamplerate-0.2.2/docs/quality.md +++ /dev/null @@ -1,84 +0,0 @@ ---- -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.** |