Author:
 Erik de Castro Lopo

SRC Quality

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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 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 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 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.