SRC.png


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 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 high 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:

  • Julius O Smiths's resampler which performs an emuluation of converting the digital signal to an analogure one an resampling. SRC uses this same algorithm with slightly improved filters.
  • Shibach - a frequency domain converter written by .

Methodology

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 20000 samples.

The bandwidth of a sample rate converter is much more difficult to measure. Currently this is done by generating a file containing many short (approx 1000 samples) sine wave bursts separated by silence. The sine wave bursts a evenly spaced between 0.75 times the sample rate and 0.95 times the sample rate. This file is then processed and analysed to determine the bandwidth of the converter.

The speed of a sample rate converter is easy to measure; simply perform a conversion on a large file and time the conversion process.

SoX

SoX provides three methods of resampling; a linear interpolator, a polyphase resampler and the Julius O. Smith simulated analogue filter method.

Shibach

Shibach

More Coming Soon.