Sun, 14 Dec 2008

Rabbit Optimisation.

For some time I have known that the sinc based converters in my sample rate conversion library Secret Rabbit Code might benefit from a particular optimisation which reuses the filter coefficients where possible, rather than recalculating them. The good news is that I finally managed to get around to performing this optimisation work and the results are reasonably impressive.

The old converter used what was basically a single channel converter on each channel in turn. Since part of this single channel operation involved calculating the filter coefficients on the fly it was pretty obvious that if more than one channel was being processed, it would be possible to calculate the coefficients for the first channel and reuse them for all the rest. Once that was done, the inner loop can be special cased for commonly used channel counts like 1, 2 and 4 while having other channel counts handled by a generic function that can handle any number of channels. These special cases contain straight line code where the generic function contains a loop.

As can bee seen in the throughput graph for the SRC_SINC_BEST_QUALITY converter, this optimisation resulted in modest throughput improvements for 1 channel, a big improvement for 2 channels and an outrageous more than doubling of throughput for the 4 channel case. Even the generic multi-channel case, represented here by 3 and 5 channels showed a modest improvement. It should be noted that the filter coefficients are the same for the old and new versions so the only changes in performance are due to the optimisation.

[Throughput graphs for old and new Best Sinc converter]

These results were obtained on a 3 plus year old 1.1 GHz Pentium M laptop. More recent machines should show much better absolute results but similar proportional improvements. The other sinc-based converters will also show similar proportional improvements.

A new version of Secret Rabbit Code containing this optimisation should probably be released some time during the next couple of days.

Posted at: 08:16 | Category: CodeHacking/SecretRabbitCode | Permalink