From: Marcus Geelnard <mage@opera.com>

Date: Thu, 19 Jul 2012 21:03:38 +0000

Message-ID: <20120719210338.0lheeius96h8oc0w@staff.opera.com>

To: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>

Cc: public-audio@w3.org

Date: Thu, 19 Jul 2012 21:03:38 +0000

Message-ID: <20120719210338.0lheeius96h8oc0w@staff.opera.com>

To: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>

Cc: public-audio@w3.org

Citerar Jussi Kalliokoski <jussi.kalliokoski@gmail.com>: > Obviously SIMD code is faster than addition in JS now, for example. And > yes, IIR filter is a type of a convolution, but I don't think it's possible > to write an efficient IIR filter algorithm using a convolution engine -- > after all, a convolution engine should be designed to deal with a FIRs. You can split the IIR filter into two parts: an FIR part and a recursive part. If you skip the recursive part, you get plain convolution. > Not to mention that common IIR filters have 4 (LP, HP, BP, N) kernels, I was thinking more along the lines of using the more generic form of the filter function (see the Matlab filter() function [1]), which supports everything from first order IIR filters to FIR-filters with 1000s of taps (or more, obviously). > And as far as IIR filter performance goes, I think SIMD > instructions offer very little usefulness in IIR algorithms, since they're > so linear. Actually, I made a simple benchmark on my AMD x2, 64-bit Linux, using an IIR filter function identical to the filter function in the BiquadFilterNode (excluding buffer edge handling). JavaScript code: function filter (y, x, b, a) { var b0 = b[0], b1 = b[1], b2 = b[2]; var a1 = a[0], a2 = a[2]; var len = x.length; for (var k = 2; k < len; ++k) y[k] = b0 * x[k] + b1 * x[k-1] + b2 * x[k-2] - a1 * y[k-1] - a2 * y[k-2]; } C++ code: void filter(float* y, const float* x, unsigned len, const float* b, const float* a) { float b0 = b[0], b1 = b[1], b2 = b[2]; float a1 = a[0], a2 = a[2]; // FIR part for (unsigned k = 2; k < len; ++k) y[k] = b0 * x[k] + b1 * x[k-1] + b2 * x[k-2]; // Recursive part for (unsigned k = 2; k < len; ++k) y[k] -= a1 * y[k-1] + a2 * y[k-2]; } Note: In the C++ version I split the FIR and recursive parts into two separate loops because somehow g++ managed to optimize that better than doing a single loop (looking at the assembler output, it actually SIMD:ified the FIR part for me - impressive!). Also, it illustrates that you can do the FIR part first, with optimizations (e.g. FFT-based convolution or SIMD), and then append the recursive part as a second pass. Benchmark results (using 128 samples long buffers): C++: 529.4 Ms/s Chrome: 22.0 Ms/s FF: 13.0 Ms/s Opera: 9.0 Ms/s So, we actually get a >20x speedup when going from the fastest JS engine to native C++ code (g++ -O3 -ffast-math). I think that's motivation enough to implement it as a native function ;) Regards, Marcus [1] http://www.mathworks.se/help/techdoc/ref/filter.html -- Marcus Geelnard Core Graphics Developer Opera Software ASAReceived on Thursday, 19 July 2012 21:12:23 UTC

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