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62 lines
2.5 KiB
Text
62 lines
2.5 KiB
Text
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This is a snapshot of my current work developing an audio
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resampling library. While working on this library, I started
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writing lots of general purpose functions that should really
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be part of a larger library. Rather than have a constantly
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changing library, and since the current code is capable, I
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decided to freeze this codebase for use with gstreamer, and
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move active development of the code elsewhere.
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The algorithm used is based on Shannon's theorem, which says
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that you can recreate an input signal from equidistant samples
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using a sin(x)/x filter; thus, you can create new samples from
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the regenerated input signal. Since sin(x)/x is expensive to
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evaluate, an interpolated lookup table is used. Also, a
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windowing function (1-x^2)^2 is used, which aids the convergence
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of sin(x)/x for lower frequencies at the expense of higher.
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There is one tunable parameter, which is the filter length.
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Longer filter lengths are obviously slower, but more accurate.
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There's not much reason to use a filter length longer than 64,
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since other approximations start to dominate. Filter lengths
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as short as 8 are audially acceptable, but should not be
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considered for serious work.
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Performance: A PowerPC G4 at 400 Mhz can resample 2 audio
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channels at almost 10x speed with a filter length of 64, without
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using Altivec extensions. (My goal was 10x speed, which I almost
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reached. Maybe later.)
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Limitations: Currently only supports streams in the form of
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interleaved signed 16-bit samples.
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The test.c program is a simple regression test. It creates a
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test input pattern (1 sec at 48 khz) that is a frequency ramp
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from 0 to 24000 hz, and then converts it to 44100 hz using a
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filter length of 64. It then compares the result to the same
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pattern generated at 44100 hz, and outputs the result to the
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file "out".
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A graph of the correct output should have field 2 and field 4
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almost equal (plus/minus 1) up to about sample 40000 (which
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corresponds to 20 khz), and then field 2 should be close to 0
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above that. Running the test program will print to stdout
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something like the following:
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time 0.112526
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average error 10k=0.4105 22k=639.34
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The average error is RMS error over the range [0-10khz] and
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[0-22khz], and is expressed in sample values, for an input
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amplitude of 16000. Note that RMS errors below 1.0 can't
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really be compared, but basically this shows that below
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10 khz, the resampler is nearly perfect. Most of the error
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is concentrated above 20 khz.
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If the average error is significantly larger after modifying
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the code, it's probably not good.
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dave...
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