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added an resampler for arbitrary rate differences

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Ahmet Inan 2018-09-13 11:23:10 +02:00
commit 4ff2e0f148
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README.md
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@ -76,3 +76,8 @@ Sometimes we need a sequence of ["random enough"](https://en.wikipedia.org/wiki/
Here a [Pseudorandom number generator](https://en.wikipedia.org/wiki/Pseudorandom_number_generator) can help by prodiving a deterministic and thus repeatable sequence of numbers.
[George Marsaglia](https://en.wikipedia.org/wiki/George_Marsaglia) discovered a class of simple and fast pseudorandom number generators, which he called [Xorshift](https://en.wikipedia.org/wiki/Xorshift).
### [resampler.hh](resampler.hh)
When working with [Analog-to-digital](https://en.wikipedia.org/wiki/Analog-to-digital_converter) and [Digital-to-analog](https://en.wikipedia.org/wiki/Digital-to-analog_converter) converters, we often face the ugly truth, that we can't always have a precise [Sampling](https://en.wikipedia.org/wiki/Sampling_(signal_processing)) rate.
But if we can estimate the Sampling frequency offset, we can correct it by [Resampling](https://en.wikipedia.org/wiki/Sample-rate_conversion) the sampled data.

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resampler.hh Normal file
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@ -0,0 +1,60 @@
/*
Resampling by an arbitrary rate difference
Copyright 2018 Ahmet Inan <inan@aicodix.de>
*/
#ifndef RESAMPLER_HH
#define RESAMPLER_HH
#include "window.hh"
#include "spline.hh"
#include "const.hh"
namespace DSP {
template <typename TYPE, int RATE, int TAPS, int OVER>
class Resampler
{
typedef DSP::UniformNaturalCubicSpline<TAPS * OVER, TYPE, TYPE> spline_type;
spline_type lpf;
static TYPE sinc(TYPE x)
{
return TYPE(0) == x ? TYPE(1) : std::sin(DSP::Const<TYPE>::Pi() * x) / (DSP::Const<TYPE>::Pi() * x);
}
public:
Resampler(TYPE cutoff, TYPE alpha)
{
TYPE tmp[TAPS * OVER];
DSP::Kaiser<TYPE> window(alpha);
for (int n = 0; n < TAPS * OVER; ++n) {
TYPE x = TYPE(n) / TYPE(OVER) - TYPE(TAPS - 1) / TYPE(2);
TYPE y = TYPE(2) * cutoff / TYPE(RATE) * sinc(TYPE(2) * cutoff / TYPE(RATE) * x);
tmp[n] = y * window(n, TAPS * OVER);
}
lpf = spline_type(tmp, 0, 1.0 / OVER);
}
template <typename IO>
void operator ()(IO *output, IO *input, TYPE diff, int samples, int stride = 1)
{
TYPE ratio = (RATE + diff) / RATE;
TYPE recip = RATE / (RATE + diff);
for (int i = 0; i < samples - TAPS; ++i) {
IO sum = 0;
int s0 = nearbyint(i * ratio);
int s1 = s0 + TAPS;
for (int s = s0; s < s1; ++s) {
TYPE k = s * recip - i;
sum += lpf(k) * input[s * stride];
}
output[(i + (TAPS - 1) / 2) * stride] = sum;
}
}
};
}
#endif