Example 2: Frequency-Domain Filtering

Introduction

This example shows how to apply a filter to an audio file using the Gaborator library, by turning the audio into spectrogram coefficients, modifying the coefficients, and resynthesizing audio from them.

The specific filter implemented here is a 3 dB/octave lowpass filter. This is sometimes called a pinking filter because it can be used to produce pink noise from white noise. In practice, the 3 dB/octave slope is only applied above some minimum frequency, for example 20 Hz, because otherwise the gain of the filter would approach infinity as the frequency approaches 0, and the impulse response would have to be infinitely wide.

Since the slope of this filter is not a multiple of 6 dB/octave, it is difficult to implement as an analog filter, but by filtering digitally in the frequency domain, arbitrary filter responses such as this can easily be achieved.

Preamble

#include <memory.h>
#include <iostream>
#include <sndfile.h>
#include <gaborator/gaborator.h>

int main(int argc, char **argv) {
    if (argc < 3) {
        std::cerr << "usage: filter input.wav output.wav\n";
        exit(1);
    }

Reading the Audio

The code for reading the input audio file is identical to that in Example 1:

    SF_INFO sfinfo;
    memset(&sfinfo, 0, sizeof(sfinfo));
    SNDFILE *sf_in = sf_open(argv[1], SFM_READ, &sfinfo);
    if (! sf_in) {
        std::cerr << "could not open input audio file\n";
        exit(1);
    }
    double fs = sfinfo.samplerate;
    sf_count_t n_frames = sfinfo.frames;
    sf_count_t n_samples = sfinfo.frames * sfinfo.channels;
    std::vector<float> audio(n_samples);
    sf_count_t n_read = sf_readf_float(sf_in, audio.data(), n_frames);
    if (n_read != n_frames) {
        std::cerr << "read error\n";
        exit(1);
    }
    sf_close(sf_in);

Spectrum Analysis Parameters

The spectrum analysis works much the same as in Example 1, but uses slightly different parameters. We use a larger number of frequency bands per octave (100) to minimize ripple in the frequency response, and the reference frequency argument is omitted as we don't care about the exact alignment of the bands with respect to a musical scale.

    gaborator::parameters params(100, 20.0 / fs);
    gaborator::analyzer<float> analyzer(params);

Precalculating Gains

The filtering will be done by multiplying each spectrogram coefficient with a frequency-dependent gain. To avoid having to calculate the gain on the fly for each coefficient, which would be slow, we will precalculate the gains into a vector band_gains of one gain value per band, including one for the special lowpass band that contains the frequencies from 0 to 20 Hz.

    std::vector<float> band_gains(analyzer.bands_end());

First, we calculate the gains for the bandpass bands. For a 3 dB/octave lowpass filter, the voltage gain needs to be proportional to the square root of the inverse of the frequency. To get the frequency of each band, we call the analyzer method band_ff(), which returns the center frequency of the band in units of the sampling frequency. The gain is normalized to unity at 20 Hz.

    for (int band = analyzer.bandpass_bands_begin(); band < analyzer.bandpass_bands_end(); band++) {
        float f_hz = analyzer.band_ff(band) * fs;
        band_gains[band] = 1.0 / sqrt(f_hz / 20.0);
    }

The gain of the lowpass band is set to the the same value as the lowest-frequency bandpass band, so that the overall filter gain plateaus smoothly to a constant value below 20 Hz.

    band_gains[analyzer.band_lowpass()] = band_gains[analyzer.bandpass_bands_end() - 1];

De-interleaving

To handle stereo and other multi-channel audio files, we will loop over the channels and filter each channel separately. Since libsndfile produces interleaved samples, we first de-interleave the current channel into a temporary vector called channel:

    for (sf_count_t ch = 0; ch < sfinfo.channels; ch++) {
        std::vector<float> channel(n_frames);
        for (sf_count_t i = 0; i < n_frames; i++)
            channel[i] = audio[i * sfinfo.channels + ch];

Spectrum Analysis

Now we can spectrum analyze the current channel, producing a set of coefficients:

        gaborator::coefs<float> coefs(analyzer);
        analyzer.analyze(channel.data(), 0, channel.size(), coefs);

Filtering

The filtering is done using the function gaborator::apply(), which applies a user-defined function to each spectrogram coefficient. Here, that user-defined function is a lambda expression that multiplies the coefficient by the appropriate precalculated frequency-dependent gain, modifying the coefficient in place. The unused int64_t argument is the time in units of samples; this could be use to implement a time-varying filter if desired.

        apply(analyzer, coefs,
            [&](std::complex<float> &coef, int band, int64_t) {
                coef *= band_gains[band];
            });

Resynthesis

We can now resynthesize audio from the filtered coefficients by calling synthesize(). This is a mirror image of the call to analyze(): now the coefficients are the input, and the buffer of samples is the output. The channel vector that originally contained the input samples for the channel is now reused to hold the output samples.

        analyzer.synthesize(coefs, 0, channel.size(), channel.data());

Re-interleaving

The audio vector that contained the original interleaved audio is reused for the interleaved filtered audio. This concludes the loop over the channels.

        for (sf_count_t i = 0; i < n_frames; i++)
            audio[i * sfinfo.channels + ch] = channel[i];
    }

Writing the Audio

The filtered audio is written using libsndfile, using code that closely mirrors that for reading. Note that we use SFC_SET_CLIPPING to make sure that any samples too loud for the file format will saturate; by default, libsndfile makes them wrap around, which sounds really bad.

    SNDFILE *sf_out = sf_open(argv[2], SFM_WRITE, &sfinfo);
    if (! sf_out) {
        std::cerr << "could not open output audio file\n";
        exit(1);
    }
    sf_command(sf_out, SFC_SET_CLIPPING, NULL, SF_TRUE);
    sf_count_t n_written = sf_writef_float(sf_out, audio.data(), n_frames);
    if (n_written != n_frames) {
        std::cerr << "write error\n";
        exit(1);
    }
    sf_close(sf_out);

Postamble

We need a couple more lines of boilerplate to make the example a complete program:

    return 0;
}

Compiling

Like Example 1, this example can be built using a one-line build command:

c++ -std=c++11 -I.. -O3 -ffast-math `pkg-config --cflags sndfile` filter.cc `pkg-config --libs sndfile` -o filter

Or using the vDSP FFT on macOS:

c++ -std=c++11 -I.. -O3 -ffast-math -DGABORATOR_USE_VDSP `pkg-config --cflags sndfile` filter.cc `pkg-config --libs sndfile` -framework Accelerate -o filter

Or using PFFFT (see Example 1 for how to download and build PFFFT):

c++ -std=c++11 -I.. -Ipffft -O3 -ffast-math -DGABORATOR_USE_PFFFT `pkg-config --cflags sndfile` filter.cc pffft/pffft.o pffft/fftpack.o `pkg-config --libs sndfile` -o filter

Running

To filter the file guitar.wav that was downloaded in Example 1, simply run

./filter guitar.wav guitar_filtered.wav

The resulting lowpass filtered audio in guitar_filtered.wav will sound muffled compared to the original, but less so than it would with a 6 dB/octave filter.

Frequency response

The following plot shows the actual measured frequency response of the filter, with the expected 3 dB/octave slope above 20 Hz and minimal ripple:

Frequency response plot