HRTFKernel::HRTFKernel(AudioChannel* channel, size_t fftSize, float sampleRate, bool bassBoost)
: m_frameDelay(0)
, m_sampleRate(sampleRate)
{
ASSERT(channel);
// Determine the leading delay (average group delay) for the response.
m_frameDelay = extractAverageGroupDelay(channel, fftSize / 2);
float* impulseResponse = channel->data();
size_t responseLength = channel->length();
if (bassBoost) {
// Run through some post-processing to boost the bass a little -- the HRTF's seem to be a little bass-deficient.
// FIXME: this post-processing should have already been applied to the HRTF file resources. Once the files are put into this form,
// then this code path can be removed along with the bassBoost parameter.
Biquad filter;
filter.setLowShelfParams(700.0 / nyquist(), 6.0); // boost 6dB at 700Hz
filter.process(impulseResponse, impulseResponse, responseLength);
}
// We need to truncate to fit into 1/2 the FFT size (with zero padding) in order to do proper convolution.
size_t truncatedResponseLength = min(responseLength, fftSize / 2); // truncate if necessary to max impulse response length allowed by FFT
// Quick fade-out (apply window) at truncation point
unsigned numberOfFadeOutFrames = static_cast<unsigned>(sampleRate / 4410); // 10 sample-frames @44.1KHz sample-rate
ASSERT(numberOfFadeOutFrames < truncatedResponseLength);
if (numberOfFadeOutFrames < truncatedResponseLength) {
for (unsigned i = truncatedResponseLength - numberOfFadeOutFrames; i < truncatedResponseLength; ++i) {
float x = 1.0f - static_cast<float>(i - (truncatedResponseLength - numberOfFadeOutFrames)) / numberOfFadeOutFrames;
impulseResponse[i] *= x;
}
}
m_fftFrame = adoptPtr(new FFTFrame(fftSize));
m_fftFrame->doPaddedFFT(impulseResponse, truncatedResponseLength);
}
