diff options
| -rw-r--r-- | core/uhjfilter.cpp | 36 | ||||
| -rw-r--r-- | utils/uhjencoder.cpp | 4 |
2 files changed, 31 insertions, 9 deletions
diff --git a/core/uhjfilter.cpp b/core/uhjfilter.cpp index 7298a9fc..14fe28f8 100644 --- a/core/uhjfilter.cpp +++ b/core/uhjfilter.cpp @@ -40,7 +40,7 @@ constexpr std::array<float,4> Filter1Coeff{{ square(0.6923878f), square(0.9360654322959f), square(0.9882295226860f), square(0.9987488452737f) }}; -/* Filter coefficients for the shifted all-pass IIR, which applies a frequency- +/* Filter coefficients for the offset all-pass IIR, which applies a frequency- * dependent phase-shift of N+90 degrees. */ constexpr std::array<float,4> Filter2Coeff{{ @@ -48,9 +48,9 @@ constexpr std::array<float,4> Filter2Coeff{{ square(0.9952884791278f) }}; -/* This applies the base all-pass filter in reverse, resulting in a phase-shift - * of -N degrees. Extra samples are provided at the back of the input to reduce - * the amount of error at the back of the output. +/* This applies the base all-pass filter in reverse. No state is kept between + * calls since the samples aren't processed in a linear fashion, the state from + * a previous call wouldn't be valid. */ void allpass1_process_rev(const al::span<const float> src, float *RESTRICT dst) { @@ -71,8 +71,14 @@ void allpass1_process_rev(const al::span<const float> src, float *RESTRICT dst) proc_sample); } -/* This applies the shifted all-pass filter to the output of the base filter, - * resulting in a phase shift of -N + N + 90 degrees, or just 90 degrees. +/* This applies the offset all-pass filter. 'forwardSamples' is the number of + * samples to process and keep the state of, while the remaining samples are + * processed but the state is discarded. This essentially "rewinds" the filter + * since the decoder will re-process some of the previous samples (as the + * decoding is done prior to resampling, which need future samples that are + * re-read on the following update, or because the source is being paused, + * which fades out from the current position and will replay the same samples + * when resumed). */ void allpass2_process(const al::span<UhjAllPassState,4> state, const al::span<const float> src, const size_t forwardSamples, float *RESTRICT dst) @@ -190,6 +196,22 @@ void UhjEncoder<N>::encode(float *LeftOut, float *RightOut, right[i] += (mS[i] - mD[i]) * 0.5f; } +/* This encoding implementation uses a dual IIR filter to produce the desired + * phase shift, preserving the phase of the non-filtered signal. + * + * The first filter applies a frequency-dependent phase shift of N degrees. + * Applied in reverse over the input signal results in a phase shift of -N + * degrees. Extra samples are utilized at the back of the input to reduce the + * amount of error at the back of the output (resulting in a small bit of + * latency). + * + * The second filter applies a phase shift like first, but with a +90 degree + * offset in each frequency band. + * + * Applying the first filter backward, followed by the second filter forward, + * results in a total phase shift of -N + N + 90 degrees, i.e. a frequency- + * independent phase shift of 90 degrees. + */ void UhjEncoderIIR::encode(float *LeftOut, float *RightOut, const al::span<const float *const, 3> InSamples, const size_t SamplesToDo) { @@ -208,7 +230,7 @@ void UhjEncoderIIR::encode(float *LeftOut, float *RightOut, mS[i] = 0.9396926f*mW[i] + 0.1855740f*mX[i]; /* Precompute j(-0.3420201*W + 0.5098604*X) and store in mD. */ - std::transform(winput, winput+SamplesToDo, mX.cbegin(), mWX.begin()+sFilterDelay, + std::transform(winput, winput+SamplesToDo, xinput, mWX.begin()+sFilterDelay, [](const float w, const float x) noexcept { return -0.3420201f*w + 0.5098604f*x; }); /* Shift the input ahead by one sample, so that the output is delayed by * one sample in the reverse. diff --git a/utils/uhjencoder.cpp b/utils/uhjencoder.cpp index 5dc3b949..b2fc6dce 100644 --- a/utils/uhjencoder.cpp +++ b/utils/uhjencoder.cpp @@ -128,9 +128,9 @@ void UhjEncoder::encode(const al::span<FloatBufferLine> OutSamples, std::copy_n(mTemp.cbegin()+SamplesToDo, mWXHistory1.size(), mWXHistory1.begin()); PShift.process({mD.data(), SamplesToDo}, mTemp.data()); - /* D = 0.6554516*Y + j(-0.3420201*W + 0.5098604*X) */ + /* D = j(-0.3420201*W + 0.5098604*X) + 0.6554516*Y */ for(size_t i{0};i < SamplesToDo;++i) - mD[i] = 0.6554516f*mY[i] + mD[i]; + mD[i] = mD[i] + 0.6554516f*mY[i]; /* Left = (S + D)/2.0 */ float *RESTRICT left{al::assume_aligned<16>(OutSamples[0].data())}; |