Improve the front stablizer

Apply the all-pass+band-split only once, after generating the mid and side
signals separately.

alc/alc.cpp

@@ -2096,13 +2096,10 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const int *attrList)
         if(GetConfigValueBool(device->DeviceName.c_str(), nullptr, "front-stablizer", 0))
         {
             auto stablizer = std::make_unique<FrontStablizer>();
-            /* Initialize band-splitting filters for the front-left and front-
+            /* Initialize band-splitting filter for the mid signal, with a
-             * right channels, with a crossover at 5khz (could be higher).
+             * crossover at 5khz (could be higher).
              */
-            const float scale{5000.0f / static_cast<float>(device->Frequency)};
+            stablizer->MidFilter.init(5000.0f / static_cast<float>(device->Frequency));
-
-            stablizer->LFilter.init(scale);
-            stablizer->RFilter = stablizer->LFilter;
 
             device->Stablizer = std::move(stablizer);
             /* NOTE: Don't know why this has to be "copied" into a local static

alc/alu.cpp

@@ -1830,45 +1830,51 @@ void ApplyStablizer(FrontStablizer *Stablizer, const al::span<FloatBufferLine> B
         }
     }
 
-    float (&lsplit)[2][BUFFERSIZE] = Stablizer->LSplit;
+    al::span<float> tmpbuf{Stablizer->TempBuf, SamplesToDo+FrontStablizer::DelayLength};
-    float (&rsplit)[2][BUFFERSIZE] = Stablizer->RSplit;
-    const al::span<float> tmpbuf{Stablizer->TempBuf, SamplesToDo+FrontStablizer::DelayLength};
 
-    /* This applies the band-splitter, preserving phase at the cost of some
+    /* Use the right delay buf for the side signal delay. Combine the delayed
-     * delay. The shorter the delay, the more error seeps into the result.
+     * signal with the incoming signal.
      */
-    auto apply_splitter = [tmpbuf,SamplesToDo](const FloatBufferLine &InBuf,
+    auto tmpiter = std::copy_n(std::begin(Stablizer->DelayBuf[ridx]), FrontStablizer::DelayLength,
-        const al::span<float,FrontStablizer::DelayLength> DelayBuf, BandSplitter &Filter,
-        float (&splitbuf)[2][BUFFERSIZE]) -> void
-    {
-        /* Combine the input and delayed samples into a temp buffer in reverse,
-         * then copy the final samples into the delay buffer for next time.
-         * Note that the delay buffer's samples are stored backwards here.
-         */
-        auto tmp_iter = std::reverse_copy(InBuf.cbegin(), InBuf.cbegin()+SamplesToDo,
         tmpbuf.begin());
-        std::copy(DelayBuf.cbegin(), DelayBuf.cend(), tmp_iter);
+    for(size_t i{0};i < SamplesToDo;++i,++tmpiter)
-        std::copy_n(tmpbuf.cbegin(), DelayBuf.size(), DelayBuf.begin());
+        *tmpiter = Buffer[lidx][i] - Buffer[ridx][i];
+    /* Hold on to the beginning for later, and save the end for next time. */
+    std::copy_n(tmpbuf.begin(), SamplesToDo, std::begin(Stablizer->Side));
+    std::copy_n(tmpbuf.begin()+SamplesToDo, FrontStablizer::DelayLength,
+        std::begin(Stablizer->DelayBuf[ridx]));
 
-        /* Apply an all-pass on the reversed signal, then reverse the samples
+    /* Use the left delay buf for the mid signal delay. Combine the delayed
-         * to get the forward signal with a reversed phase shift.
+     * signal with the incoming signal. Note that the samples are stored and
+     * combined in reverse, so the newest samples are at the front and the
+     * oldest at the back.
      */
-        Filter.applyAllpass(tmpbuf);
+    tmpiter = tmpbuf.begin() + SamplesToDo;
+    std::copy_n(std::cbegin(Stablizer->DelayBuf[lidx]), FrontStablizer::DelayLength, tmpiter);
+    for(size_t i{0};i < SamplesToDo;++i)
+        *--tmpiter = Buffer[lidx][i] + Buffer[ridx][i];
+    /* Save the newest samples for next time. */
+    std::copy_n(tmpbuf.cbegin(), FrontStablizer::DelayLength,
+        std::begin(Stablizer->DelayBuf[lidx]));
+
+    /* Apply an all-pass on the reversed signal, then reverse the samples to
+     * get the forward signal with a reversed phase shift. The future samples
+     * are included with the all-pass to reduce the error in the output
+     * samples (the smaller the delay, the more error is introduced).
+     */
+    Stablizer->MidFilter.applyAllpass(tmpbuf);
+    tmpbuf = tmpbuf.subspan<FrontStablizer::DelayLength>();
     std::reverse(tmpbuf.begin(), tmpbuf.end());
 
-        /* Now apply the band-splitter, combining its phase shift with the
+    /* Now apply the band-splitter, combining its phase shift with the reversed
-         * reversed phase shift, restoring the original phase on the split
+     * phase shift, restoring the original phase on the split signal.
-         * signal.
      */
-        Filter.process(tmpbuf.first(SamplesToDo), splitbuf[1], splitbuf[0]);
+    Stablizer->MidFilter.process(tmpbuf, Stablizer->MidHF, Stablizer->MidLF);
-    };
-    apply_splitter(Buffer[lidx], Stablizer->DelayBuf[lidx], Stablizer->LFilter, lsplit);
-    apply_splitter(Buffer[ridx], Stablizer->DelayBuf[ridx], Stablizer->RFilter, rsplit);
 
-    /* This pans the separate low- and high-frequency sums between being on the
+    /* This pans the separate low- and high-frequency signals between being on
-     * center channel and the left/right channels. The low-frequency sum is
+     * the center channel and the left+right channels. The low-frequency signal
-     * 1/3rd toward center (2/3rds on left/right) and the high-frequency sum is
+     * is panned 1/3rd toward center and the high-frequency signal is panned
-     * 1/4th toward center (3/4ths on left/right). These values can be tweaked.
+     * 1/4th toward center. These values can be tweaked.
      */
     const float cos_lf{std::cos(1.0f/3.0f * (al::MathDefs<float>::Pi()*0.5f))};
     const float cos_hf{std::cos(1.0f/4.0f * (al::MathDefs<float>::Pi()*0.5f))};
@@ -1876,12 +1882,9 @@ void ApplyStablizer(FrontStablizer *Stablizer, const al::span<FloatBufferLine> B
     const float sin_hf{std::sin(1.0f/4.0f * (al::MathDefs<float>::Pi()*0.5f))};
     for(ALuint i{0};i < SamplesToDo;i++)
     {
-        float lfsum{lsplit[0][i] + rsplit[0][i]};
+        const float m{Stablizer->MidLF[i]*cos_lf + Stablizer->MidHF[i]*cos_hf};
-        float hfsum{lsplit[1][i] + rsplit[1][i]};
+        const float c{Stablizer->MidLF[i]*sin_lf + Stablizer->MidHF[i]*sin_hf};
-        float s{lsplit[0][i] + lsplit[1][i] - rsplit[0][i] - rsplit[1][i]};
+        const float s{Stablizer->Side[i]};
-
-        float m{lfsum*cos_lf + hfsum*cos_hf};
-        float c{lfsum*sin_lf + hfsum*sin_hf};
 
         /* The generated center channel signal adds to the existing signal,
          * while the modified left and right channels replace.

alc/front_stablizer.h

@@ -10,14 +10,15 @@
 struct FrontStablizer {
     static constexpr size_t DelayLength{256u};
 
-    alignas(16) float DelayBuf[MAX_OUTPUT_CHANNELS][DelayLength];
+    BandSplitter MidFilter;
-
+    alignas(16) float MidLF[BUFFERSIZE];
-    BandSplitter LFilter, RFilter;
+    alignas(16) float MidHF[BUFFERSIZE];
-    alignas(16) float LSplit[2][BUFFERSIZE];
+    alignas(16) float Side[BUFFERSIZE];
-    alignas(16) float RSplit[2][BUFFERSIZE];
 
     alignas(16) float TempBuf[BUFFERSIZE + DelayLength];
 
+    alignas(16) float DelayBuf[MAX_OUTPUT_CHANNELS][DelayLength];
+
     DEF_NEWDEL(FrontStablizer)
 };