add downmix-audio-to-stereo-rfc7845.py

Author: milahu

ffmpeg/downmix-audio-to-stereo-rfc7845.py

@@ -0,0 +1,138 @@
+#!/usr/bin/env python3
+
+# https://superuser.com/a/1616102/951886
+# Properly downmix 5.1 to stereo using ffmpeg
+
+# https://www.rfc-editor.org/rfc/rfc7845#section-5.1.1.5
+# RFC 7845 Section 5.1.1.5 Downmixing
+
+# https://trac.ffmpeg.org/wiki/AudioChannelManipulation
+
+# https://mediaarea.net/AudioChannelLayout # speaker positions
+
+# test:
+# for L in 3.0 4.0 5.0 5.1 6.1 7.1; do echo; echo $L; ./downmix-audio-to-stereo-rfc7845.py $L; done
+
+# TODO 5.1(side) versus 5.1
+
+# note: ffmpeg says "back" instead of "rear"
+
+
+
+from math import sqrt
+
+
+
+def get_coefficients_for_downmix_to_stereo(
+        input_channel_layout,
+        scale_center = 1.0,
+        scale_lfe = 1.0,
+        scale_front = 1.0,
+        scale_rear = 1.0,
+        scale_side1 = 1.0,
+        scale_side2 = 1.0,
+    ):
+
+    if input_channel_layout in ("mono", "monophonic", "1ch", "1.0"):
+        return None # noop
+
+    if input_channel_layout in ("stereo", "2ch", "2.0"):
+        return None # noop
+
+    if input_channel_layout in ("linear surround", "3ch", "3.0"):
+        # Linear Surround Channel Mapping: L C R
+        # left, center, right
+        sides = 1
+        center = (1/sqrt(2)) * scale_center
+        n = 1/(sides + center)
+        return dict(
+            FL = dict(FL=sides*n, FC=center*n, FR=0),
+            FR = dict(FL=0, FC=center*n, FR=sides*n),
+        )
+
+    if input_channel_layout in ("quadraphonic", "4ch", "4.0"):
+        # Quadraphonic Channel Mapping: FL FR RL RR
+        # front left, front right, rear left, rear right
+        front = 1 * scale_front
+        side1 = (sqrt(3)/2) * scale_side1
+        side2 = (1/2) * scale_side2
+        n = 1/(front + side1 + side2)
+        return dict(
+            FL = dict(FL=front*n, FR=0, BL=side1*n, BR=side2*n),
+            FR = dict(FL=0, FR=front*n, BL=side2*n, BR=side1*n),
+        )
+
+    if input_channel_layout in ("5.0", "5ch"):
+        # 5.0 Surround Mapping: FL FC FR RL RR
+        # front left, front center, front right, rear left, rear right
+        # 5.1 without subwoofer (LFE)
+        front = 1 * scale_front
+        center = (1/sqrt(2)) * scale_center
+        side1 = (sqrt(3)/2) * scale_side1
+        side2 = (1/2) * scale_side2
+        n = 2/(front + center + side1 + side2) # note: 2/
+        return dict(
+            FL = dict(FL=front*n, FC=center*n, FR=0, BL=side1*n, BR=side2*n),
+            FR = dict(FL=0, FC=center*n, FR=front*n, BL=side2*n, BR=side1*n),
+        )
+
+    if input_channel_layout in ("5.1", "6ch"):
+        # 5.1 Surround Mapping: FL FC FR RL RR LFE
+        # front left, front center, front right, rear left, rear right, LFE
+        front = 1 * scale_front
+        center = (1/sqrt(2)) * scale_center
+        side1 = (sqrt(3)/2) * scale_side1
+        side2 = (1/2) * scale_side2
+        lfe = (1/sqrt(2)) * scale_lfe
+        n = 2/(front + center + side1 + side2 + lfe) # note: 2/
+        return dict(
+            FL = dict(FL=front*n, FC=center*n, FR=0, BL=side1*n, BR=side2*n, LFE=lfe*n),
+            FR = dict(FL=0, FC=center*n, FR=front*n, BL=side2*n, BR=side1*n, LFE=lfe*n),
+        )
+
+    if input_channel_layout in ("6.1", "7ch"):
+        # 6.1 Surround Mapping: FL FC FR SL SR RC LFE
+        # front left, front center, front right, side left, side right, rear center, LFE
+        # 5.1 + rear center, "rear" -> "side"
+        front = 1 * scale_front
+        center = (1/sqrt(2)) * scale_center
+        side1 = (sqrt(3)/2) * scale_side1
+        side2 = (1/2) * scale_side2
+        rearC = (sqrt(3)/2)/sqrt(2)
+        lfe = (1/sqrt(2)) * scale_lfe
+        n = 2/(front + center + side1 + side2 + rearC + lfe) # note: 2/
+        return dict(
+            FL = dict(FL=front*n, FC=center*n, FR=0, SL=side1*n, SR=side2*n, BC=rearC*n, LFE=lfe*n),
+            FR = dict(FL=0, FC=center*n, FR=front*n, SL=side2*n, SR=side1*n, BC=rearC*n, LFE=lfe*n),
+        )
+
+    if input_channel_layout in ("7.1", "8ch"):
+        # 7.1 Surround Mapping: FL FC FR SL SR RL RR LFE
+        # front left, front center, front right, side left, side right, rear left, rear right, LFE
+        # 6.1 + RC -> RL RR
+        front = 1 * scale_front
+        center = (1/sqrt(2)) * scale_center
+        side1 = (sqrt(3)/2) * scale_side1
+        side2 = (1/2) * scale_side2
+        lfe = (1/sqrt(2)) * scale_lfe
+        n = 2/(front + center + 2*side1 + 2*side2 + lfe) # note: 2/
+        return dict(
+            FL = dict(FL=front*n, FC=center*n, FR=0, SL=side1*n, SR=side2*n, BL=side1*n, BR=side2*n, LFE=lfe*n),
+            FR = dict(FL=0, FC=center*n, FR=front*n, SL=side2*n, SR=side1*n, BL=side2*n, BR=side1*n, LFE=lfe*n),
+        )
+
+    raise ValueError(f"unknown input_channel_layout {input_channel_layout}")
+
+
+
+def get_ffmpeg_audio_filter_for_downmix_to_stereo(input_channel_layout, **kwargs):
+        coefficients = get_coefficients_for_downmix_to_stereo(input_channel_layout, **kwargs)
+        return "pan=stereo|" + "|".join(map(lambda kv: kv[0] + "=" + "+".join(map(lambda kv2: f"{kv2[1]}*{kv2[0]}", kv[1].items())), coefficients.items()))
+
+
+
+if __name__ == "__main__":
+    import sys
+    input_channel_layout = sys.argv[1]
+    #print(get_coefficients_for_downmix_to_stereo(input_channel_layout))
+    print(get_ffmpeg_audio_filter_for_downmix_to_stereo(input_channel_layout))