Fix loss of volume when using a mono track as master audio

By default, when converting or playing mono audio tracks as stereo,
ffmpeg halves the power of the left/right audio channels. This is
undesirable since it makes mono tracks quieter than intended.

This commit makes corrscope detect mono master audio, and tell ffmpeg to
play audio at full volume in both channels. We cannot enable this for
stereo master audio, since it will discard the right input channel.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Major Changes
 
 - Encode audio using libopus instead of aac (#487)
+- Fix loss of volume when using a mono track as master audio (#488)
 
 ## 0.10.1
 

corrscope/outputs.py

@@ -1,6 +1,7 @@
 import errno
 import shlex
 import subprocess
+import wave
 from abc import ABC, abstractmethod
 from os.path import abspath
 from typing import TYPE_CHECKING, Type, List, Union, Optional, ClassVar, Callable
@@ -87,8 +88,11 @@ class _FFmpegProcess:
 
             self.templates.append(f"-ss {corr_cfg.begin_time}")
 
+            with wave.open(corr_cfg.master_audio, "rb") as master_wav:
+                self.mono = master_wav.getnchannels() <= 1
+
             audio_path = shlex.quote(abspath(corr_cfg.master_audio))
-            self.templates += ffmpeg_input_audio(audio_path)  # audio
+            self.templates += ffmpeg_input_audio(audio_path, self.mono)  # audio
 
             if corr_cfg.end_time is not None:
                 dur = corr_cfg.end_time - corr_cfg.begin_time
@@ -97,7 +101,13 @@ class _FFmpegProcess:
     def add_output(self, cfg: "Union[FFmpegOutputConfig, FFplayOutputConfig]") -> None:
         self.templates.append(cfg.video_template)  # video
         if self.corr_cfg.master_audio:
-            self.templates.append(cfg.audio_template)  # audio
+            audio_template = cfg.audio_template
+
+            # When upmixing mono to stereo (in ffmpeg_input_audio()), we need to
+            # reencode audio.
+            if self.mono and audio_template.endswith(" copy"):
+                audio_template = "-c:a pcm_s16le"
+            self.templates.append(audio_template)  # audio
 
     def popen(self, extra_args: List[str], bufsize: int, **kwargs) -> subprocess.Popen:
         """Raises FileNotFoundError if FFmpeg missing"""
@@ -126,8 +136,14 @@ def ffmpeg_input_video(cfg: "Config") -> List[str]:
     ]
 
 
-def ffmpeg_input_audio(audio_path: str) -> List[str]:
-    return ["-i", audio_path]
+def ffmpeg_input_audio(audio_path: str, mono: bool) -> List[str]:
+    out = ["-i", audio_path]
+
+    # Preserve mono volume and upmix to stereo. If we don't do this explicitly, most
+    # players will attenuate stereo playback by 3 dB during encoding/playback.
+    if mono:
+        out += ["-af", "pan=stereo|c0=c0|c1=c0"]
+    return out
 
 
 class PipeOutput(Output):