Support for custom output bitrate, e.g. to target //gs in 2.8MHz mode.

For some reason playback speed is only about 1.6x (probably due to
slowing down accesses to the I/O page to 1MHz, so as not to mess up
hardware timings), but happily this comes within 3% of being 44100/2.

transcoder/audio.py

@@ -8,21 +8,38 @@ import numpy as np
 
 
 class Audio:
+    """
+    Decodes audio stream from input file and resamples.
+
+    Notes on audio bitrate:
+
+    At 73 cycles/tick, true audio playback sample rate is
+    roughly 1024*1024/73 = 14364 Hz (ignoring ACK slow path).
+    Typical audio encoding is 44100Hz which is close to 14700*3
+    Downscaling by 3x gives better results than trying to resample
+    to a non-divisor.  So we cheat a bit and play back the video a
+    tiny bit (<2%) faster.
+
+    For //gs playback at 2.8MHz, the effective speed increase is only about
+    1.6x.  This is probably because accessing the I/O page is done at 1MHz
+    to not mess up hardware timings.
+
+    This is close (2.1%) to 22500Hz which is again a simple divisor of the
+    base frequency (1/2).
+    """
+
     def __init__(
-            self, filename: str, normalization: float = None):
+            self,
+            filename: str,
+            bitrate: int = 14700,
+            normalization: float = None):
         self.filename = filename  # type: str
 
         # TODO: take into account that the available range is slightly offset
         # as fraction of total cycle count?
         self._tick_range = [4, 66]
 
-        # At 73 cycles/tick, true audio playback sample rate is
+        self.sample_rate = float(bitrate)  # type: float
-        # roughly 1024*1024/73 = 14364 Hz (ignoring ACK slow path).
-        # Typical audio encoding is 44100Hz which is close to 14700*3
-        # Downscaling by 3x gives better results than trying to resample
-        # to a non-divisor.  So we cheat a bit and play back the video a tiny
-        # bit (<2%) faster.
-        self.sample_rate = 14700.  # type: float
 
         self.normalization = (
                 normalization or self._normalization())  # type: float

transcoder/main.py

@@ -20,6 +20,11 @@ parser.add_argument(
     '--audio_normalization', type=float, default=None,
     help='Override auto-detected multiplier for audio normalization.'
 )
+parser.add_argument(
+    '--audio_bitrate', type=int, default=14700,
+    help='Select output audio bitrate (Hz), controls video speed (Default: '
+         '14700; try 22500 for //gs 2.8MHz mode)'
+)
 parser.add_argument(
     '--every_n_video_frames', type=int, default=2,
     help='Allows skipping frames of input video to lower effective output '
@@ -42,6 +47,7 @@ def main(args):
     m = movie.Movie(
         filename,
         every_n_video_frames=args.every_n_video_frames,
+        audio_bitrate=args.audio_bitrate,
         audio_normalization=args.audio_normalization,
         max_bytes_out=1024. * 1024 * args.max_output_mb,
         video_mode=video_mode.VideoMode[args.video_mode],

transcoder/movie.py

@@ -15,6 +15,7 @@ class Movie:
     def __init__(
             self, filename: str,
             every_n_video_frames: int = 1,
+            audio_bitrate: int = 14700,
             audio_normalization: float = None,
             max_bytes_out: int = None,
             video_mode: VideoMode = VideoMode.HGR,
@@ -27,7 +28,8 @@ class Movie:
         self.palette = palette  # type: Palette
 
         self.audio = audio.Audio(
-            filename, normalization=audio_normalization)  # type: audio.Audio
+            filename, bitrate=audio_bitrate,
+            normalization=audio_normalization)  # type: audio.Audio
 
         self.frame_grabber = frame_grabber.FileFrameGrabber(
             filename, mode=video_mode, palette=self.palette)