Improve audio transcoding

ffmpeg aac encoder is not very good so prefer to keep the same bitrate
as mp3
we also use the max available bitrate with a flac input that has an
unknown bitrate

packages/ffmpeg/src/ffmpeg-default-transcoding-profile.ts

@@ -2,7 +2,7 @@ import { getAverageTheoreticalBitrate, getMaxTheoreticalBitrate, getMinTheoretic
 import {
   buildStreamSuffix,
   getAudioStream,
-  getMaxAudioBitrate,
+  getMaxAudioKBitrate,
   getVideoStream,
   getVideoStreamBitrate,
   getVideoStreamDimensionsInfo,
@@ -52,7 +52,7 @@ const defaultAACOptionsBuilder: EncoderOptionsBuilder = async ({ input, streamNu
 
   const audioCodecName = parsedAudio.audioStream['codec_name']
 
-  const bitrate = getMaxAudioBitrate(audioCodecName, parsedAudio.bitrate)
+  const bitrate = getMaxAudioKBitrate(audioCodecName, parsedAudio.bitrate)
 
   // Force stereo as it causes some issues with HLS playback in Chrome
   const base = [ '-channel_layout', 'stereo' ]
@@ -116,7 +116,7 @@ export async function canDoQuickAudioTranscode (path: string, probe?: FfprobeDat
   const audioBitrate = parsedAudio.bitrate
   if (!audioBitrate) return false
 
-  const maxAudioBitrate = getMaxAudioBitrate('aac', audioBitrate)
+  const maxAudioBitrate = getMaxAudioKBitrate('aac', audioBitrate)
   if (maxAudioBitrate !== -1 && audioBitrate > maxAudioBitrate) return false
 
   const channelLayout = parsedAudio.audioStream['channel_layout']

packages/ffmpeg/src/ffprobe.ts

@@ -6,7 +6,7 @@ import ffmpeg, { FfprobeData } from 'fluent-ffmpeg'
  * Helpers to run ffprobe and extract data from the JSON output
  */
 
-function ffprobePromise (path: string) {
+export function ffprobePromise (path: string) {
   return new Promise<FfprobeData>((res, rej) => {
     ffmpeg.ffprobe(path, [ '-show_chapters' ], (err, data) => {
       if (err) return rej(err)
@@ -62,7 +62,7 @@ const imageCodecs = new Set([
   'xwd'
 ])
 
-async function isAudioFile (path: string, existingProbe?: FfprobeData) {
+export async function isAudioFile (path: string, existingProbe?: FfprobeData) {
   const videoStream = await getVideoStream(path, existingProbe)
   if (!videoStream) return true
 
@@ -71,13 +71,13 @@ async function isAudioFile (path: string, existingProbe?: FfprobeData) {
   return false
 }
 
-async function hasAudioStream (path: string, existingProbe?: FfprobeData) {
+export async function hasAudioStream (path: string, existingProbe?: FfprobeData) {
   const { audioStream } = await getAudioStream(path, existingProbe)
 
   return !!audioStream
 }
 
-async function getAudioStream (videoPath: string, existingProbe?: FfprobeData) {
+export async function getAudioStream (videoPath: string, existingProbe?: FfprobeData) {
   const data = existingProbe || await ffprobePromise(videoPath)
 
   if (Array.isArray(data.streams)) {
@@ -95,45 +95,34 @@ async function getAudioStream (videoPath: string, existingProbe?: FfprobeData) {
   return { absolutePath: data.format.filename }
 }
 
-function getMaxAudioBitrate (type: string, bitrate: number) {
+export function getMaxAudioKBitrate (type: string, bitrate: number) {
   const maxKBitrate = 384
-  const kToBits = (kbits: number) => kbits * 1000
 
   // If we did not manage to get the bitrate, use an average value
-  if (!bitrate) return 256
+  if (!bitrate) {
+    // We expect uploader wants a high quality audio if the input is in FLAC format
+    if (type === 'flac') return maxKBitrate
+
+    return 256
+  }
+
+  const kBitrate = Math.round(bitrate / 1000)
 
   if (type === 'aac') {
-    switch (true) {
+    if (kBitrate > maxKBitrate) return maxKBitrate
-      case bitrate > kToBits(maxKBitrate):
-        return maxKBitrate
 
-      default:
+    // We interpret it as a signal to copy the audio stream as is
-        return -1 // we interpret it as a signal to copy the audio stream as is
+    return -1
-    }
   }
 
-  /*
+  return Math.min(maxKBitrate, kBitrate)
-    a 192kbit/sec mp3 doesn't hold as much information as a 192kbit/sec aac.
-    That's why, when using aac, we can go to lower kbit/sec. The equivalences
-    made here are not made to be accurate, especially with good mp3 encoders.
-    */
-  switch (true) {
-    case bitrate <= kToBits(192):
-      return 128
-
-    case bitrate <= kToBits(384):
-      return 256
-
-    default:
-      return maxKBitrate
-  }
 }
 
 // ---------------------------------------------------------------------------
 // Video
 // ---------------------------------------------------------------------------
 
-async function getVideoStreamDimensionsInfo (path: string, existingProbe?: FfprobeData) {
+export async function getVideoStreamDimensionsInfo (path: string, existingProbe?: FfprobeData) {
   const videoStream = await getVideoStream(path, existingProbe)
   if (!videoStream) {
     return {
@@ -164,7 +153,7 @@ async function getVideoStreamDimensionsInfo (path: string, existingProbe?: Ffpro
   }
 }
 
-async function getVideoStreamFPS (path: string, existingProbe?: FfprobeData) {
+export async function getVideoStreamFPS (path: string, existingProbe?: FfprobeData) {
   const videoStream = await getVideoStream(path, existingProbe)
   if (!videoStream) return 0
 
@@ -182,7 +171,7 @@ async function getVideoStreamFPS (path: string, existingProbe?: FfprobeData) {
   return 0
 }
 
-async function getVideoStreamBitrate (path: string, existingProbe?: FfprobeData): Promise<number> {
+export async function getVideoStreamBitrate (path: string, existingProbe?: FfprobeData): Promise<number> {
   const metadata = existingProbe || await ffprobePromise(path)
 
   let bitrate = metadata.format.bit_rate
@@ -197,19 +186,19 @@ async function getVideoStreamBitrate (path: string, existingProbe?: FfprobeData)
   return undefined
 }
 
-async function getVideoStreamDuration (path: string, existingProbe?: FfprobeData) {
+export async function getVideoStreamDuration (path: string, existingProbe?: FfprobeData) {
   const metadata = existingProbe || await ffprobePromise(path)
 
   return Math.round(metadata.format.duration)
 }
 
-async function getVideoStream (path: string, existingProbe?: FfprobeData) {
+export async function getVideoStream (path: string, existingProbe?: FfprobeData) {
   const metadata = existingProbe || await ffprobePromise(path)
 
   return metadata.streams.find(s => s.codec_type === 'video')
 }
 
-async function hasVideoStream (path: string, existingProbe?: FfprobeData) {
+export async function hasVideoStream (path: string, existingProbe?: FfprobeData) {
   const videoStream = await getVideoStream(path, existingProbe)
 
   return !!videoStream
@@ -219,7 +208,7 @@ async function hasVideoStream (path: string, existingProbe?: FfprobeData) {
 // Chapters
 // ---------------------------------------------------------------------------
 
-async function getChaptersFromContainer (options: {
+export async function getChaptersFromContainer (options: {
   path: string
   maxTitleLength: number
   ffprobe?: FfprobeData
@@ -236,20 +225,3 @@ async function getChaptersFromContainer (options: {
       title: (c['TAG:title'] || '').slice(0, maxTitleLength)
     }))
 }
-
-// ---------------------------------------------------------------------------
-
-export {
-  ffprobePromise,
-  getAudioStream,
-  getChaptersFromContainer,
-  getMaxAudioBitrate,
-  getVideoStream,
-  getVideoStreamBitrate,
-  getVideoStreamDimensionsInfo,
-  getVideoStreamDuration,
-  getVideoStreamFPS,
-  hasAudioStream,
-  hasVideoStream,
-  isAudioFile
-}