Make the player's output sample rate configurable

AudioStreaming/Streaming/AudioPlayer/AudioPlayer.swift

@@ -145,8 +145,9 @@ open class AudioPlayer {
         frameFilterProcessor
     }
 
-    /// An `AVAudioFormat` object for the canonical audio stream
-    private var outputAudioFormat: AVAudioFormat = .init(commonFormat: .pcmFormatFloat32, sampleRate: 44100.0, channels: 2, interleaved: true)!
+    /// An `AVAudioFormat` object for the canonical audio stream.
+    /// Built from `configuration.outputSampleRate` in `init` and fixed for the player's lifetime.
+    private let outputAudioFormat: AVAudioFormat
 
     /// Keeps track of the player's state before being paused.
     private var stateBeforePaused: InternalState = .initial
@@ -185,7 +186,13 @@ open class AudioPlayer {
 
     public init(configuration: AudioPlayerConfiguration = .default,
                 clientCertificateProvider: ClientCertificateProvider? = nil) {
-        self.configuration = configuration.normalizeValues()
+        let normalizedConfiguration = configuration.normalizeValues()
+        self.configuration = normalizedConfiguration
+        // `normalizeValues` guarantees a positive rate, so this cannot fail.
+        outputAudioFormat = .init(commonFormat: .pcmFormatFloat32,
+                                  sampleRate: normalizedConfiguration.outputSampleRate,
+                                  channels: 2,
+                                  interleaved: true)!
         let engine = AVAudioEngine()
         audioEngine = engine
         rendererContext = AudioRendererContext(configuration: configuration, outputAudioFormat: outputAudioFormat)

AudioStreaming/Streaming/AudioPlayer/AudioPlayerConfiguration.swift

@@ -20,6 +20,15 @@ public struct AudioPlayerConfiguration: Equatable {
     /// - note: Must be larger that `bufferSizeInSeconds`
     let secondsRequiredToStartPlayingAfterBufferUnderrun: Int
 
+    /// The sample rate, in hertz, of the player's output format.
+    ///
+    /// Decoded audio is converted to this rate before it reaches the engine, so it should match
+    /// the rate the output hardware is actually running at — on iOS,
+    /// `AVAudioSession.sharedInstance().sampleRate`. Leaving it at the 44100 default on hardware
+    /// running at another rate makes the system resample a second time: a 48 kHz source is
+    /// converted down to 44100 here and back up to 48000 by the output unit.
+    let outputSampleRate: Double
+
     /// Enables the internal logs
     let enableLogs: Bool
 
@@ -28,6 +37,7 @@ public struct AudioPlayerConfiguration: Equatable {
                                                            secondsRequiredToStartPlaying: 1,
                                                            gracePeriodAfterSeekInSeconds: 0.5,
                                                            secondsRequiredToStartPlayingAfterBufferUnderrun: 7,
+                                                           outputSampleRate: 44100,
                                                            enableLogs: false)
     /// Initializes the configuration for the `AudioPlayer`
     ///
@@ -38,20 +48,23 @@ public struct AudioPlayerConfiguration: Equatable {
     /// - parameter secondsRequiredToStartPlaying: Number of seconds of audio required to before playback first starts.
     /// - parameter gracePeriodAfterSeekInSeconds: Number of seconds of audio required after seek occurs.
     /// - parameter secondsRequiredToStartPlayingAfterBufferUnderrun: Number of seconds of audio required to before playback resumes after a buffer underrun
+    /// - parameter outputSampleRate: Sample rate, in hertz, of the player's output format
     /// - parameter enableLogs: Enables the internal logs
     ///
     public init(flushQueueOnSeek: Bool = true,
                 bufferSizeInSeconds: Double = 10,
                 secondsRequiredToStartPlaying: Double = 1,
                 gracePeriodAfterSeekInSeconds: Double = 0.5,
                 secondsRequiredToStartPlayingAfterBufferUnderrun: Int = 1,
+                outputSampleRate: Double = 44100,
                 enableLogs: Bool = true)
     {
         self.flushQueueOnSeek = flushQueueOnSeek
         self.bufferSizeInSeconds = bufferSizeInSeconds
         self.secondsRequiredToStartPlaying = secondsRequiredToStartPlaying
         self.gracePeriodAfterSeekInSeconds = gracePeriodAfterSeekInSeconds
         self.secondsRequiredToStartPlayingAfterBufferUnderrun = secondsRequiredToStartPlayingAfterBufferUnderrun
+        self.outputSampleRate = outputSampleRate
         self.enableLogs = enableLogs
         Logger.isEnabled = enableLogs
     }
@@ -76,11 +89,17 @@ public struct AudioPlayerConfiguration: Equatable {
             ? defaults.secondsRequiredToStartPlayingAfterBufferUnderrun
             : self.secondsRequiredToStartPlayingAfterBufferUnderrun
 
+        // A non-positive rate would make `AVAudioFormat` fail to initialise.
+        let outputSampleRate = self.outputSampleRate > 0
+            ? self.outputSampleRate
+            : defaults.outputSampleRate
+
         return AudioPlayerConfiguration(flushQueueOnSeek: flushQueueOnSeek,
                                         bufferSizeInSeconds: bufferSizeInSeconds,
                                         secondsRequiredToStartPlaying: secondsRequiredToStartPlaying,
                                         gracePeriodAfterSeekInSeconds: gracePeriodAfterSeekInSeconds,
                                         secondsRequiredToStartPlayingAfterBufferUnderrun: secondsRequiredToStartPlayingAfterBufferUnderrun,
+                                        outputSampleRate: outputSampleRate,
                                         enableLogs: enableLogs)
     }
 }