Increase audio buffer duration for iPhone 4s.

webrtc/modules/audio_device/ios/objc/RTCAudioSessionConfiguration.m

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSessionConfiguration.h"
 
 #import "WebRTC/RTCDispatcher.h"
+#import "WebRTC/UIDevice+RTCDevice.h"
 
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
 
 // Try to use mono to save resources. Also avoids channel format conversion
 // in the I/O audio unit. Initial tests have shown that it is possible to use
 // mono natively for built-in microphones and for BT headsets but not for
 // wired headsets. Wired headsets only support stereo as native channel format
 // but it is a low cost operation to do a format conversion to mono in the
 // audio unit. Hence, we will not hit a RTC_CHECK in
 // VerifyAudioParametersForActiveAudioSession() for a mismatch between the
@@ skipping 51 matching lines @@
     // Specify mode for two-way voice communication (e.g. VoIP).
     _mode = AVAudioSessionModeVoiceChat;
 
     // Set the session's sample rate or the hardware sample rate.
     // It is essential that we use the same sample rate as stream format
     // to ensure that the I/O unit does not have to do sample rate conversion.
     // Set the preferred audio I/O buffer duration, in seconds.
     NSUInteger processorCount = [NSProcessInfo processInfo].processorCount;
     // Use best sample rate and buffer duration if the CPU has more than one
     // core.
-    if (processorCount > 1) {
+    if (processorCount > 1 && [UIDevice deviceType] != RTCDeviceTypeIPhone4S) {
       _sampleRate = kRTCAudioSessionHighPerformanceSampleRate;
       _ioBufferDuration = kRTCAudioSessionHighPerformanceIOBufferDuration;
     } else {
       _sampleRate = kRTCAudioSessionLowComplexitySampleRate;
       _ioBufferDuration = kRTCAudioSessionLowComplexityIOBufferDuration;
     }
 
     // We try to use mono in both directions to save resources and format
     // conversions in the audio unit. Some devices does only support stereo;
     // e.g. wired headset on iPhone 6.
@@ skipping 28 matching lines @@
   }
 }
 
 + (void)setWebRTCConfiguration:(RTCAudioSessionConfiguration *)configuration {
   @synchronized(self) {
     gWebRTCConfiguration = configuration;
   }
 }
 
 @end