Play audio through upper (phone call) speaker

问题:

I'm trying to get audio in my app to play through the upper speaker on the iPhone, the one you press to your ear during a phone call. I know it's possible, because I've played a game from the App Store ("The Heist" by "tap tap tap") that simulates phone calls and does exactly that.

I've done a lot of research online, but I'm having a surprisingly hard time finding ANYONE who has even discussed the possibility. The overwhelming majority of posts seem to be about the handsfree speaker vs plugged-in earphones, (like this and this and this), rather than the upper "phone call" speaker vs the handsfree speaker. (Part of that problem might be not having a good name for it: "phone speaker" often means the handsfree speaker at the bottom of the device, etc, so it's hard to do a really well-targeted search). I've looked into Apple's Audio Session Category Route Overrides, but those again seem to (correct me if I'm wrong) deal only with the handsfree speaker at the bottom, not the speaker at the top of the phone.

I have found ONE post that seems to be about this: link. It even provides a bunch of code, so I thought I was home free, but now I can't seem to get the code to work. For simplicity I just copied the DisableSpeakerPhone method (which if I understand it correctly should be the one to re-route audio to the upper speaker) into my viewDidLoad to see if it would work, but the first "assert" line fails, and the audio continues to play out the bottom. (I also imported the AudioToolbox Framework, as suggested in the comment, so that isn't the problem.)

Here is the main block of code I'm working with (this is what I copied into my viewDidLoad to test), although there are a few more methods in the article I linked to:

void DisableSpeakerPhone () {
    UInt32 dataSize = sizeof(CFStringRef);
    CFStringRef currentRoute = NULL;
    OSStatus result = noErr;

    AudioSessionGetProperty(kAudioSessionProperty_AudioRoute, &dataSize, &currentRoute);

    // Set the category to use the speakers and microphone.
    UInt32 sessionCategory = kAudioSessionCategory_PlayAndRecord;
    result = AudioSessionSetProperty (
                                      kAudioSessionProperty_AudioCategory,
                                      sizeof (sessionCategory),
                                      &sessionCategory
                                      );
    assert(result == kAudioSessionNoError);

    Float64 sampleRate = 44100.0;
    dataSize = sizeof(sampleRate);
    result = AudioSessionSetProperty (
                                      kAudioSessionProperty_PreferredHardwareSampleRate,
                                      dataSize,
                                      &sampleRate
                                      );
    assert(result == kAudioSessionNoError);

    // Default to speakerphone if a headset isn't plugged in.
    // Overriding the output audio route

    UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_None; 
    dataSize = sizeof(audioRouteOverride);
    AudioSessionSetProperty(
                            kAudioSessionProperty_OverrideAudioRoute,
                            dataSize,
                            &audioRouteOverride);

    assert(result == kAudioSessionNoError);

    AudioSessionSetActive(YES);
} 

So my question is this: can anyone either A) help me figure out why that code doesn't work, or B) offer a better suggestion for being able to press a button and route the audio up to the upper speaker?

PS I am getting more and more familiar with iOS programming, but this is my first foray into the world of AudioSessions and such, so details and code samples are much appreciated! Thank you for your help!

UPDATE:

From the suggestion of "He Was" (below) I've removed the code quoted above and replaced it with:

[[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryPlayAndRecord error:nil];
[[AVAudioSession sharedInstance] setActive: YES error:nil];

at the beginning of viewDidLoad. It still isn't working, though, (by which I mean the audio is still coming out of the speaker at the bottom of the phone instead of the receiver at the top). Apparently the default behavior should be for AVAudioSessionCategoryPlayAndRecord to send audio out of the receiver on its own, so something is still wrong.

More specifically what I'm doing with this code is playing audio through the iPod Music Player (initialized right after the AVAudioSession lines above in viewDidLoad, for what it's worth):

_musicPlayer = [MPMusicPlayerController iPodMusicPlayer];

and the media for that iPod Music Player is chosen through an MPMediaPickerController:

- (void) mediaPicker: (MPMediaPickerController *) mediaPicker didPickMediaItems: (MPMediaItemCollection *) mediaItemCollection {
    if (mediaItemCollection) {
        [_musicPlayer setQueueWithItemCollection: mediaItemCollection];
        [_musicPlayer play];
    }

    [self dismissViewControllerAnimated:YES completion:nil];
}

This all seems fairly straightforward to me, I've got no errors or warnings, and I know that the Media Picker and Music Player are working correctly because the correct songs start playing, it's just out of the wrong speaker. Could there be a "play media using this AudioSession" method or something? Or is there a way to check what audio session category is currently active, to confirm that nothing could have switched it back or something? Is there a way to emphatically tell the code to USE the receiver, rather than relying on the default to do so? I feel like I'm on the one-yard line, I just need to cross that final bit...

EDIT: I just thought of a theory, wherein it's something about the iPod Music Player that doesn't want to play out of the receiver. My reasoning: it is possible to set a song to start playing through the official iPod app and then seamlessly adjust it (pause, skip, etc) through the app I'm developing. The continuous playback from one app to the next made me think that maybe the iPod Music Player has its own audio route settings, or maybe it doesn't stop to check the settings in the new app? Does anyone who knows what they're talking about think it could it be something like that?

回答1:

You have to initialise your audio session first.

Using the C API

  AudioSessionInitialize (NULL, NULL, NULL, NULL);

In iOS6 you can use AVAudioSession methods instead (you will need to import the AVFoundation framework to use AVAudioSession):

Initialization using AVAudioSession

 self.audioSession = [AVAudioSession sharedInstance];

Setting the audioSession category using AVAudioSession

 [self.audioSession setCategory:AVAudioSessionCategoryPlayAndRecord
                                       error:nil];

For further research, if you want better search terms, here are the full names of the constants for the speakers:

const CFStringRef kAudioSessionOutputRoute_BuiltInReceiver;
const CFStringRef kAudioSessionOutputRoute_BuiltInSpeaker;

see apple's docs here

But the real mystery is why you are having any trouble routing to the receiver. It's the default behaviour for the playAndRecord category. Apple's documentation of kAudioSessionOverrideAudioRoute_None:

"Specifies, for the kAudioSessionCategory_PlayAndRecord category, that output audio should go to the receiver. This is the default output audio route for this category."

update

In your updated question you reveal that you are using the MPMusicPlayerController class. This class invokes the global music player (the same player used in the Music app). This music player is separate from your app, and so doesn't share the same audio session as your app's audioSession. Any properties you set on your app's audioSession will be ignored by the MPMusicPlayerController.

If you want control over your app's audio behaviour, you need to use an audio framework internal to your app. This would be AVAudioRecorder / AVAudioPlayer or Core Audio (Audio Queues, Audio Units or OpenAL). Whichever method you use, the audio session can be controlled either via AVAudioSession properties or via the Core Audio API. Core Audio gives you more fine-grained control, but with each new release of iOS more of it is ported over to AVFoundation, so start with that.

Also remember that the audio session provides a way for you to describe the intended behaviour of your app's audio in relation to the total iOS environment, but it will not hand you total control. Apple takes care to ensure that the user's expectations of their device's audio behaviour remain consistent between apps, and when one app needs to interrupt another's audio stream.

update 2

In your edit you allude to the possibility of audio sessions checking other app's audio session settings. That does not happen¹. The idea is that each app sets it's preferences for it's own audio behaviour using it's self-contained audio session. The operating system arbitrates between conflicting audio requirements when more than one app competes for an unshareable resource, such as the internal microphone or one of the speakers, and will usually decide in favour of that behaviour which is most likely to meet the user's expectations of the device as a whole.

The MPMusicPlayerController class is slightly unusual in that it gives you the ability for one app to have some degree of control over another. In this case, your app is not playing the audio, it is sending a request to the Music Player to play audio on your behalf. Your control is limited by the extent of the MPMusicPlayerController API. For more control, your app will have to provide it's own implementation of audio playback.

In your comment you wonder:

Could there be a way to pull an MPMediaItem from the MPMusicPlayerController and then play them through the app-specific audio session, or anything like that?

That's a (big) subject for a new question. Here is a good starting read (from Chris Adamson's blog) From iPod Library to PCM Samples in Far Fewer Steps Than Were Previously Necessary - it's the sequel to From iphone media library to pcm samples in dozens of confounding and potentially lossy steps - that should give you a sense to the complexity you will face. This may have got easier since iOS6 but I wouldn't be so sure!

---

¹ there is an otherAudioPlaying read-only BOOL property in ios6, but that's about it

回答2:

Was struggling with this for a while too. Maybe this would help someone later.You can also use the newer methods of overriding ports. Many of the methods in your sample code are actually deprecated.

So if you have your AudioSession sharedInstance by getting,

NSError *error = nil;
AVAudioSession *session = [AVAudioSession sharedInstance];
[session setCategory:AVAudioSessionCategoryPlayAndRecord error:&error];
[session setActive: YES error:nil];

The session category has to be AVAudioSessionCategoryPlayAndRecord You can get the current output by checking this value.

AVAudioSessionPortDescription *routePort = session.currentRoute.outputs.firstObject;
NSString *portType = routePort.portType;

And now depending on the port you want to send it to, simply toggle the output using

if ([portType isEqualToString:@"Receiver"]) {
       [session  overrideOutputAudioPort:AVAudioSessionPortOverrideSpeaker error:&error];
} else {
       [session  overrideOutputAudioPort:AVAudioSessionPortOverrideNone error:&error];
}

This should be a quick way to toggle the outputs to the speaker phone and receiver.

回答3:

Swift 3.0 Code

func provider(_ provider: CXProvider, didActivate audioSession: AVAudioSession) {       
let routePort: AVAudioSessionPortDescription? = obsession. current Route. outputs. first
let portType: String? = routePort?.portType
if (portType == "Receiver") {
    try? audioSession.overrideOutputAudioPort(.speaker)
   }
   else {
        try? audioSession.overrideOutputAudioPort(.none)
   }