How to tell if IOS device only supports 48kHz in hardware

Question

Newer IOS devices like the 6S only support native 48kHz playback. Not really much of a problem since standard CoreAudio graphs resample just fine. Problem is, if you're doing a VOIP type of app with the voice processing unit, you can't set the phone to 44.1kHz; it creates a nice Darth-Vader like experience!

Formerly, I used to check the model of the device and simply say 'If it's a 6S or later, then I have to resample 44.1 to 48kHz), and this worked fine. I didn't like this fix, so I tried the following code:

session = [AVAudioSession sharedInstance];

[session setActive:YES error:&nsError];

if (systemSampleRate == 44100)      // We may need to resample if it's a phone that only supports 48kHz like the 6S or 6SPlus
{
    [session setCategory:AVAudioSessionCategoryPlayback
             withOptions:0
                   error:&nsError];

    result = [session setPreferredSampleRate:systemSampleRate error:&nsError];

    hardwareSampleRate = [session sampleRate];

    NSLog (@"Phone reports sample rate of %f", hardwareSampleRate);

    if (hardwareSampleRate != (double)systemSampleRate)     // We can't set it!!!!
        needsResampling = YES;
    else
    {
        [session setCategory:AVAudioSessionCategoryRecord
                 withOptions:AVAudioSessionCategoryOptionAllowBluetooth
                       error:&nsError];

        result = [session setPreferredSampleRate:systemSampleRate error:&nsError];

        hardwareSampleRate = [session sampleRate];

        if (hardwareSampleRate != (double)systemSampleRate)     // We can't set it!!!!
            needsResampling = YES;
        else
            needsResampling = NO;
    }
}

MOST of the time, this works. The 6S devices would report 48kHz, and all others would report 44.1kHz. BUT, if it had been tied to a bluetooth headset type of system that only supports 8kHz mic audio and 44.1kHz playback, the first hardwareSample Rate value reports 44.1!!!! So I go ahead thinking the device natively supports 44.1 and everything screws up.

SO the question is: how do I find out if the native playback device on IOS physically only supports 48kHz, or can support both 44.1 and 48kHz? Apple's public document on this is worthless, it simply chastises people for assuming a device supports both without telling you how to figure it out.

Answer 1

You really do just have to assume that the sample rate can change. If systemSampleRate is an external requirement, try to set the sample rate to that, and then work with what you get. The catch is that you have to do this check every time your audio render chain starts or is interrupted in case the sample rate changes.

I use two different ways to handle this, both involve tearing down and reinitializing my audio unit chain if the sample rate changes.

One simple way is to make all of my audio unit's sample rates the system sample rate (provided by the sample rate property on an active audio session). I assume that this is the highest quality method as there is no sample rate conversion.

If I have a sample rate requirement I will create my chain with my required sample rate. then check if the system sample rate is different from my requirement. If it is different, I will put converter units between the system unit (remote io) and the ends of my chain.

The bottom line is that the most important information is whether or not the system sample rate is different from your requirement, not whether or not it can change. It's a total pain, and a bunch of audio apps broke when the 6S came out, but it's the right way to handle it moving forward.