78 4. ANALOG-TO-DIGITAL SIGNAL CONVERSION
• Navigate to the GlobalVariable.m file in the Xcode project used above.
• e function printTime has the code that offloads the GUI update code to the main thread.
void printTime(double duration) {
dispatch_async(dispatch_get_main_queue(), ^(void) {
textView = (__bridge UITextView *)(view);
NSString *status = [NSString stringWithFormat:@"Average
Frame Processing Time : %f milliseconds
",
(double)duration*1e3];
textView.text = [textView.text
stringByAppendingString:status];
[textView scrollRangeToVisible:NSMakeRange([[textView text]
length], 0)];
});
}
In this code, the dispatch_async function runs the code inside it on the main thread. is
allows the audio processing code to run while the GUI is updated at the same time.
L4.6 MULTI-RATE SIGNAL PROCESSING
Similar to Android smarpthones, iPhones also suffer from audio latency. e latency can be
minimized by running at a sampling rate of 48 kHz similar to Android smartphones. e op-
timum frame size for all iOS devices is 64 samples and the round-trip latency ranges from
9–11 ms. e same multi-rate signal processing approach in Section L3.8 can be used here to
synchronize the audio i/o and processing modules while generating a low-latency outcome.
L4.7 LAB EXERCISES
1 How much memory is required to buffer a 1-s length recording of audio signal samples?
2 Experiment with various frame sizes and computation delays to find the maximum al-
lowable computation delay for a given frame size. Explain the situations when real-time
processing breaks down.
Hints—Here are the steps that need to be taken for running the prebuilt app code on your
iOS smartphone target.
• Select your device in the scheme window of the Xcode.
• Run the project. is will install the app.
L4. LAB 4: IPHONE AUDIO SIGNAL SAMPLIN 79
You should then be able to run the app. You can record your own audio samples by select-
ing a sampling rate from the app Settings menu. Test the case when recording audio and the
processing takes too long, and then the case when reading from a file and the processing takes
too long.
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