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Media: audio and recording
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12.2 Recording audio
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In past versions of the SDK, audio recording was a daunting task. It required intimate knowledge of audio processing as well as a lot of code. With the release of the 3.0 SDK, Apple has provided a much simpler way of achieving this functionality. You can find all the classes for recording audio in the AV Foundation framework. In order to use these classes, you must add AVFoundation.framework to your project. We ll look at how to initialize and control the audio reader and also how to respond to its associated events.
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12.2.1 Initializing the audio recorder
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When you re initializing a new AVAudioRecorder object, you should avoid using the default constructor init. This is to reduce complexity, because this class requires quite a bit of configuration. The constructor you should use is initWithURL:settings: error. It allows you to specify the location on disk to record the audio to as well as provide various audio settings. The first parameter is the location where the recording will be stored. Although it s expressed as an NSURL, it s really a local path that points to a location on disk. In most cases, you ll want to store recordings in the Documents directory. The next parameter is an NSDictionary that contains the settings for the recording. Table 12.5 lists some of the settings that you may want to consider when setting up your recorder.
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Table 12.5 Basic audio settings for AVAudioRecorder Setting key AVSampleRateKey Description A sample rate, in Hertz, expressed as an NSNumber floatingpoint value. A format identifier. A common value for this is kAudioFormatAppleLossless. The number of channels expressed as an NSNumber integer value. You can set this value to 1. A key that refers to the quality of the audio being played.
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AVFormatIDKey
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AVNumberOfChannelsKey
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AVEncoderAudioQualityKey
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You can specify quite a few other settings when creating your recorder. These settings are all optional and have default values; you can use them to fine-tune your audio recording. Listing 12.3 demonstrates how to build an AVAudioRecorder object with some basic settings.
Listing 12.3 Initialization code for AVAudioRecorder
NSString * filePath = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents/recording.caf"]; NSDictionary *recordSettings =
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Recording audio
[[NSDictionary alloc] initWithObjectsAndKeys: [NSNumber numberWithFloat: 44100.0], AVSampleRateKey, [NSNumber numberWithInt: kAudioFormatAppleLossless],AVFormatIDKey, [NSNumber numberWithInt: 1], AVNumberOfChannelsKey, [NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,nil]; AVAudioRecorder * soundRecorder = [[AVAudioRecorder alloc] initWithURL: [NSURL fileURLWithPath:filePath] settings: recordSettings error: nil];
Note the filePath. This is an NSString that points to a file named recording.caf in the Documents directory. This path is converted to an NSURL during the construction of the recorder.
12.2.2 Controlling the audio recorder
After you construct an AVAudioRecorder, you have quite a bit of control over it. Table 12.6 lists all the methods you can call on a recorder to control the recording session. The following code shows how to make a simple toggleRecord method that can be used as an IBAction for a button. The code assumes you ve created a few global properties. Theses properties include recording of type BOOL and soundRecorder of type AVAudioRecorder:
- (IBAction) toggleRecord:(id) sender { if (recording) { [soundRecorder stop]; } else { [soundRecorder record]; } recording = !recording; } Table 12.6 Methods to control audio recording Method - (BOOL)prepareToRecord Description Creates the recording file on disk at the specified URL path. This method also prepares the system for recording. Starts or resumes recording. This method implicitly calls the prepareToRecord method. Starts the recorder and records for a specified amount of time. Pauses a recording. To resume recording, call the record method again. Stops the recording and closes the audio file. Deletes the current recording. For this method to work, the recording must be stopped.
- (BOOL)record
- (BOOL)recordForDuration: (NSTimeInterval)duration - (void)pause
- (void)stop - (BOOL)deleteRecording
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Media: audio and recording
When toggleRecord is called for the first time, record is set to NO. This starts the audio recording and sets the recording property to YES. The system creates the recording file and begins receiving input from the built-in microphone. If the device s headset is plugged in, the system uses the headset s microphone instead. The second time toggleRecord is called, the recorder stops recording. This closes the audio file and allows it to be played. The recording property is also set to NO.
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