This post discusses how to record PCM audio on Android with the android.media.AudioRecord class. If you’re not familiar with PCM, please read a previous post PCM Audio Format.
For the source code, please refer to AndroidPCMRecorder.
1. State Transition
The source code records the PCM audio with PcmAudioRecorder.java, which uses Android AudioRecord internally. Similar to the Android MediaRecorder class, PcmAudioRecorder follows a simple state machine as shown below.
Figure 1. State Transition of PcmAudioRecorder Class
As indicated in the diagram, we initialize a PcmAudioRecorder class by either calling the getInstance static method or the constructor to get into INITIALIZING state. We can then set the output file path and call prepare to get into PERPARED state. We can then start recording by calling start method and finally call stop to stop the recording. At any state except ERROR, we can call reset to get back to INITIALIZING state. When we’re done with recording, we can call release to discard the PcmAudioRecorder object.
2. Filling the Buffer with Data and Write to File
One particular part of the code requires a bit attention is the updateListener. We register the listener with the AudioRecord object using setRecordPositionUpdateListener method. The listener is an interface with two abstract methods, namely onMarkerReached and onPeriodicNotification. We implemented the onPeriodicNotification method to pull the audio data from AudioRecord object and save it to the output file.
In order for the listener to work, we need to specify the notification period by calling AudioRecord.setPositionNotificationPeriod(int) to specify how frequently we want the listener to be triggered and pull data. The method accepts a single argument, which indicates the update period in number of frames. This leads us to next section.
3. Frame vs Sample
For PCM audio, a frame consists of the set of samples from all channels at a given point of time. In other words, the number of frames in a second is equal to sample rate.
However, when the audio is compressed (encoded further to mp3, aac etc.), a frame consists of compressed data for a whole series of samples with additional, non-sample data. For such audio formats, the sample rate and sample size refer to data after decoded to PCM, and it’s completely different from frame rate and frame size.
In our sample code, we set the update period for setPositionNotificationPeriod as number of frames in every 100 millisecond, therefore the listener will be triggered every 100 milliseconds, and we can pull data and update the recording file every 100 milliseconds.
Note that source code is modified based on ExtAudioRecorder.java.