need to understad how AudioRecord and AudioTrack work for raw PCM capture and playback

问题

I use the following code in a Thread to capture raw audio samples from the microphone and play it back through the speaker.

public void run(){
            short[] lin = new short[SIZE_OF_RECORD_ARRAY];
            int num = 0;
            // am = (AudioManager) this.getSystemService(Context.AUDIO_SERVICE); // -> MOVED THESE TO init()
            // am.setMode(AudioManager.MODE_IN_COMMUNICATION);
            record.startRecording();
            track.play();
            while (passThroughMode) {
            // while (!isInterrupted()) {
                num = record.read(lin, 0, SIZE_OF_RECORD_ARRAY);
                for(i=0;i<lin.length;i++)
                    lin[i] *= WAV_SAMPLE_MULTIPLICATION_FACTOR; 
                track.write(lin, 0, num);
            }
            // /*
            record.stop();
            track.stop();
            record.release();
            track.release();
            // */
        }  

where record is an AudioRecord and track is an Audiotrack. I need to know in detail (and in a simplified way if possible) how the AudioRecord stores PCM data and AudioTrack plays PCM data. This is how I have understood it so far:

As the while() loop is continuously running, record obtains SIZE_OF_RECORD_ARRAY number of samples (which is 1024 for now) as shown in the figure. The samples get saved contiguously in the lin[] array of shorts (16 bit shorts, as I am using 16 bit PCM encoding). This is done by record.read(). Then track.write() places these samples in the speaker which is played by the hardware. Is this correct or am I missing something here?

回答1:

As for how the samples are laid out in memory; they're just arrays of linear approximations to a sound wave, taken at discrete times (like your figure shows). In the case of stereo, the samples will be interleaved (LRLRLRLR...).

When it comes to the path the audio takes, you're essentially right, although there are a few more steps involved:

• Writing data to your Java AudioTrack causes it to make a JNI (Java Native Interface) call to a native helper class, which in turn calls the native AudioTrack class.

• The AudioTracks are owned by the AudioFlinger, which periodically takes data from all the AudioTracks on a given output thread (which have been mixed by the AudioMixer) and writes it to the audio HAL output stream class.

• From there the data goes to the user-space ALSA library, and through a couple of intermediate steps to the kernel-space PCM driver. Then further on from there; typically going through some kind of DSP that applies various acoustic compensation filters, and eventually making it's way to the hardware codec, which controls the speaker DAC and amplifiers.

When recording from the internal microphone(s) you'd have more or less the same steps, except that they'd be done in the opposite order.

Note that some of these steps (essentially everything from the audio HAL and below) are platform-specific, and therefore might differ between platforms from different vendors (and even different platforms from the same vendor).

来源：https://stackoverflow.com/questions/18349054/need-to-understad-how-audiorecord-and-audiotrack-work-for-raw-pcm-capture-and-pl