This sample demonstrates how to build a simple musical game. The objective of the game is to copy the clap patterns you hear by tapping on the screen.
The UI is deliberately very simple - just tap anywhere in the grey area after hearing the claps.
The game plays the clap sounds on the first 3 beats of the bar. These are played in time with the backing track.
When the user taps on the screen, a clap sound is played and the game checks whether the tap occurred within an acceptable time window.
Oboe provides the AudioStream class and associated objects to allow the sample to output audio data to the audio device. All other objects are provided by the sample.
Each time the AudioStream needs more audio data it calls AudioDataCallback::onAudioReady. This passes a container array named audioData to the Game object which must then fill the array with numFrames of audio frames.
The sample uses the following optimizations to obtain a low latency audio stream:
- Performance mode set to Low Latency
- Sharing mode set to Exclusive
- Buffer size set to twice the number of frames in a burst (double buffering)
The RenderableAudio interface (abstract class) represents objects which can produce frames of audio data. The SoundRecording and Mixer objects both implement this interface.
Both the clap sound and backing tracks are represented by SoundRecording objects which are then mixed together using a Mixer.
It is very important that the audio thread (which calls the onAudioReady method) is never blocked. Blocking can cause underruns and audio glitches. To avoid blocking we use a LockFreeQueue to share information between the audio thread and other threads. The following diagram shows how claps are enqueued by pushing the clap times (in frames) onto the queue, then dequeuing the clap time when the clap is played.
We also use atomics to ensure that threads see a consistent view of any shared primitives.
When a tap event arrives on the UI thread it only contains the time (milliseconds since boot) that the event occurred. We need to figure out how far along the audio track was when the tap occurred. Put another way, we need to convert milliseconds since boot to audio frames.
To do this we use a reference point which stores the current frame number in the audio timeline and the milliseconds since boot which the frame was rendered. This reference point is updated each time the onAudioReady method is called. This enables us to keep the UI in sync with the audio timeline.
- The game will only work with audio devices which have a sample rate of 48,000 samples per second and 2 output channels (stereo). Overcoming this limitation requires either resampling the raw PCM files to match the target audio device, or resampling the audio data on-the-fly before sending it to the audio stream. A later version of this sample may include this feature.