5.6. Audio Latency
Audio latency is the time delay as an audio signal passes through a system. Many classes of applications rely on short latencies, to achieve real-time sound effects.
For the purposes of this section, use the following definitions:
- output latency. The interval between when an application writes a frame of PCM-coded data and when the corresponding sound is presented to environment at an on-device transducer or signal leaves the device via a port and can be observed externally.
- cold output latency. The output latency for the first frame, when the audio output system has been idle and powered down prior to the request.
- continuous output latency. The output latency for subsequent frames, after the device is playing audio.
- input latency. The interval between when a sound is presented by environment to device at an on-device transducer or signal enters the device via a port and when an application reads the corresponding frame of PCM-coded data.
- lost input. The initial portion of an input signal that is unusable or unavailable.
- cold input latency. The sum of lost input time and the input latency for the first frame, when the audio input system has been idle and powered down prior to the request.
- continuous input latency. The input latency for subsequent frames, while the device is capturing audio.
- cold output jitter. The variability among separate measurements of cold output latency values.
- cold input jitter. The variability among separate measurements of cold input latency values.
- continuous round-trip latency. The sum of continuous input latency plus continuous output latency plus one buffer period. The buffer period allows time for the app to process the signal and time for the app to mitigate phase difference between input and output streams.
- OpenSL ES PCM buffer queue API. The set of PCM-related OpenSL ES APIs within Android NDK.
- AAudio native audio API. The set of AAudio APIs within Android NDK.
- Timestamp. A pair consisting of a relative frame position within a stream and the estimated time when that frame enters or leaves the audio processing pipeline on the associated endpoint. See also AudioTimestamp.
If device implementations declare
android.hardware.audio.output they are STRONGLY RECOMMENDED to meet or exceed the following requirements:
- [C-SR] Cold output latency of 100 milliseconds or less
- [C-SR] Continuous output latency of 45 milliseconds or less
- [C-SR] Minimize the cold output jitter
- [C-SR] The output timestamp returned by AudioTrack.getTimestamp and
AAudioStream_getTimestamp is accurate to +/- 1 ms.
If device implementations meet the above requirements, after any initial calibration, when using both the OpenSL ES PCM buffer queue and AAudio native audio APIs, for continuous output latency and cold output latency over at least one supported audio output device, they are:
If device implementations do not meet the requirements for low-latency audio via both the OpenSL ES PCM buffer queue and AAudio native audio APIs, they:
- [C-1-1] MUST NOT report support for low-latency audio.
If device implementations include
android.hardware.microphone, they are STRONGLY RECOMMENDED to meet these input audio requirements:
- [C-SR] Cold input latency of 100 milliseconds or less.
- [C-SR] Continuous input latency of 30 milliseconds or less.
- [C-SR] Continuous round-trip latency of 50 milliseconds or less.
- [C-SR] Minimize the cold input jitter.
- [C-SR] Limit the error in input timestamps, as returned by AudioRecord.getTimestamp or
AAudioStream_getTimestamp, to +/- 1 ms.