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114 lines
6.9 KiB
114 lines
6.9 KiB
7 months ago
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## 5.6\. Audio Latency
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Audio latency is the time delay as an audio signal passes through a system.
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Many classes of applications rely on short latencies, to achieve real-time
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sound effects.
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For the purposes of this section, use the following definitions:
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* **output latency**. The interval between when an application writes a frame
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of PCM-coded data and when the corresponding sound is presented to environment
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at an on-device transducer or signal leaves the device via a port and can be
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observed externally.
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* **cold output latency**. The output latency for the first frame, when the
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audio output system has been idle and powered down prior to the request.
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* **continuous output latency**. The output latency for subsequent frames,
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after the device is playing audio.
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* **input latency**. The interval between when a sound is presented by
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environment to device at an on-device transducer or signal enters the device via
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a port and when an application reads the corresponding frame of PCM-coded data.
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* **lost input**. The initial portion of an input signal that is unusable or
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unavailable.
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* **cold input latency**. The sum of lost input time and the input latency
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for the first frame, when the audio input system has been idle and powered down
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prior to the request.
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* **continuous input latency**. The input latency for subsequent frames,
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while the device is capturing audio.
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* **cold output jitter**. The variability among separate measurements of cold
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output latency values.
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* **cold input jitter**. The variability among separate measurements of cold
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input latency values.
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* **continuous round-trip latency**. The sum of continuous input latency plus
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continuous output latency plus one buffer period. The buffer period allows
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time for the app to process the signal and time for the app to mitigate phase
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difference between input and output streams.
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* **OpenSL ES PCM buffer queue API**. The set of PCM-related
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[OpenSL ES](https://developer.android.com/ndk/guides/audio/opensl/index.html)
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APIs within [Android NDK](https://developer.android.com/ndk/index.html).
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* **AAudio native audio API**. The set of
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[AAudio](https://developer.android.com/ndk/guides/audio/aaudio/aaudio.html) APIs
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within [Android NDK](https://developer.android.com/ndk/index.html).
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* **Timestamp**. A pair consisting of a relative frame position within a
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stream and the estimated time when that frame enters or leaves the
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audio processing pipeline on the associated endpoint. See also
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[AudioTimestamp](https://developer.android.com/reference/android/media/AudioTimestamp).
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* **glitch**. A temporary interruption or incorrect sample value in the audio signal,
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typically caused by a
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[buffer underrun](https://en.wikipedia.org/wiki/Buffer_underrun) for output,
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buffer overrun for input, or any other source of digital or analog noise.
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If device implementations declare `android.hardware.audio.output`, they
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MUST meet or exceed the following requirements:
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* [C-1-1] The output timestamp returned by
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[AudioTrack.getTimestamp](https://developer.android.com/reference/android/media/AudioTrack.html#getTimestamp(android.media.AudioTimestamp))
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and `AAudioStream_getTimestamp` is accurate to +/- 2 ms.
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* [C-1-2] Cold output latency of 500 milliseconds or less.
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If device implementations declare `android.hardware.audio.output` they are
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STRONGLY RECOMMENDED to meet or exceed the following requirements:
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* [C-SR] Cold output latency of 100 milliseconds or less. Existing and new
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devices that run this version of Android are VERY STRONGLY RECOMMENDED
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to meet these requirements now. In a future platform release in 2021, we
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will require Cold output latency of 200 ms or less as a MUST.
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* [C-SR] Continuous output latency of 45 milliseconds or less.
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* [C-SR] Minimize the cold output jitter.
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* [C-SR] The output timestamp returned by
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[AudioTrack.getTimestamp](https://developer.android.com/reference/android/media/AudioTrack.html#getTimestamp(android.media.AudioTimestamp))
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and `AAudioStream_getTimestamp` is accurate to +/- 1 ms.
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If device implementations meet the above requirements, after any initial
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calibration, when using both the OpenSL ES PCM buffer queue and AAudio native audio APIs,
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for continuous output latency and cold output latency over at least one supported audio
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output device, they are:
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* [C-SR] STRONGLY RECOMMENDED to report low-latency audio by declaring
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`android.hardware.audio.low_latency` feature flag.
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* [C-SR] STRONGLY RECOMMENDED to meet the requirements for low-latency
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audio via the AAudio API.
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* [C-SR] STRONGLY RECOMMENDED to ensure that for streams that return
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[`AAUDIO_PERFORMANCE_MODE_LOW_LATENCY`](https://developer.android.com/ndk/guides/audio/aaudio/aaudio#performance-mode)
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from [`AAudioStream_getPerformanceMode()`](https://developer.android.com/ndk/reference/group/audio#aaudiostream_getperformancemode),
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the value returned by [`AAudioStream_getFramesPerBurst()`](https://developer.android.com/ndk/reference/group/audio#aaudiostream_getframesperburst)
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is less than or equal to the value returned by [`android.media.AudioManager.getProperty(String)`](https://developer.android.com/reference/android/media/AudioManager.html#getProperty%28java.lang.String%29)
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for property key [`AudioManager.PROPERTY_OUTPUT_FRAMES_PER_BUFFER`](https://developer.android.com/reference/android/media/AudioManager.html#PROPERTY_OUTPUT_FRAMES_PER_BUFFER).
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If device implementations do not meet the requirements for low-latency audio
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via both the OpenSL ES PCM buffer queue and AAudio native audio APIs, they:
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* [C-2-1] MUST NOT report support for low-latency audio.
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If device implementations include `android.hardware.microphone`, they
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MUST meet these input audio requirements:
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* [C-3-1] Limit the error in input timestamps, as returned by
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[AudioRecord.getTimestamp](https://developer.android.com/reference/android/media/AudioRecord.html#getTimestamp(android.media.AudioTimestamp,%20int))
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or `AAudioStream_getTimestamp`, to +/- 2 ms.
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"Error" here means the deviation from the correct value.
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* [C-3-2] Cold input latency of 500 milliseconds or less.
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If device implementations include `android.hardware.microphone`, they are
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STRONGLY RECOMMENDED to meet these input audio requirements:
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* [C-SR] Cold input latency of 100 milliseconds or less. Existing and new
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devices that run this version of Android are VERY STRONGLY RECOMMENDED
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to meet these requirements now. In a future platform release in 2021 we
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will require Cold input latency of 200 ms or less as a MUST.
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* [C-SR] Continuous input latency of 30 milliseconds or less.
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* [C-SR] Continuous round-trip latency of 50 milliseconds or less.
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* [C-SR] Minimize the cold input jitter.
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* [C-SR] Limit the error in input timestamps, as returned by
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[AudioRecord.getTimestamp](https://developer.android.com/reference/android/media/AudioRecord.html#getTimestamp(android.media.AudioTimestamp,%20int))
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or `AAudioStream_getTimestamp`, to +/- 1 ms.
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