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Audio Latency Tracing

The “audio.latency” tracing category is a tool to measure internal chromium audio latency, without any hardware dependencies. This document outlines how to record and analyse a latency trace, and offers suggestions as to how to construct test pages.

[TOC]

Overview

audio.latency tracing overview

When an input stream detects an audio signal peak (e.g. clapping hands in front of microphone), we start an “audio.latency” trace event. When the output stream detects an audio peak, we stop the tracing event. The duration of the trace event should give us the total internal Chrome latency. This excludes any latency incured from hardware such as a microphone or sound card's internal buffers.

Measuring Latency

Prerequisites

  • The computer must have a microphone input accessible through getUserMedia()
  • Only the test page and chrome://tracing should be open.
  • The test page must have one call to getUserMedia() + one audio output (e.g. one media element OR one WebAudio graph).
  • The surrounding area should be relatively quiet.
  • If echo cancellation is disabled, consider lowering speaker volume or OS volume.
  • Trace recording must be started before any streams are created by the test page, or there will be no traces. One can check that there are no Input Controllers or Output Streams in the chrome://media-internals audio tab, as pictured below. No streams

Steps

  • Open the latest version of Chrome (114.0.5695.0 or later).
  • Open chrome://tracing.
  • Record a new trace selecting the “audio” and the “audio.latency” categories. Selecting tracing categories
  • In a separate tab, open the test page.
  • Initialize the test page, so that it is capturing audio using getUserMedia() and playing out the captured stream.
  • Repeatedly clap next to the microphone, clearly and loudly, waiting roughly 1 second between claps.
  • Stop the trace.
  • Look for the AmplitudePeak trace event: the wall duration corresponds to the measured latency Viewing results

Test pages

Baseline latency test page

The simplest latency test page involves directly routing a getUserMedia() MediaStream to an <audio> element or WebAudio. Such a web page should allow us to measure the round trip latency within Chrome, between the audio service and the renderer process. For convenience, a simple test page is checked in under third_party/blink/manual_tests/audio_latency.html. The audio signal flow for this test pages matches the diagram in the overview.

Codec latency test page

Adding an encoding and decoding step to a simple baseline test web page should allow us to measure the extra latency incurred by encoding and decoding audio. No such test page is provided, but the signal flow should follow this diagram:

Codec latency test page diagram

Network end-to-end latency test

Adding a network round trip to a codec latency test page should simulate the expected end-to-end latency in real-world scenarios. The sender and receiver test pages can live in two tabs, if only one page calls getUserMedia() and one page plays out audio. Again, no example test page is provided, but the signal flow should follow this diagram:

Network latency test page diagram