Some special terms for Android audio development (to be translated)

Audio terminologyIn this DOCUMENT

1. Generic Terms

1. Digital Audio

2. Hardware and Accessories

3. Audio Signal Path

2. Android-specific Terms

3. Sample Rate Conversion

This document provides a glossary of audio-related terminology, including a list of widely used, generic terms and a list Of terms that is specific to Android.

Generic Terms

These is audio terms that is widely used, with their conventional meanings.

Digital Audio

• Acoustics

• The study of the mechanical properties of the, for example how the physical placement of transducers such as speakers an D microphones on a device affects perceived audio quality.

• Attenuation

• A multiplicative factor less than or equal to 1.0, applied to an audio signal to decrease the signal level. Compare to "gain".

• Bits per sample or bit depth

• Number of bits of information per sample.

• Channel

• A single stream of audio information, usually corresponding to one location of recording or playback.

• Downmixing

• To decrease the number of channels, e.g. from stereo to mono, or from 5.1 to stereo. This can is accomplished by dropping some channels, mixing channels, or more advanced signal processing. Simple mixing without attenuation or limiting have the potential for overflow and clipping. Compare to "upmixing".

• Duck

• To temporarily reduce the volume of one stream, when another stream becomes active. For example, if music was playing and a notification arrives, then the music stream could being ducked while the notification Plays. Compare to "mute".

• Frame

• A set of samples, one per channel, at a point in time.

• Frames per buffer

• The number of frames handed from one module to the next at once; For example the audio HAL interface uses this concept.

• Gain

• A multiplicative factor greater than or equal to 1.0, applied to an audio signal to increase the signal level. Compare to "attenuation".

• Hz

• The units for sample rate or frame rate.

• Latency

• Time delay as a signal passes through a system.

• Mono

• One channel.

• Multichannel

• See ' Surround sound '. Strictly, since stereo is more than one channel, it's also "multi" channel. But the usage would be confusing.

• Mute

• to (temporarily) force volume to is zero, independently from the usual volume controls.

• Pcm

• Pulse Code modulation, the most common low-level encoding of digital audio. The audio signal is sampled at a regular interval, called the sample rate, and then quantized to discrete values within a Particular range depending on the bit depth. For example, for 16-bit PCM, the sample values is integers between-32768 and +32767.

• Ramp

• To gradually increase or decrease the level of a particular audio parameter, for example volume or the strength of an Effe Ct. A volume ramp is commonly applied when pausing and resuming music, to avoid a hard audible transition.

• Sample

• A number representing the audio value for a, single channel at A, in time.

• Sample rate or frame rate

• Number of frames per second; Note that ' frame rate ' is thus more accurate, but ' sample rate ' is conventionally used to mean ' frame rate. '

• Sonification

• The use of sound to express feedback or information, for example touch sounds and keyboard sounds.

• Stereo

• The channels.

• Stereo widening

• An effect applied to a stereo signal, to make another stereo signal which sounds fuller and richer. The effect can also was applied to a mono signal, in which case it was a type of upmixing.

• Surround sound

• Various techniques for increasing the ability of a listener to perceive sound position beyond stereo left and right.

• Upmixing

• To increase the number of channels, e.g. from mono to stereo, or from stereo to surround. This can is accomplished by duplication, panning, or more advanced signal processing. Compare to "downmixing".

• Virtualizer

• An effect this attempts to spatialize audio channels, such as trying to simulate more speakers, or give the illusion Various sound sources has position.

• Volume

• Loudness, the subjective strength of an audio signal.

Hardware and Accessories

These terms is related to audio hardware and accessories.

Inter-device Interconnect

These technologies connect audio and video components between devices, and is readily visible at the external connectors. The HAL implementor may need to being aware of these, as well as the end user.

• Bluetooth

• A Short Range wireless technology. The major audio-related Bluetooth profiles and Bluetooth protocols is described at these Wikipedia articles:

• A2DP for Music

• SCO for telephony

• DisplayPort

• Digital display interface by VESA.

• Hdmi

• High-definition Multimedia Interface, an Interface for transferring audio and video data. For mobile devices, either a MICRO-HDMI (type D) or MHL connector is used.

• MHL

• Mobile High-definition Link is a mobile Audio/video interface, often over MICRO-USB connector.

• Phone connector

• A mini or Sub-mini phone connector connects a device to wired headphones, headset, or line-level amplifier.

• SlimPort

• An adapter from Micro-usb to HDMI.

• S/PDIF

• Sony/philips Digital Interface Format is a interconnect for uncompressed PCM. See Wikipedia article S/PDIF.

• Usb

• Universal Serial Bus. See Wikipedia article USB.

Intra-device Interconnect

These technologies connect internal audio components within a given device, and is not visible without disassembling the Device. The HAL implementor may need to being aware of these, but not the end user.

See these Wikipedia articles:

• GPIO

• I²C

• I²S

• McASP

• SLIMbus

• Spi

Audio Signal Path

These terms is related to the signal path, which audio data follows from a application to the transducer, or VICE-VERSA.

• Adc

• Analog to Digital Converter, a module this converts an Analog signal (continuous in both time and amplitude) to a digital Signal (discrete in both time and amplitude). Conceptually, an ADC consists of a periodic sample-and-hold followed by a quantizer, although it does not having to be imple Mented that. An ADC was usually preceded by a low-pass filter to remove any high frequency the components of is not representable using T He desired sample rate. See Wikipedia article Analog-to-digital_converter.

• Ap

• Application processor, the main general-purpose computer on a mobile device.

• Codec

• Coder-decoder, a module, encodes and/or decodes an audio signal from one representation to another. Typically this was analog to PCM, or PCM to analog. Strictly, the term "codec" was reserved for modules that both encode and decode, however it can also more loosely refer to Only one of these. See Wikipedia article Audio codec.

• Dac

• Digital to Analog converter, a-module that converts a digital signal (discrete-both time and amplitude) to an analog Si Gnal (continuous in both time and amplitude). A DAC is usually followed by a low-pass filter to remove any high frequency components introduced by digital quantization. See Wikipedia article Digital-to-analog Converter.

• Dsp

• Digital Signal Processor, an optional component which are typically located after the application Processor (for output), O R before the application processor (for input). The primary purpose of a DSP is to off-load the application processor, and provide signal processing-features at a lower p Ower cost.

• Pdm

• Pulse-density modulation is a form of modulation used to represent an analog signal by a digital signal, where the relativ e density of 1s versus 0s indicates the signal level. It is commonly used by digital to analog converters. See Wikipedia article pulse-density modulation.

• Pwm

• Pulse-width modulation is a form of modulation used to represent an analog signal by a digital signal, where the relative Width of a digital pulse indicates the signal level. It is commonly used by analog to digital converters. See Wikipedia article Pulse-width modulation.

Android-specific Terms

These is terms specific to the Android audio framework, or which may have a special meaning within Android beyond their GE Neral meaning.

• ALSA

• Advanced Linux Sound Architecture. As the name suggests, it is a audio framework primarily for Linux, but it had influenced other systems. See Wikipedia article ALSA for the general definition. As used within Android, it refers primarily to the kernel audio framework and drivers, not to the User-mode API. See Tinyalsa.

• Audioeffect

• An APIs and implementation framework for output (post-processing) effects and input (pre-processing) effects. The API is defined at Android.media.audiofx.AudioEffect.

• Audioflinger

• The sound server implementation for Android. Audioflinger runs within the mediaserver process. See Wikipedia article Sound server for the generic definition.

• Audio Focus

• A set of APIs for managing audio interactions across multiple independent apps. See Managing Audio Focus and the focus-related methods and constants of Android.media.AudioManager.

• Audiomixer

• The module within Audioflinger responsible for combining multiple tracks and applying attenuation (volume) and certain EFF ECTs. The Wikipedia article Audio mixing (recorded music) may be useful for understanding the generic concept. But this article describes a mixer more as a hardware device or a software application, rather than a software module with In a system.

• Audio policy

• Service responsible for all actions, require a policy decision to be made first, such as opening a new I/O stream, re- Routing after a change and stream volume management.

• Audiorecord

• The primary low-level client API for receiving data from an audio input device such as microphone. The data is usually in Pulse-code modulation (PCM) format. The API is defined Atandroid.media.AudioRecord.

• Audioresampler

• The module within Audioflinger responsible for sample rate conversion.

• Audiotrack

• The primary low-level client API for sending data to an audio output device such as a speaker. The data is usually in PCM format. The API is defined at Android.media.AudioTrack.

• Client

• Usually same as application or app, but sometimes the ' client ' of Audioflinger is actually a thread running within the Med Iaserver system process. An example of this is when playing media, is decoded by a MediaPlayer object.

• HAL

• Hardware abstraction Layer. HAL is a generic term in Android. With respect to audio, it's a layer between Audioflinger and the kernel device driver with a C API, which replaces the EA Rlier C + + Libaudio.

• Fastmixer

• A thread within Audioflinger that services lower latency "fast tracks" and drives the primary output device.

• Fast Track

• An Audiotrack client with lower latency but fewer features, on some devices.

• MediaPlayer

• A higher-level client API than Audiotrack, for playing either encoded content, or content which includes multimedia audio and video tracks.

• Media.log

• An Audioflinger debugging feature, available on custom builds only, for logging audio events to a circular buffer where th EY can then is dumped retroactively when needed.

• MediaServer

• An Android system process, which contains a number of media-related services, including Audioflinger.

• Nbaio

• An abstraction for "non-blocking" audio input/output ports used within Audioflinger. The name can be misleading, as some implementations of the Nbaio API actually does support blocking. The key implementations of Nbaio is for pipes of various kinds.

• Normal mixer

• A thread within Audioflinger that services very full-featured Audiotrack clients, and either directly drives an output dev Ice or feeds its sub-mix into fastmixer via a pipe.

• OpenSL ES

• An audio API standard by the Khronos Group. Android versions since API Level 9 support a native audio API which are based on a subset of OpenSL ES 1.0.1.

• Silent mode

• A user-settable feature to mute the phone ringer and notifications, without affecting media playback (music, videos, games ) or alarms.

• Soundpool

• A higher-level client API than Audiotrack, used for playing sampled audio clips. It's useful for triggering UI feedback, game sounds, etc. The API is defined at Android.media.SoundPool.

• Stagefright

• See Media.

• Statequeue

• A module within Audioflinger responsible for synchronizing state among threads. Whereas Nbaio is used to pass data, Statequeue was used to pass control information.

• Strategy

• A grouping of stream types with similar behavior, used by the audio policy service.

• Stream type

• An enumeration, expresses a use case for audio output. The audio policy implementation uses the stream type, along with other parameters, to determine volume and routing Decisio Ns. Specific stream types is listed at Android.media.AudioManager.

• Tee sink

• See the separate article on tee sink in Audio debugging.

• Tinyalsa

• A small User-mode API above ALSA kernel with BSD license, recommended for use in HAL implementations.

• Tonegenerator

• A higher-level client API than Audiotrack, used for playing DTMF signals. See the Wikipedia article Dual-tone multi-frequency signaling, and the API definition atandroid.media.ToneGenerator.

• Track

• An audio stream, controlled by the Audiotrack API.

• Volume attenuation curve

• A device-specific mapping from a generic volume index to a particular attenuation factor for a given output.

• Volume index

• A unitless integer that expresses the desired relative volume of a stream. The volume-related APIs of Android.media.AudioManager operate in volume indices rather than absolute attenuation factors.

Sample Rate Conversion

• Downsample

• To Resample, where sink the sample rate < source sample rate.

• Nyquist frequency

• The Nyquist frequency, equal to $ A given sample rate, was the maximum frequency component that can being represented by A discretized signal at this sample rate. For example, the human hearing range are typically assumed to extend up to approximately-kHz, and so a digital audio sig NAL must has a sample rate of at least-kHz to represent that range. In practice, sample rates of 44.1 khz and khz is commonly used, with Nyquist frequencies of 22.05 khz and khz RESPE Ctively. See Nyquist frequency and hearing range for more information.

• Resampler

• Synonym for sample rate converter.

• Resampling

• The process of converting sample rate.

• Sample Rate Converter

• A module that resamples.

• Sink

• The output of a resampler.

• Source

• The input to a resampler.

• Upsample

• To Resample, where sink the sample rate > Source sample rate.

Some special terms for Android audio development (to be translated)