Sound +Driver Technology

Audio hardware +device

An audio hardware device is an individual hardware codec +device together with any associated controller hardware that allows the CPU +to communicate with it.

A basic audio hardware device typically provides +two communication paths: an input path for the audio recording and an output +path for audio playback.

Most basic audio hardware devices support +full duplex data transfer although some are only half-duplex or maybe just +simplex. Each input or output path may be used to transfer mono data or stereo +data. In the case of stereo this consists of two audio channels, the left +and the right; mono data consists of just a single audio channel.

A +more complex audio hardware device could be an AC 97 codec or similar, plus +its associated controller, which can support multiple input and output paths. +Each input or output path may be used to transfer mono data, stereo data or +'multichannel data'. For example, left, right, centre, left and right surround +and Low-Frequency Effects (LFE).

Unit

Units +are used to provide access to the various audio hardware devices. Each unit +supports just one communication path this is either input or output.

Clients +of the audio hardware system can open a separate connection to each unit. +The mapping between the units on a given phone and the audio hardware devices +themselves is platform specific; this is determined by the implementer of +the Sound Driver PDD for that platform.

A basic full-duplex audio +hardware device is presented as two units, one input/record unit and one output/playback +unit. A more complex audio hardware device such as an AC 97 codec may be represented +to the rest of the OS as a number of audio input and output units.

Audio channel

An +audio channel is a data stream between a client and an audio unit. There are +one or more audio channels per driver channel.

Driver channel

A +driver channel is a session between a client and an audio unit. A client may +have driver channels open on more than one unit.

The difference +between a driver channel and an audio channel. A driver channel is a session +between the client and an audio device which can consist of one or more audio +channels. An audio channel refers to the audio stream, for example, left or +right output.

+ The relationship between audio channels and device channels. + + +

Mono to stereo +conversion

Many codecs that support stereo playback can only accept +audio data that is delivered with the samples for each of the channels interleaved, +for example, LRLRLR. For these audio hardware devices, in order to operate +the channel in mono mode and to play audio data which contains only samples +for a single channel it is necessary to perform mono-to-stereo conversion +on the audio data before delivering it to the codec. So, for a section of +mono audio data that contains three samples, lets call them S1, S2 and S3, +each sample is duplicated, so we have S1,S1,S2,S2,S3,S3, with identical samples +being delivered to each channel.

Unfortunately, the only way for the +PDD to implement this conversion is for it to allocate a conversion buffer +and to copy each sample twice into this buffer. The PDD has to allocate a +separate conversion buffer for each simultaneous transfer operation it supports, +for example, for the template playback driver, a conversion buffer count equal +to KTemplateMaxTxDmaRequests.

When performing conversion +in this manner, the maximum transfer size that the PDD can accept from the +LDD becomes half the size of each conversion buffer, and when configured in +this mode, the value returned to the LDD in response to DSoundScPdd::MaxTransferLen must +equal this value.

Likewise, the record data may be delivered by the +audio hardware device only in stereo format with the data samples for each +channel interleaved. If the driver channel is configured in mono record mode, +stereo to mono conversion has to be performed in the PDD to discard each alternate +sample.