|
1 <?xml version="1.0" encoding="utf-8"?> |
|
2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
|
3 <!-- This component and the accompanying materials are made available under the terms of the License |
|
4 "Eclipse Public License v1.0" which accompanies this distribution, |
|
5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
|
6 <!-- Initial Contributors: |
|
7 Nokia Corporation - initial contribution. |
|
8 Contributors: |
|
9 --> |
|
10 <!DOCTYPE concept |
|
11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
|
12 <concept id="GUID-E2641957-8163-5EF4-B282-FC3FD9CA75A6" xml:lang="en"><title>Sound |
|
13 Driver Technology</title><shortdesc>Describes the technology concepts that are used in the Sound Driver. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
|
14 <section id="GUID-CBC17614-34B3-52A1-9C74-F7093A175398"><title>Audio hardware |
|
15 device</title> <p>An audio hardware device is an individual hardware codec |
|
16 device together with any associated controller hardware that allows the CPU |
|
17 to communicate with it. </p> <p>A basic audio hardware device typically provides |
|
18 two communication paths: an input path for the audio recording and an output |
|
19 path for audio playback. </p> <p>Most basic audio hardware devices support |
|
20 full duplex data transfer although some are only half-duplex or maybe just |
|
21 simplex. Each input or output path may be used to transfer mono data or stereo |
|
22 data. In the case of stereo this consists of two audio channels, the left |
|
23 and the right; mono data consists of just a single audio channel. </p> <p>A |
|
24 more complex audio hardware device could be an AC 97 codec or similar, plus |
|
25 its associated controller, which can support multiple input and output paths. |
|
26 Each input or output path may be used to transfer mono data, stereo data or |
|
27 'multichannel data'. For example, left, right, centre, left and right surround |
|
28 and Low-Frequency Effects (LFE). </p> </section> |
|
29 <section id="GUID-7C8FF32F-2E84-57B1-8962-9EDB4D51FEE6"><title>Unit</title> <p>Units |
|
30 are used to provide access to the various audio hardware devices. Each unit |
|
31 supports just one communication path this is either input or output. </p> <p>Clients |
|
32 of the audio hardware system can open a separate connection to each unit. |
|
33 The mapping between the units on a given phone and the audio hardware devices |
|
34 themselves is platform specific; this is determined by the implementer of |
|
35 the Sound Driver PDD for that platform. </p> <p>A basic full-duplex audio |
|
36 hardware device is presented as two units, one input/record unit and one output/playback |
|
37 unit. A more complex audio hardware device such as an AC 97 codec may be represented |
|
38 to the rest of the OS as a number of audio input and output units. </p> </section> |
|
39 <section id="GUID-74B14664-8D9D-57D2-B090-709136FEB5E3"><title>Audio channel</title> <p>An |
|
40 audio channel is a data stream between a client and an audio unit. There are |
|
41 one or more audio channels per driver channel. </p> </section> |
|
42 <section id="GUID-4D6A01FE-47E7-5A6F-9DFE-F612B880165B"><title>Driver channel</title> <p>A |
|
43 driver channel is a session between a client and an audio unit. A client may |
|
44 have driver channels open on more than one unit. </p> <p><note> The difference |
|
45 between a driver channel and an audio channel. A driver channel is a session |
|
46 between the client and an audio device which can consist of one or more audio |
|
47 channels. An audio channel refers to the audio stream, for example, left or |
|
48 right output.</note> </p> <fig id="GUID-C3B51755-EDB0-5518-83D1-886E9148D65A"> |
|
49 <title> The relationship between audio channels and device channels. |
|
50 </title> |
|
51 <image href="GUID-2D98DB88-BA48-5EF8-A5F9-0CB8688B0B63_d0e32099_href.png" placement="inline"/> |
|
52 </fig> </section> |
|
53 <section id="GUID-5C66667B-55C0-521D-86E3-67593DB381C9"><title>Mono to stereo |
|
54 conversion</title> <p>Many codecs that support stereo playback can only accept |
|
55 audio data that is delivered with the samples for each of the channels interleaved, |
|
56 for example, LRLRLR. For these audio hardware devices, in order to operate |
|
57 the channel in mono mode and to play audio data which contains only samples |
|
58 for a single channel it is necessary to perform mono-to-stereo conversion |
|
59 on the audio data before delivering it to the codec. So, for a section of |
|
60 mono audio data that contains three samples, lets call them S1, S2 and S3, |
|
61 each sample is duplicated, so we have S1,S1,S2,S2,S3,S3, with identical samples |
|
62 being delivered to each channel. </p> <p>Unfortunately, the only way for the |
|
63 PDD to implement this conversion is for it to allocate a conversion buffer |
|
64 and to copy each sample twice into this buffer. The PDD has to allocate a |
|
65 separate conversion buffer for each simultaneous transfer operation it supports, |
|
66 for example, for the template playback driver, a conversion buffer count equal |
|
67 to <xref href="GUID-EE69DF4F-3341-355A-837A-5DA1C8A20A67.dita"><apiname>KTemplateMaxTxDmaRequests</apiname></xref>. </p> <p>When performing conversion |
|
68 in this manner, the maximum transfer size that the PDD can accept from the |
|
69 LDD becomes half the size of each conversion buffer, and when configured in |
|
70 this mode, the value returned to the LDD in response to <xref href="GUID-61CC68CB-A01D-3CA0-93D9-F3717ABD6424.dita#GUID-61CC68CB-A01D-3CA0-93D9-F3717ABD6424/GUID-A47005E5-36B4-3795-A4C0-CB67B43802B5"><apiname>DSoundScPdd::MaxTransferLen</apiname></xref> must |
|
71 equal this value. </p> <p>Likewise, the record data may be delivered by the |
|
72 audio hardware device only in stereo format with the data samples for each |
|
73 channel interleaved. If the driver channel is configured in mono record mode, |
|
74 stereo to mono conversion has to be performed in the PDD to discard each alternate |
|
75 sample. </p> </section> |
|
76 </conbody></concept> |