annotate src/cdrom/macosx/SDL_syscdrom_c.h @ 773:da0a2ad35bf4

Date: Sun, 4 Jan 2004 23:48:19 +0100 From: Max Horn Subject: Re: Again Audio CD patch Am 04.01.2004 um 22:38 schrieb Sam Lantinga: > > Okay, I fixed the buffering problems by simply using a 4 second buffer > instead of a 1 second buffer. However, using your code I can't play an > entire CD - the playback stops after the first song. > Found the problem: FSReadFork returns eofErr when the file is finished. However, we check its return value for errors, and if anything but noErr occurs, the reader thread aborts its current iteration. That is bad, because it aborts before it can ever set the flag which tells that the file is over (also, any remaining data which FSRead did return is lost - so you'd not hear about to 4 seconds from the end of the file. Furthermore, the computed data size was 8 bytes to high (I forgot to account for the fact that the size of an (A)IFF chunk always contains the chunk header & size fields, too). This is enough to make it work. However, the end condition is rather fragile, so I tuned some other things to be pessimistic (check for <= 0 instead of == 0, when eofErr is encountered enforce mReadFilePosition == mFileLength). You never know... The attached patch fixes the issue for me.
author Sam Lantinga <slouken@libsdl.org>
date Mon, 05 Jan 2004 00:57:51 +0000
parents b8d311d90021
children 19418e4422cb
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1 /*
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2 SDL - Simple DirectMedia Layer
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3 Copyright (C) 1997-2004 Sam Lantinga
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4
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5 This library is free software; you can redistribute it and/or
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6 modify it under the terms of the GNU Library General Public
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7 License as published by the Free Software Foundation; either
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8 version 2 of the License, or (at your option) any later version.
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9
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10 This library is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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13 Library General Public License for more details.
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14
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15 You should have received a copy of the GNU Library General Public
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16 License along with this library; if not, write to the Free
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17 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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19 Sam Lantinga
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20 slouken@libsdl.org
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21 */
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22
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23 /* This is the Mac OS X / CoreAudio specific header for the SDL CD-ROM API
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24 Contributed by Darrell Walisser and Max Horn
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25 */
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26
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27 /***********************************************************************************
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28 Implementation Notes
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29 *********************
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30
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31 This code has several limitations currently (all of which are proabaly fixable):
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32
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33 1. A CD-ROM device is inferred from a mounted cdfs volume, so device 0 is
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34 not necessarily the first CD-ROM device on the system. (Somewhat easy to fix
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35 by useing the device name from the volume id's to reorder the volumes)
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36
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37 2. You can only open and control 1 CD-ROM device at a time. (Challenging to fix,
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38 due to extensive code restructuring)
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39
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40 3. The status reported by SDL_CDStatus only changes to from CD_PLAYING to CD_STOPPED in
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41 1-second intervals (because the audio is buffered in 1-second chunks) If
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42 the audio data is less than 1 second, the remainder is filled with silence.
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43
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44 If you need to play sequences back-to-back that are less that 1 second long,
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45 use the frame position to determine when to play the next sequence, instead
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46 of SDL_CDStatus.
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47
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48 This may be possible to fix with a clever usage of the AudioUnit API.
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49
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50 4. When new volumes are inserted, our volume information is not updated. The only way
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51 to refresh this information is to reinit the CD-ROM subsystem of SDL. To fix this,
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52 one would probably have to fix point 1 above first, then figure out how to register
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53 for a notification when new media is mounted in order to perform an automatic
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54 rescan for cdfs volumes.
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55
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57
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58 So, here comes a description of how this all works.
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59
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60 < Initializing >
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61
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62 To get things rolling, we have to locate mounted volumes that contain
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63 audio (since nearly all Macs don't have analog audio-in on the sound card).
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64 That's easy, since these volumes have a flag that indicates this special
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65 filesystem. See DetectAudioCDVolumes() in CDPlayer.cpp for this code.
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66
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67 Next, we parse the invisible .TOC.plist in the root of the volume, which gets us
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68 the track information (number, offset, length, leadout, etc). See ReadTOCData() in
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69 CDPlayer.cpp for the skinny on this.
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70
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71
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72 < The Playback Loop >
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73
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74 Now come the tricky parts. Let's start with basic audio playback. When a frame
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75 range to play is requested, we must first find the .aiff files on the volume,
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76 hopefully in the right order. Since these files all begin with a number "1 Audio Track",
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77 etc, this is used to determine the correct track order.
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78
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79 Once all files are determined, we have to find what file corresponds to the start
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80 and length parameter to SDL_SYS_CDPlay(). Again, this is quite simple by walking the
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81 cdrom's track list. At this point, we also save the offset to the next track and frames
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82 remaining, if we're going to have to play another file after the first one. See
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83 GetFileForOffset() for this code.
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84
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85 At this point we have all info needed to start playback, so we hand off to the LoadFile()
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86 function, which proceeds to do its magic and plays back the file.
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87
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88 When the file is finished playing, CompletionProc() is invoked, at which time we can
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89 play the next file if the previously saved next track and frames remaining
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90 indicates that we should.
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91
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92
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93 < Magic >
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94
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95 OK, so it's not really magic, but since I don't fully understand all the hidden details it
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96 seems like it to me ;-) The API's involved are the AudioUnit and AudioFile API's. These
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97 appear to be an extension of CoreAudio for creating modular playback and f/x entities.
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98 The important thing is that CPU usage is very low and reliability is very high. You'd
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99 be hard-pressed to find a way to stutter the playback with other CPU-intensive tasks.
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100
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101 One part of this magic is that it uses multiple threads, which carries the usual potential
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102 for disaster if not handled carefully. Playback currently requires 4 additional threads:
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103 1. The coreaudio runloop thread
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104 2. The coreaudio device i/o thread
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105 3. The file streaming thread
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106 4. The notification/callback thread
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107
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108 The first 2 threads are necessary evil - CoreAudio creates this no matter what the situation
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109 is (even the SDL sound implementation creates theses suckers). The last two are are created
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110 by us.
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111
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112 The file is streamed from disk using a threaded double-buffer approach.
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113 This way, the high latency operation of reading from disk can be performed without interrupting
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114 the real-time device thread (which amounts to avoiding dropouts). The device thread grabs the
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115 buffer that isn't being read and sends it to the CoreAudio mixer where it eventually gets
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116 to the sound card.
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117
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118 The device thread posts a notification when the file streaming thread is out of data. This
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119 notification must be handled in a separate thread to avoid potential deadlock in the
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120 device thread. That's where the notification thread comes in. This thread is signaled
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121 whenever a notification needs to be processed, so another file can be played back if need be.
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122
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123 The API in CDPlayer.cpp contains synchronization because otherwise both the notification thread
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124 and main thread (or another other thread using the SDL CD api) can potentially call it at the same time.
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125
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126 ************************************************************************************/
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127
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128
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129 #include "SDL_cdrom.h"
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130 #include "SDL_syscdrom.h"
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131
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132 #include "CDPlayer.h"
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133
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134 #define kErrorFakeDevice "Error: Cannot proceed since we're faking a CD-ROM device. Reinit the CD-ROM subsystem to scan for new volumes."
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135