audio_policy.conf文件分析，不同的Android產品在音頻的設計上通常是存在差異的，而這些差異可以同過Audio的配置文件audio_policy.conf來獲得。在Android系統中音頻配置文件存放路徑有兩處，存放地址可以從AudioPolicyManagerBase.cpp文件中知道：
#define AUDIO_POLICY_VENDOR_CONFIG_FILE "/vendor/etc/audio_policy.conf" #define AUDIO_POLICY_CONFIG_FILE "/system/etc/audio_policy.conf"
在AudioPolicyManager.cpp文件中可以知道系統會首先加載vendor/etc目錄下的configure文件，再加載system/etc目錄下的configure文件。若這兩者加載都發生錯誤的話，系統會加載default配置文件，並命名為primary module，從這可以看出，音頻系統中一定必須存在的module就是primary瞭。
1 if (loadAudioPolicyConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE) != NO_ERROR) {
2 if (loadAudioPolicyConfig(AUDIO_POLICY_CONFIG_FILE) != NO_ERROR) {
3 ALOGE("could not load audio policy configuration file, setting defaults");
4 defaultAudioPolicyConfig();
5 }
6 }
1 # Global configuration section: lists input and output devices always present on the device
2 # as well as the output device selected by default.
3 # Devices are designated by a string that corresponds to the enum in audio.h
4
5 global_configuration {
6 attached_output_devices AUDIO_DEVICE_OUT_SPEAKER
7 default_output_device AUDIO_DEVICE_OUT_SPEAKER
8 attached_input_devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_VOICE_CALL|AUDIO_DEVICE_IN_REMOTE_SUBMIX
9 custom_properties {
10 voice_volume_applied_after_mixing_in_call true
11 voice_volume_applied_after_mixing_in_communication false
12 in_call_music_attenuation_dB 6
13 }
14 }
15
16 # audio hardware module section: contains descriptors for all audio hw modules present on the
17 # device. Each hw module node is named after the corresponding hw module library base name.
18 # For instance, "primary" corresponds to audio.primary..so.
19 # The "primary" module is mandatory and must include at least one output with
20 # AUDIO_OUTPUT_FLAG_PRIMARY flag.
21 # Each module descriptor contains one or more output profile descriptors and zero or more
22 # input profile descriptors. Each profile lists all the parameters supported by a given output
23 # or input stream category.
24 # The "channel_masks", "formats", "devices" and "flags" are specified using strings corresponding
25 # to enums in audio.h and audio_policy.h. They are concatenated by use of "|" without space or "\n".
26
27 audio_hw_modules {
28 primary {
29 outputs {
30 primary {
31 sampling_rates 48000
32 channel_masks AUDIO_CHANNEL_OUT_STEREO
33 formats AUDIO_FORMAT_PCM_16_BIT
34 devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE|AUDIO_DEVICE_OUT_ALL_SCO
35 flags AUDIO_OUTPUT_FLAG_PRIMARY
36 }
37 deep_buffer {
38 sampling_rates 48000
39 channel_masks AUDIO_CHANNEL_OUT_STEREO
40 formats AUDIO_FORMAT_PCM_16_BIT
41 devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE
42 flags AUDIO_OUTPUT_FLAG_DEEP_BUFFER
43 }
44 }
45 inputs {
46 primary {
47 sampling_rates 8000|11025|12000|16000|22050|24000|32000|44100|48000
48 channel_masks AUDIO_CHANNEL_IN_MONO|AUDIO_CHANNEL_IN_STEREO
49 formats AUDIO_FORMAT_PCM_16_BIT
50 devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET|AUDIO_DEVICE_IN_WIRED_HEADSET|AUDIO_DEVICE_IN_VOICE_CALL
51 }
52 }
53 }
54 a2dp {
55 outputs {
56 a2dp {
57 sampling_rates 44100
58 channel_masks AUDIO_CHANNEL_OUT_STEREO
59 formats AUDIO_FORMAT_PCM_16_BIT
60 devices AUDIO_DEVICE_OUT_ALL_A2DP
61 }
62 }
63 }
64 hdmi {
65 outputs {
66 hdmi_stereo {
67 sampling_rates 48000
68 channel_masks AUDIO_CHANNEL_OUT_STEREO
69 formats AUDIO_FORMAT_PCM_16_BIT
70 devices AUDIO_DEVICE_OUT_AUX_DIGITAL
71 }
72 hdmi_multi {
73 sampling_rates 32000|44100|48000|88200|96000
74 # channel_masks AUDIO_CHANNEL_OUT_5POINT1
75 channel_masks dynamic
76 formats AUDIO_FORMAT_PCM_16_BIT
77 devices AUDIO_DEVICE_OUT_AUX_DIGITAL
78 flags AUDIO_OUTPUT_FLAG_DIRECT
79 }
80 }
81 }
82 r_submix {
83 outputs {
84 submix {
85 sampling_rates 48000
86 channel_masks AUDIO_CHANNEL_OUT_STEREO
87 formats AUDIO_FORMAT_PCM_16_BIT
88 devices AUDIO_DEVICE_OUT_REMOTE_SUBMIX
89 }
90 }
91 inputs {
92 submix {
93 sampling_rates 48000
94 channel_masks AUDIO_CHANNEL_IN_STEREO
95 formats AUDIO_FORMAT_PCM_16_BIT
96 devices AUDIO_DEVICE_IN_REMOTE_SUBMIX
97 }
98 }
99 }
100 hs_usb {
101 outputs {
102 usb_dgtl {
103 sampling_rates 44100|48000
104 channel_masks AUDIO_CHANNEL_OUT_STEREO
105 formats AUDIO_FORMAT_PCM_16_BIT
106 devices AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET
107 }
108 }
109 }
110 usb {
111 outputs {
112 usb_accessory {
113 sampling_rates 44100
114 channel_masks AUDIO_CHANNEL_OUT_STEREO
115 formats AUDIO_FORMAT_PCM_16_BIT
116 devices AUDIO_DEVICE_OUT_USB_ACCESSORY
117 }
118 # usb_device {
119 # sampling_rates 44100
120 # channel_masks AUDIO_CHANNEL_OUT_STEREO
121 # formats AUDIO_FORMAT_PCM_16_BIT
122 # devices AUDIO_DEVICE_OUT_USB_DEVICE
123 # }
124 }
125 } }
從audio_policy.conf文件中可以發現，系統包含瞭primary、a2dp、usb等音頻接口，對應著系統中的audio…so。每個音頻接口中又包含瞭若幹個outputs & inputs，並且每個output or input又包含瞭若幹個devices，且還有采樣頻率，聲道數等信息。這些devices信息、采樣頻率信息 & 聲道信息等都會保存在各自module的IOProfile中。按上文中audio_policy.conf配置文件所描述，系統最後會生成6個modules(eg.primary,a2dp,hdmi,r_submix,hs_usb & usb)以及7個outputs。以AUDIO_DEVICE_OUT_SPEAKER為例，該device會定義在primary模塊中outputs所屬的IOProfile1中，其它設備依次類推。
根據audio_policy.conf文件可以簡化Audio module的架構，如下圖1所示：
圖1
