1 ============================
2 ALSA PCM channel-mapping API
3 ============================
4
5 Takashi Iwai <tiwai@suse.de>
6
7 General
8 =======
9
10 The channel mapping API allows user to query the possible channel maps
11 and the current channel map, also optionally to modify the channel map
12 of the current stream.
13
14 A channel map is an array of position for each PCM channel.
15 Typically, a stereo PCM stream has a channel map of
16 ``{ front_left, front_right }``
17 while a 4.0 surround PCM stream has a channel map of
18 ``{ front left, front right, rear left, rear right }.``
19
20 The problem, so far, was that we had no standard channel map
21 explicitly, and applications had no way to know which channel
22 corresponds to which (speaker) position. Thus, applications applied
23 wrong channels for 5.1 outputs, and you hear suddenly strange sound
24 from rear. Or, some devices secretly assume that center/LFE is the
25 third/fourth channels while others that C/LFE as 5th/6th channels.
26
27 Also, some devices such as HDMI are configurable for different speaker
28 positions even with the same number of total channels. However, there
29 was no way to specify this because of lack of channel map
30 specification. These are the main motivations for the new channel
31 mapping API.
32
33
34 Design
35 ======
36
37 Actually, "the channel mapping API" doesn't introduce anything new in
38 the kernel/user-space ABI perspective. It uses only the existing
39 control element features.
40
41 As a ground design, each PCM substream may contain a control element
42 providing the channel mapping information and configuration. This
43 element is specified by:
44
45 * iface = SNDRV_CTL_ELEM_IFACE_PCM
46 * name = "Playback Channel Map" or "Capture Channel Map"
47 * device = the same device number for the assigned PCM substream
48 * index = the same index number for the assigned PCM substream
49
50 Note the name is different depending on the PCM substream direction.
51
52 Each control element provides at least the TLV read operation and the
53 read operation. Optionally, the write operation can be provided to
54 allow user to change the channel map dynamically.
55
56 TLV
57 ---
58
59 The TLV operation gives the list of available channel
60 maps. A list item of a channel map is usually a TLV of
61 ``type data-bytes ch0 ch1 ch2...``
62 where type is the TLV type value, the second argument is the total
63 bytes (not the numbers) of channel values, and the rest are the
64 position value for each channel.
65
66 As a TLV type, either ``SNDRV_CTL_TLVT_CHMAP_FIXED``,
67 ``SNDRV_CTL_TLV_CHMAP_VAR`` or ``SNDRV_CTL_TLVT_CHMAP_PAIRED`` can be used.
68 The ``_FIXED`` type is for a channel map with the fixed channel position
69 while the latter two are for flexible channel positions. ``_VAR`` type is
70 for a channel map where all channels are freely swappable and ``_PAIRED``
71 type is where pair-wise channels are swappable. For example, when you
72 have {FL/FR/RL/RR} channel map, ``_PAIRED`` type would allow you to swap
73 only {RL/RR/FL/FR} while ``_VAR`` type would allow even swapping FL and
74 RR.
75
76 These new TLV types are defined in ``sound/tlv.h``.
77
78 The available channel position values are defined in ``sound/asound.h``,
79 here is a cut:
80
81 ::
82
83 /* channel positions */
84 enum {
85 SNDRV_CHMAP_UNKNOWN = 0,
86 SNDRV_CHMAP_NA, /* N/A, silent */
87 SNDRV_CHMAP_MONO, /* mono stream */
88 /* this follows the alsa-lib mixer channel value + 3 */
89 SNDRV_CHMAP_FL, /* front left */
90 SNDRV_CHMAP_FR, /* front right */
91 SNDRV_CHMAP_RL, /* rear left */
92 SNDRV_CHMAP_RR, /* rear right */
93 SNDRV_CHMAP_FC, /* front center */
94 SNDRV_CHMAP_LFE, /* LFE */
95 SNDRV_CHMAP_SL, /* side left */
96 SNDRV_CHMAP_SR, /* side right */
97 SNDRV_CHMAP_RC, /* rear center */
98 /* new definitions */
99 SNDRV_CHMAP_FLC, /* front left center */
100 SNDRV_CHMAP_FRC, /* front right center */
101 SNDRV_CHMAP_RLC, /* rear left center */
102 SNDRV_CHMAP_RRC, /* rear right center */
103 SNDRV_CHMAP_FLW, /* front left wide */
104 SNDRV_CHMAP_FRW, /* front right wide */
105 SNDRV_CHMAP_FLH, /* front left high */
106 SNDRV_CHMAP_FCH, /* front center high */
107 SNDRV_CHMAP_FRH, /* front right high */
108 SNDRV_CHMAP_TC, /* top center */
109 SNDRV_CHMAP_TFL, /* top front left */
110 SNDRV_CHMAP_TFR, /* top front right */
111 SNDRV_CHMAP_TFC, /* top front center */
112 SNDRV_CHMAP_TRL, /* top rear left */
113 SNDRV_CHMAP_TRR, /* top rear right */
114 SNDRV_CHMAP_TRC, /* top rear center */
115 SNDRV_CHMAP_LAST = SNDRV_CHMAP_TRC,
116 };
117
118 When a PCM stream can provide more than one channel map, you can
119 provide multiple channel maps in a TLV container type. The TLV data
120 to be returned will contain such as:
121 ::
122
123 SNDRV_CTL_TLVT_CONTAINER 96
124 SNDRV_CTL_TLVT_CHMAP_FIXED 4 SNDRV_CHMAP_FC
125 SNDRV_CTL_TLVT_CHMAP_FIXED 8 SNDRV_CHMAP_FL SNDRV_CHMAP_FR
126 SNDRV_CTL_TLVT_CHMAP_FIXED 16 NDRV_CHMAP_FL SNDRV_CHMAP_FR \
127 SNDRV_CHMAP_RL SNDRV_CHMAP_RR
128
129 The channel position is provided in LSB 16bits. The upper bits are
130 used for bit flags.
131 ::
132
133 #define SNDRV_CHMAP_POSITION_MASK 0xffff
134 #define SNDRV_CHMAP_PHASE_INVERSE (0x01 << 16)
135 #define SNDRV_CHMAP_DRIVER_SPEC (0x02 << 16)
136
137 ``SNDRV_CHMAP_PHASE_INVERSE`` indicates the channel is phase inverted,
138 (thus summing left and right channels would result in almost silence).
139 Some digital mic devices have this.
140
141 When ``SNDRV_CHMAP_DRIVER_SPEC`` is set, all the channel position values
142 don't follow the standard definition above but driver-specific.
143
144 Read Operation
145 --------------
146
147 The control read operation is for providing the current channel map of
148 the given stream. The control element returns an integer array
149 containing the position of each channel.
150
151 When this is performed before the number of the channel is specified
152 (i.e. hw_params is set), it should return all channels set to
153 ``UNKNOWN``.
154
155 Write Operation
156 ---------------
157
158 The control write operation is optional, and only for devices that can
159 change the channel configuration on the fly, such as HDMI. User needs
160 to pass an integer value containing the valid channel positions for
161 all channels of the assigned PCM substream.
162
163 This operation is allowed only at PCM PREPARED state. When called in
164 other states, it shall return an error.
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