channel-mapping-api.rst (6015B)
1============================ 2ALSA PCM channel-mapping API 3============================ 4 5Takashi Iwai <tiwai@suse.de> 6 7General 8======= 9 10The channel mapping API allows user to query the possible channel maps 11and the current channel map, also optionally to modify the channel map 12of the current stream. 13 14A channel map is an array of position for each PCM channel. 15Typically, a stereo PCM stream has a channel map of 16``{ front_left, front_right }`` 17while a 4.0 surround PCM stream has a channel map of 18``{ front left, front right, rear left, rear right }.`` 19 20The problem, so far, was that we had no standard channel map 21explicitly, and applications had no way to know which channel 22corresponds to which (speaker) position. Thus, applications applied 23wrong channels for 5.1 outputs, and you hear suddenly strange sound 24from rear. Or, some devices secretly assume that center/LFE is the 25third/fourth channels while others that C/LFE as 5th/6th channels. 26 27Also, some devices such as HDMI are configurable for different speaker 28positions even with the same number of total channels. However, there 29was no way to specify this because of lack of channel map 30specification. These are the main motivations for the new channel 31mapping API. 32 33 34Design 35====== 36 37Actually, "the channel mapping API" doesn't introduce anything new in 38the kernel/user-space ABI perspective. It uses only the existing 39control element features. 40 41As a ground design, each PCM substream may contain a control element 42providing the channel mapping information and configuration. This 43element 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 50Note the name is different depending on the PCM substream direction. 51 52Each control element provides at least the TLV read operation and the 53read operation. Optionally, the write operation can be provided to 54allow user to change the channel map dynamically. 55 56TLV 57--- 58 59The TLV operation gives the list of available channel 60maps. A list item of a channel map is usually a TLV of 61``type data-bytes ch0 ch1 ch2...`` 62where type is the TLV type value, the second argument is the total 63bytes (not the numbers) of channel values, and the rest are the 64position value for each channel. 65 66As a TLV type, either ``SNDRV_CTL_TLVT_CHMAP_FIXED``, 67``SNDRV_CTL_TLV_CHMAP_VAR`` or ``SNDRV_CTL_TLVT_CHMAP_PAIRED`` can be used. 68The ``_FIXED`` type is for a channel map with the fixed channel position 69while the latter two are for flexible channel positions. ``_VAR`` type is 70for a channel map where all channels are freely swappable and ``_PAIRED`` 71type is where pair-wise channels are swappable. For example, when you 72have {FL/FR/RL/RR} channel map, ``_PAIRED`` type would allow you to swap 73only {RL/RR/FL/FR} while ``_VAR`` type would allow even swapping FL and 74RR. 75 76These new TLV types are defined in ``sound/tlv.h``. 77 78The available channel position values are defined in ``sound/asound.h``, 79here 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 118When a PCM stream can provide more than one channel map, you can 119provide multiple channel maps in a TLV container type. The TLV data 120to 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 129The channel position is provided in LSB 16bits. The upper bits are 130used 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). 139Some digital mic devices have this. 140 141When ``SNDRV_CHMAP_DRIVER_SPEC`` is set, all the channel position values 142don't follow the standard definition above but driver-specific. 143 144Read Operation 145-------------- 146 147The control read operation is for providing the current channel map of 148the given stream. The control element returns an integer array 149containing the position of each channel. 150 151When 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 155Write Operation 156--------------- 157 158The control write operation is optional, and only for devices that can 159change the channel configuration on the fly, such as HDMI. User needs 160to pass an integer value containing the valid channel positions for 161all channels of the assigned PCM substream. 162 163This operation is allowed only at PCM PREPARED state. When called in 164other states, it shall return an error.