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authorTakashi Iwai <tiwai@suse.de>2012-09-15 14:32:31 (GMT)
committerTakashi Iwai <tiwai@suse.de>2012-09-15 14:32:31 (GMT)
commite5d6db8e6039d7f51a9a648d74db1109c7fdc746 (patch)
tree1f8ec13b6bda70fdf6c607a86d02c3e974e4c469 /Documentation/sound
parent62cbde1868b16e7cf1ed115cdfb9cbe82e230f0a (diff)
parent5efbc2610a7b2aac6c51f8fc943c019106568939 (diff)
Merge branch 'topic/tlv-chmap' into for-next
This is a merge of a topic branch containing the support for the new channel map API using control elements.
Diffstat (limited to 'Documentation/sound')
-rw-r--r--Documentation/sound/alsa/Channel-Mapping-API.txt153
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diff --git a/Documentation/sound/alsa/Channel-Mapping-API.txt b/Documentation/sound/alsa/Channel-Mapping-API.txt
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+ALSA PCM channel-mapping API
+============================
+ Takashi Iwai <tiwai@suse.de>
+
+GENERAL
+-------
+
+The channel mapping API allows user to query the possible channel maps
+and the current channel map, also optionally to modify the channel map
+of the current stream.
+
+A channel map is an array of position for each PCM channel.
+Typically, a stereo PCM stream has a channel map of
+ { front_left, front_right }
+while a 4.0 surround PCM stream has a channel map of
+ { front left, front right, rear left, rear right }.
+
+The problem, so far, was that we had no standard channel map
+explicitly, and applications had no way to know which channel
+corresponds to which (speaker) position. Thus, applications applied
+wrong channels for 5.1 outputs, and you hear suddenly strange sound
+from rear. Or, some devices secretly assume that center/LFE is the
+third/fourth channels while others that C/LFE as 5th/6th channels.
+
+Also, some devices such as HDMI are configurable for different speaker
+positions even with the same number of total channels. However, there
+was no way to specify this because of lack of channel map
+specification. These are the main motivations for the new channel
+mapping API.
+
+
+DESIGN
+------
+
+Actually, "the channel mapping API" doesn't introduce anything new in
+the kernel/user-space ABI perspective. It uses only the existing
+control element features.
+
+As a ground design, each PCM substream may contain a control element
+providing the channel mapping information and configuration. This
+element is specified by:
+ iface = SNDRV_CTL_ELEM_IFACE_PCM
+ name = "Playback Channel Map" or "Capture Channel Map"
+ device = the same device number for the assigned PCM substream
+ index = the same index number for the assigned PCM substream
+
+Note the name is different depending on the PCM substream direction.
+
+Each control element provides at least the TLV read operation and the
+read operation. Optionally, the write operation can be provided to
+allow user to change the channel map dynamically.
+
+* TLV
+
+The TLV operation gives the list of available channel
+maps. A list item of a channel map is usually a TLV of
+ type data-bytes ch0 ch1 ch2...
+where type is the TLV type value, the second argument is the total
+bytes (not the numbers) of channel values, and the rest are the
+position value for each channel.
+
+As a TLV type, either SNDRV_CTL_TLVT_CHMAP_FIXED,
+SNDRV_CTL_TLV_CHMAP_VAR or SNDRV_CTL_TLVT_CHMAP_PAIRED can be used.
+The _FIXED type is for a channel map with the fixed channel position
+while the latter two are for flexible channel positions. _VAR type is
+for a channel map where all channels are freely swappable and _PAIRED
+type is where pair-wise channels are swappable. For example, when you
+have {FL/FR/RL/RR} channel map, _PAIRED type would allow you to swap
+only {RL/RR/FL/FR} while _VAR type would allow even swapping FL and
+RR.
+
+These new TLV types are defined in sound/tlv.h.
+
+The available channel position values are defined in sound/asound.h,
+here is a cut:
+
+/* channel positions */
+enum {
+ SNDRV_CHMAP_UNKNOWN = 0,
+ SNDRV_CHMAP_NA, /* N/A, silent */
+ SNDRV_CHMAP_MONO, /* mono stream */
+ /* this follows the alsa-lib mixer channel value + 3 */
+ SNDRV_CHMAP_FL, /* front left */
+ SNDRV_CHMAP_FR, /* front right */
+ SNDRV_CHMAP_RL, /* rear left */
+ SNDRV_CHMAP_RR, /* rear right */
+ SNDRV_CHMAP_FC, /* front center */
+ SNDRV_CHMAP_LFE, /* LFE */
+ SNDRV_CHMAP_SL, /* side left */
+ SNDRV_CHMAP_SR, /* side right */
+ SNDRV_CHMAP_RC, /* rear center */
+ /* new definitions */
+ SNDRV_CHMAP_FLC, /* front left center */
+ SNDRV_CHMAP_FRC, /* front right center */
+ SNDRV_CHMAP_RLC, /* rear left center */
+ SNDRV_CHMAP_RRC, /* rear right center */
+ SNDRV_CHMAP_FLW, /* front left wide */
+ SNDRV_CHMAP_FRW, /* front right wide */
+ SNDRV_CHMAP_FLH, /* front left high */
+ SNDRV_CHMAP_FCH, /* front center high */
+ SNDRV_CHMAP_FRH, /* front right high */
+ SNDRV_CHMAP_TC, /* top center */
+ SNDRV_CHMAP_TFL, /* top front left */
+ SNDRV_CHMAP_TFR, /* top front right */
+ SNDRV_CHMAP_TFC, /* top front center */
+ SNDRV_CHMAP_TRL, /* top rear left */
+ SNDRV_CHMAP_TRR, /* top rear right */
+ SNDRV_CHMAP_TRC, /* top rear center */
+ SNDRV_CHMAP_LAST = SNDRV_CHMAP_TRC,
+};
+
+When a PCM stream can provide more than one channel map, you can
+provide multiple channel maps in a TLV container type. The TLV data
+to be returned will contain such as:
+ SNDRV_CTL_TLVT_CONTAINER 96
+ SNDRV_CTL_TLVT_CHMAP_FIXED 4 SNDRV_CHMAP_FC
+ SNDRV_CTL_TLVT_CHMAP_FIXED 8 SNDRV_CHMAP_FL SNDRV_CHMAP_FR
+ SNDRV_CTL_TLVT_CHMAP_FIXED 16 NDRV_CHMAP_FL SNDRV_CHMAP_FR \
+ SNDRV_CHMAP_RL SNDRV_CHMAP_RR
+
+The channel position is provided in LSB 16bits. The upper bits are
+used for bit flags.
+
+#define SNDRV_CHMAP_POSITION_MASK 0xffff
+#define SNDRV_CHMAP_PHASE_INVERSE (0x01 << 16)
+#define SNDRV_CHMAP_DRIVER_SPEC (0x02 << 16)
+
+SNDRV_CHMAP_PHASE_INVERSE indicates the channel is phase inverted,
+(thus summing left and right channels would result in almost silence).
+Some digital mic devices have this.
+
+When SNDRV_CHMAP_DRIVER_SPEC is set, all the channel position values
+don't follow the standard definition above but driver-specific.
+
+* READ OPERATION
+
+The control read operation is for providing the current channel map of
+the given stream. The control element returns an integer array
+containing the position of each channel.
+
+When this is performed before the number of the channel is specified
+(i.e. hw_params is set), it should return all channels set to
+UNKNOWN.
+
+* WRITE OPERATION
+
+The control write operation is optional, and only for devices that can
+change the channel configuration on the fly, such as HDMI. User needs
+to pass an integer value containing the valid channel positions for
+all channels of the assigned PCM substream.
+
+This operation is allowed only at PCM PREPARED state. When called in
+other states, it shall return an error.

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