sound/designs/timestamping.rst

7 - Trigger_tstamp is the system time snapshot taken when the .trigger
11   estimate with a delay. In the latter two cases, the low-level driver
17 - tstamp is the current system timestamp updated during the last
19   The difference (tstamp - trigger_tstamp) defines the elapsed time.
29 - ``avail`` reports how much can be written in the ring buffer
30 - ``delay`` reports the time it will take to hear a new sample after all
43 ascii-art, this could be represented as follows (for the playback
47   --------------------------------------------------------------> time
50    analog         link            dma              app       FullBuffer
53     |< codec delay >|<--hw delay-->|<queued samples>|<---avail->|
54     |<----------------- delay---------------------->|           |
55                                    |<----ring buffer length---->|
61 The link time is taken at the output of the SoC/chipset as the samples
62 are pushed on a link. The link time can be directly measured if
67 The DMA time is measured using counters - typically the least reliable
76 accuracy. It can also request the delay-to-analog be included in the
77 measurement. Direct access to the link time is very interesting on
78 platforms that provide an embedded DSP; measuring directly the link
84 in hardware/low-level driver, the type is overridden as DEFAULT and the
88 provide timestamp selection, with a zero-valued COMPAT timestamp type
92 The audio timestamp accuracy can be returned to user-space, so that
95 - for dma time (default), the granularity of the transfers can be
100 - the link time can be used to track long-term drifts between audio
101   and system time using the (tstamp-trigger_tstamp)/audio_tstamp
102   ratio, the precision helps define how much smoothing/low-pass
103   filtering is required. The link time can be either reset on startup
105   different streams - but may require the wallclock to be always
106   running and not wrap-around during idle periods). If supported in
107   hardware, the absolute link time could also be used to define a
110 - including the delay in the audio timestamp may
111   counter-intuitively not increase the precision of timestamps, e.g. if a
112   codec includes variable-latency DSP processing or a chain of
115 The accuracy is reported in nanosecond units (using an unsigned 32-bit
119 Due to the varied nature of timestamping needs, even for a single
121 the ``STATUS`` ioctl, the parameters are read-only and do not allow for
124 with read/write parameters. ALSA-lib will be modified to make use of
127 The ALSA API only allows for a single audio timestamp to be reported
131 are. To avoid any interpretation issues, a single (system, audio)
136 In some hardware-specific configuration, the system timestamp is
137 latched by a low-level audio subsystem, and the information provided
151   $ ./audio_time  -p --ts_type=1
152   playback: systime: 341121338 nsec, audio time 342000000 nsec, 	systime delta -878662
153   playback: systime: 426236663 nsec, audio time 427187500 nsec, 	systime delta -950837
154   playback: systime: 597080580 nsec, audio time 598000000 nsec, 	systime delta -919420
155   playback: systime: 682059782 nsec, audio time 683020833 nsec, 	systime delta -961051
156   playback: systime: 852896415 nsec, audio time 853854166 nsec, 	systime delta -957751
157   playback: systime: 937903344 nsec, audio time 938854166 nsec, 	systime delta -950822
162   $ ./audio_time  -p --ts_type=1 -d
163   playback: systime: 341053347 nsec, audio time 341062500 nsec, 	systime delta -9153
166   playback: systime: 681915317 nsec, audio time 681916666 nsec, 	systime delta -1349
167   playback: systime: 852741306 nsec, audio time 852750000 nsec, 	systime delta -8694
169 3. link timestamp, compensation for DMA+analog delay
172   $ ./audio_time  -p --ts_type=2 -d
173   playback: systime: 341060004 nsec, audio time 341062791 nsec, 	systime delta -2787
174   playback: systime: 426242074 nsec, audio time 426244875 nsec, 	systime delta -2801
175   playback: systime: 597080992 nsec, audio time 597084583 nsec, 	systime delta -3591
176   playback: systime: 682084512 nsec, audio time 682088291 nsec, 	systime delta -3779
177   playback: systime: 852936229 nsec, audio time 852940916 nsec, 	systime delta -4687
178   playback: systime: 938107562 nsec, audio time 938112708 nsec, 	systime delta -5146
183 DMA-link delay in example 2 helps remove the hardware buffering but
185 error. In example 3 where the timestamps are measured with the link
197   $ ./audio_time -p -Dhw:1 -t1
198   playback: systime: 120174019 nsec, audio time 125000000 nsec, 	systime delta -4825981
199   playback: systime: 245041136 nsec, audio time 250000000 nsec, 	systime delta -4958864
200   playback: systime: 370106088 nsec, audio time 375000000 nsec, 	systime delta -4893912
201   playback: systime: 495040065 nsec, audio time 500000000 nsec, 	systime delta -4959935
202   playback: systime: 620038179 nsec, audio time 625000000 nsec, 	systime delta -4961821
203   playback: systime: 745087741 nsec, audio time 750000000 nsec, 	systime delta -4912259
204   playback: systime: 870037336 nsec, audio time 875000000 nsec, 	systime delta -4962664
209   $ ./audio_time -p -Dhw:1 -t1 -d