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1 =====================
2 ALSA PCM Timestamping
3 =====================
5 The ALSA API can provide two different system timestamps:
7 - Trigger_tstamp is the system time snapshot taken when the .trigger
8 callback is invoked. This snapshot is taken by the ALSA core in the
9 general case, but specific hardware may have synchronization
10 capabilities or conversely may only be able to provide a correct
11 estimate with a delay. In the latter two cases, the low-level driver
12 is responsible for updating the trigger_tstamp at the most appropriate
13 and precise moment. Applications should not rely solely on the first
14 trigger_tstamp but update their internal calculations if the driver
15 provides a refined estimate with a delay.
17 - tstamp is the current system timestamp updated during the last
18 event or application query.
19 The difference (tstamp - trigger_tstamp) defines the elapsed time.
21 The ALSA API provides two basic pieces of information, avail
22 and delay, which combined with the trigger and current system
23 timestamps allow for applications to keep track of the 'fullness' of
24 the ring buffer and the amount of queued samples.
26 The use of these different pointers and time information depends on
27 the application needs:
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
31 queued samples have been played out.
33 When timestamps are enabled, the avail/delay information is reported
34 along with a snapshot of system time. Applications can select from
35 ``CLOCK_REALTIME`` (NTP corrections including going backwards),
36 ``CLOCK_MONOTONIC`` (NTP corrections but never going backwards),
37 ``CLOCK_MONOTIC_RAW`` (without NTP corrections) and change the mode
38 dynamically with sw_params
41 The ALSA API also provide an audio_tstamp which reflects the passage
42 of time as measured by different components of audio hardware. In
43 ascii-art, this could be represented as follows (for the playback
44 case):
45 ::
47 --------------------------------------------------------------> time
48 ^ ^ ^ ^ ^
49 | | | | |
50 analog link dma app FullBuffer
51 time time time time time
52 | | | | |
53 |< codec delay >|<--hw delay-->|<queued samples>|<---avail->|
54 |<----------------- delay---------------------->| |
55 |<----ring buffer length---->|
58 The analog time is taken at the last stage of the playback, as close
59 as possible to the actual transducer
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
63 supported in hardware by sample counters or wallclocks (e.g. with
64 HDAudio 24MHz or PTP clock for networked solutions) or indirectly
65 estimated (e.g. with the frame counter in USB).
67 The DMA time is measured using counters - typically the least reliable
68 of all measurements due to the bursty nature of DMA transfers.
70 The app time corresponds to the time tracked by an application after
71 writing in the ring buffer.
73 The application can query the hardware capabilities, define which
74 audio time it wants reported by selecting the relevant settings in
75 audio_tstamp_config fields, thus get an estimate of the timestamp
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
79 time with dedicated hardware, possibly synchronized with system time,
80 removes the need to keep track of internal DSP processing times and
81 latency.
83 In case the application requests an audio tstamp that is not supported
84 in hardware/low-level driver, the type is overridden as DEFAULT and the
85 timestamp will report the DMA time based on the hw_pointer value.
87 For backwards compatibility with previous implementations that did not
88 provide timestamp selection, with a zero-valued COMPAT timestamp type
89 the results will default to the HDAudio wall clock for playback
90 streams and to the DMA time (hw_ptr) in all other cases.
92 The audio timestamp accuracy can be returned to user-space, so that
93 appropriate decisions are made:
95 - for dma time (default), the granularity of the transfers can be
96 inferred from the steps between updates and in turn provide
97 information on how much the application pointer can be rewound
98 safely.
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
104 or reported as is (the latter being useful to compare progress of
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
108 precise start time (patches WIP)
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
113 hardware components the delay is typically not known with precision.
115 The accuracy is reported in nanosecond units (using an unsigned 32-bit
116 word), which gives a max precision of 4.29s, more than enough for
117 audio applications...
119 Due to the varied nature of timestamping needs, even for a single
120 application, the audio_tstamp_config can be changed dynamically. In
121 the ``STATUS`` ioctl, the parameters are read-only and do not allow for
122 any application selection. To work around this limitation without
123 impacting legacy applications, a new ``STATUS_EXT`` ioctl is introduced
124 with read/write parameters. ALSA-lib will be modified to make use of
125 ``STATUS_EXT`` and effectively deprecate ``STATUS``.
127 The ALSA API only allows for a single audio timestamp to be reported
128 at a time. This is a conscious design decision, reading the audio
129 timestamps from hardware registers or from IPC takes time, the more
130 timestamps are read the more imprecise the combined measurements
131 are. To avoid any interpretation issues, a single (system, audio)
132 timestamp is reported. Applications that need different timestamps
133 will be required to issue multiple queries and perform an
134 interpolation of the results
136 In some hardware-specific configuration, the system timestamp is
137 latched by a low-level audio subsystem, and the information provided
138 back to the driver. Due to potential delays in the communication with
139 the hardware, there is a risk of misalignment with the avail and delay
140 information. To make sure applications are not confused, a
141 driver_timestamp field is added in the snd_pcm_status structure; this
142 timestamp shows when the information is put together by the driver
143 before returning from the ``STATUS`` and ``STATUS_EXT`` ioctl. in most cases
144 this driver_timestamp will be identical to the regular system tstamp.
146 Examples of timestamping with HDAudio:
148 1. DMA timestamp, no compensation for DMA+analog delay
149 ::
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
159 2. DMA timestamp, compensation for DMA+analog delay
160 ::
162 $ ./audio_time -p --ts_type=1 -d
163 playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153
164 playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947
165 playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685
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
170 ::
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
180 Example 1 shows that the timestamp at the DMA level is close to 1ms
181 ahead of the actual playback time (as a side time this sort of
182 measurement can help define rewind safeguards). Compensating for the
183 DMA-link delay in example 2 helps remove the hardware buffering but
184 the information is still very jittery, with up to one sample of
185 error. In example 3 where the timestamps are measured with the link
186 wallclock, the timestamps show a monotonic behavior and a lower
187 dispersion.
189 Example 3 and 4 are with USB audio class. Example 3 shows a high
190 offset between audio time and system time due to buffering. Example 4
191 shows how compensating for the delay exposes a 1ms accuracy (due to
192 the use of the frame counter by the driver)
194 Example 3: DMA timestamp, no compensation for delay, delta of ~5ms
195 ::
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
206 Example 4: DMA timestamp, compensation for delay, delay of ~1ms
207 ::
209 $ ./audio_time -p -Dhw:1 -t1 -d
210 playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520
211 playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740
212 playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081
213 playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907
214 playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824
215 playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847