| The ALSA API can provide two different system timestamps: |
| |
| - Trigger_tstamp is the system time snapshot taken when the .trigger |
| callback is invoked. This snapshot is taken by the ALSA core in the |
| general case, but specific hardware may have synchronization |
| capabilities or conversely may only be able to provide a correct |
| estimate with a delay. In the latter two cases, the low-level driver |
| is responsible for updating the trigger_tstamp at the most appropriate |
| and precise moment. Applications should not rely solely on the first |
| trigger_tstamp but update their internal calculations if the driver |
| provides a refined estimate with a delay. |
| |
| - tstamp is the current system timestamp updated during the last |
| event or application query. |
| The difference (tstamp - trigger_tstamp) defines the elapsed time. |
| |
| The ALSA API provides two basic pieces of information, avail |
| and delay, which combined with the trigger and current system |
| timestamps allow for applications to keep track of the 'fullness' of |
| the ring buffer and the amount of queued samples. |
| |
| The use of these different pointers and time information depends on |
| the application needs: |
| |
| - 'avail' reports how much can be written in the ring buffer |
| - 'delay' reports the time it will take to hear a new sample after all |
| queued samples have been played out. |
| |
| When timestamps are enabled, the avail/delay information is reported |
| along with a snapshot of system time. Applications can select from |
| CLOCK_REALTIME (NTP corrections including going backwards), |
| CLOCK_MONOTONIC (NTP corrections but never going backwards), |
| CLOCK_MONOTIC_RAW (without NTP corrections) and change the mode |
| dynamically with sw_params |
| |
| |
| The ALSA API also provide an audio_tstamp which reflects the passage |
| of time as measured by different components of audio hardware. In |
| ascii-art, this could be represented as follows (for the playback |
| case): |
| |
| |
| --------------------------------------------------------------> time |
| ^ ^ ^ ^ ^ |
| | | | | | |
| analog link dma app FullBuffer |
| time time time time time |
| | | | | | |
| |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| |
| |<----------------- delay---------------------->| | |
| |<----ring buffer length---->| |
| |
| The analog time is taken at the last stage of the playback, as close |
| as possible to the actual transducer |
| |
| The link time is taken at the output of the SoC/chipset as the samples |
| are pushed on a link. The link time can be directly measured if |
| supported in hardware by sample counters or wallclocks (e.g. with |
| HDAudio 24MHz or PTP clock for networked solutions) or indirectly |
| estimated (e.g. with the frame counter in USB). |
| |
| The DMA time is measured using counters - typically the least reliable |
| of all measurements due to the bursty nature of DMA transfers. |
| |
| The app time corresponds to the time tracked by an application after |
| writing in the ring buffer. |
| |
| The application can query the hardware capabilities, define which |
| audio time it wants reported by selecting the relevant settings in |
| audio_tstamp_config fields, thus get an estimate of the timestamp |
| accuracy. It can also request the delay-to-analog be included in the |
| measurement. Direct access to the link time is very interesting on |
| platforms that provide an embedded DSP; measuring directly the link |
| time with dedicated hardware, possibly synchronized with system time, |
| removes the need to keep track of internal DSP processing times and |
| latency. |
| |
| In case the application requests an audio tstamp that is not supported |
| in hardware/low-level driver, the type is overridden as DEFAULT and the |
| timestamp will report the DMA time based on the hw_pointer value. |
| |
| For backwards compatibility with previous implementations that did not |
| provide timestamp selection, with a zero-valued COMPAT timestamp type |
| the results will default to the HDAudio wall clock for playback |
| streams and to the DMA time (hw_ptr) in all other cases. |
| |
| The audio timestamp accuracy can be returned to user-space, so that |
| appropriate decisions are made: |
| |
| - for dma time (default), the granularity of the transfers can be |
| inferred from the steps between updates and in turn provide |
| information on how much the application pointer can be rewound |
| safely. |
| |
| - the link time can be used to track long-term drifts between audio |
| and system time using the (tstamp-trigger_tstamp)/audio_tstamp |
| ratio, the precision helps define how much smoothing/low-pass |
| filtering is required. The link time can be either reset on startup |
| or reported as is (the latter being useful to compare progress of |
| different streams - but may require the wallclock to be always |
| running and not wrap-around during idle periods). If supported in |
| hardware, the absolute link time could also be used to define a |
| precise start time (patches WIP) |
| |
| - including the delay in the audio timestamp may |
| counter-intuitively not increase the precision of timestamps, e.g. if a |
| codec includes variable-latency DSP processing or a chain of |
| hardware components the delay is typically not known with precision. |
| |
| The accuracy is reported in nanosecond units (using an unsigned 32-bit |
| word), which gives a max precision of 4.29s, more than enough for |
| audio applications... |
| |
| Due to the varied nature of timestamping needs, even for a single |
| application, the audio_tstamp_config can be changed dynamically. In |
| the STATUS ioctl, the parameters are read-only and do not allow for |
| any application selection. To work around this limitation without |
| impacting legacy applications, a new STATUS_EXT ioctl is introduced |
| with read/write parameters. ALSA-lib will be modified to make use of |
| STATUS_EXT and effectively deprecate STATUS. |
| |
| The ALSA API only allows for a single audio timestamp to be reported |
| at a time. This is a conscious design decision, reading the audio |
| timestamps from hardware registers or from IPC takes time, the more |
| timestamps are read the more imprecise the combined measurements |
| are. To avoid any interpretation issues, a single (system, audio) |
| timestamp is reported. Applications that need different timestamps |
| will be required to issue multiple queries and perform an |
| interpolation of the results |
| |
| In some hardware-specific configuration, the system timestamp is |
| latched by a low-level audio subsystem, and the information provided |
| back to the driver. Due to potential delays in the communication with |
| the hardware, there is a risk of misalignment with the avail and delay |
| information. To make sure applications are not confused, a |
| driver_timestamp field is added in the snd_pcm_status structure; this |
| timestamp shows when the information is put together by the driver |
| before returning from the STATUS and STATUS_EXT ioctl. in most cases |
| this driver_timestamp will be identical to the regular system tstamp. |
| |
| Examples of typestamping with HDaudio: |
| |
| 1. DMA timestamp, no compensation for DMA+analog delay |
| $ ./audio_time -p --ts_type=1 |
| playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 |
| playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 |
| playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 |
| playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 |
| playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 |
| playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 |
| |
| 2. DMA timestamp, compensation for DMA+analog delay |
| $ ./audio_time -p --ts_type=1 -d |
| playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 |
| playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 |
| playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 |
| playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 |
| playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 |
| |
| 3. link timestamp, compensation for DMA+analog delay |
| $ ./audio_time -p --ts_type=2 -d |
| playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 |
| playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 |
| playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 |
| playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 |
| playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 |
| playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 |
| |
| Example 1 shows that the timestamp at the DMA level is close to 1ms |
| ahead of the actual playback time (as a side time this sort of |
| measurement can help define rewind safeguards). Compensating for the |
| DMA-link delay in example 2 helps remove the hardware buffering but |
| the information is still very jittery, with up to one sample of |
| error. In example 3 where the timestamps are measured with the link |
| wallclock, the timestamps show a monotonic behavior and a lower |
| dispersion. |
| |
| Example 3 and 4 are with USB audio class. Example 3 shows a high |
| offset between audio time and system time due to buffering. Example 4 |
| shows how compensating for the delay exposes a 1ms accuracy (due to |
| the use of the frame counter by the driver) |
| |
| Example 3: DMA timestamp, no compensation for delay, delta of ~5ms |
| $ ./audio_time -p -Dhw:1 -t1 |
| playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 |
| playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 |
| playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 |
| playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 |
| playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 |
| playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 |
| playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 |
| |
| Example 4: DMA timestamp, compensation for delay, delay of ~1ms |
| $ ./audio_time -p -Dhw:1 -t1 -d |
| playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 |
| playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 |
| playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 |
| playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 |
| playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 |
| playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847 |
