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