diff options
Diffstat (limited to 'Documentation/sound/alsa/timestamping.txt')
| -rw-r--r-- | Documentation/sound/alsa/timestamping.txt | 12 |
1 files changed, 6 insertions, 6 deletions
diff --git a/Documentation/sound/alsa/timestamping.txt b/Documentation/sound/alsa/timestamping.txt index 1b6473f393a8..9d579aefbffd 100644 --- a/Documentation/sound/alsa/timestamping.txt +++ b/Documentation/sound/alsa/timestamping.txt @@ -14,7 +14,7 @@ provides a refined estimate with a delay. event or application query. The difference (tstamp - trigger_tstamp) defines the elapsed time. -The ALSA API provides reports two basic pieces of information, avail +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. @@ -53,21 +53,21 @@ case): 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 +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 natured of DMA transfers. +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 what the hardware supports, define which +The application can query the hardware capabilities, define which audio time it wants reported by selecting the relevant settings in -audio_tstamp_config fields, get an estimate of the timestamp +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 @@ -169,7 +169,7 @@ playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -51 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 abut +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 |