On Mon, Jun 20, 2016 at 01:08:27PM +0200, Pierre-Louis Bossart wrote: > > >Presentation time is either set by > >a) Local sound card performing capture (in which case it will be 'capture > > time') > >b) Local media application sending a stream accross the network > > (time when the sample should be played out remotely) > >c) Remote media application streaming data *to* host, in which case it will > > be local presentation time on local soundcard > > > >>This value is dominant to the number of events included in an IEC 61883-1 > >>packet. If this TSN subsystem decides it, most of these items don't need > >>to be in ALSA. > > > >Not sure if I understand this correctly. > > > >TSN should have a reference to the timing-domain of each *local* > >sound-device (for local capture or playback) as well as the shared > >time-reference provided by gPTP. > > > >Unless an End-station acts as GrandMaster for the gPTP-domain, time set > >forth by gPTP is inmutable and cannot be adjusted. It follows that the > >sample-frequency of the local audio-devices must be adjusted, or the > >audio-streams to/from said devices must be resampled. > > The ALSA API provides support for 'audio' timestamps > (playback/capture rate defined by audio subsystem) and 'system' > timestamps (typically linked to TSC/ART) with one option to take > synchronized timestamps should the hardware support them. Ok, this sounds promising, and very much in line with what AVB would need. > The intent was that the 'audio' timestamps are translated to a > shared time reference managed in userspace by gPTP, which in turn > would define if (adaptive) audio sample rate conversion is needed. > There is no support at the moment for a 'play_at' function in ALSA, > only means to control a feedback loop. Ok, I understand that the 'play_at' is difficult to obtain, but it sounds like it is doable to achieve something useful. Looks like I will be looking into what to put in the .trigger-handler in the ALSA shim and experimenting with this to see how it make sense to connect it from the TSN-stream. Thanks! -- Henrik Austad
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