[LAD] ALSA Sequencer timestamp on event without scheduling

Felipe Ferreri Tonello eu at felipetonello.com
Wed Nov 30 12:18:12 UTC 2016

Hi Clemens and others,

Any thoughts on this? I would like to start working on it.

On 30/09/16 21:32, Felipe Ferreri Tonello wrote:
> Hi Clemens,
> On 30/09/16 19:46, Clemens Ladisch wrote:
>> Felipe Ferreri Tonello wrote:
>>> This event we are discussing is time based.
>>> The only difference is that the event time is not its delivered time,
>>> but some time in the past.
>>> I just want to make ALSA Sequencer support this idea, which is new and a
>>> requirement for MIDI over BLE to work properly.
>> I see no such requirement in the BLE-MIDI specification, which says:
>> | To maintain precise inter-event timing, this protocol uses 13-bit
>> | millisecond-resolution timestamps to express the render time and event
>> | spacing of MIDI messages.
>> and:
>> | Correlation between the receiver’s clock and the received timestamps
>> | must be performed to ensure accurate rendering of MIDI messages, and
>> | is not addressed in this document.
>> In the context of the ALSA sequencer, "rendering" means delivery.
> Yes, the correlation exists on the current implementation I am working
> on[1].
> The problem is the latency on receiving BLE packets. The timestamp the
> device sends and the "receiver's clock" (ALSA Seq) will very quickly get
> out-of-sync, causing this timestamp to be invalid. Thus we need somehow
> to set the event timestamp to any arbitrary value, in practice it will
> be the BLE-MIDI timestamps (normalized to the central timestamp, of course).
> [1] https://github.com/ftonello/bluez/blob/midi/profiles/midi/midi.c#L138

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