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Re: Audio-ISSUE-105 (MIDI timestamp resolution): timestamps in MIDI should use High Resolution Time [MIDI API]

From: Chris Wilson <cwilso@google.com>
Date: Mon, 4 Jun 2012 12:40:48 -0700
Message-ID: <CAJK2wqWBotNyNogQQt-Vyg175aEaNxX4060bHmU3=B44FEt47Q@mail.gmail.com>
To: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>
Cc: Adam Goode <agoode@google.com>, public-audio@w3.org
Has any other spec/feature depended on DOMHRTS - or more particularly, on
window.performance.now()?  I'm wondering if this is expected/approved/a
good idea for features like this.  (I'm envisioning that we need a new
setTimeout too at some point - would it rely on this?)

On Mon, Jun 4, 2012 at 12:36 PM, Jussi Kalliokoski <
jussi.kalliokoski@gmail.com> wrote:

> On Mon, Jun 4, 2012 at 8:41 PM, Chris Wilson <cwilso@google.com> wrote:
>> I had some further discussion off-list with Chris, where he helped me
>> coalesce my thinking a little better.
>> I think there are three design factors to a timestamp-
>>    1. What type the actual numerical value is (e.g. WebAudio's
>>    currentTime/time params are double, as is DOMHighResTimeStamp - some others
>>    are long ints, or long longs in Windows).  More importantly for JS devs,
>>    though is what the numeric value *represents*: i.e., number of
>>    seconds (in WebAudio) or number of milliseconds (DOMHRTS and MIDI proposal).
>> I'd go for milliseconds as that's consistent with everything else on the
> web (Date.now(), setInterval, setTimeout, DOMHRTS). As to what the type of
> the number is, I'd say preferably a double (or more precise), but
> implementations can have different underlying optimizations and I don't see
> a need (nor do I think it's even possible) to impose further limitations on
> that. If the end developer needs just pure milliseconds and the decimal
> part will make his life harder, I hope they'll use a language that allows
> you to floor numbers, heh. :)
>>    1. What the "zero reference" (or start point) is - e.g. in WA, it's
>>    when the audioContext is created, for DOMHRTS in the
>>    window.performance.now() usage it's navigationStart (although by itself,
>>    the DOMHRTS type does not imply a "start point" - we need to fix this if we
>>    want to use it in the MIDI API, even if it's just to say it's the same as
>>    window.performance.now().)  For Date.now(), the zero reference/start point
>>    is 1 January 1970 00:00:00 UTC.
>>    1. On what clock the time proceeds.  This is somewhat subtle, because
>>    this is really to account for different clocks possibly running on
>>    different crystals - e.g., for WA this is the audio hardware clock. Like
>>    Data.now(), the performance.now() clock is just the system clock (except
>>    it's explicitly not supposed to adjust for system time[1]).  But just
>>    because you're using DOMHRTS type doesn't explicitly mean you're using the
>>    system clock - sections 4.3 and 4.4 are specific to the usage in
>>    window.performance.now(), and the rest of the spec simply defines the type
>>    (= #1 above).
> For consistency, I'd go with the navigationStart, i.e.
> window.performance.now(), we should avoid respecifying things that other
> groups have already specified, such as clocks. As for audio/midi clock
> synchronization, I think it isn't anything that needs to be visible to the
> end developer, the implementations should just synchronize with these
> internally, imho.
>> For the purposes of the MIDI API, I don't have any real problem with
>> using the DOMHRTS type - but I'm a) not really psyched about having to use
>> window.performance.now() in a MIDI application to get the zero-reference,
>> because it seems like that is really designed to be a performance-timing
>> thing, and b) a little less pleased that Web Audio times are in seconds,
>> and this timestamp would be in milliseconds.  Neither of those are
>> completely disastrous, but they both seem less than optimal.  Anyone else
>> have thoughts on that?  Any thoughts about how developers usually think?
> Personally I think it's a bit confusing that Web Audio API uses seconds
> instead of milliseconds, because like I said, most time related values on
> the web are presented as milliseconds.
> Cheers,
> Jussi
Received on Monday, 4 June 2012 19:41:18 UTC

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