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

From: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>
Date: Fri, 1 Jun 2012 23:12:14 +0300
Message-ID: <CAJhzemWX1JK4bGz4grvksxtK8=DoX2r05=4c8eRq_3Q+XpnXeQ@mail.gmail.com>
To: Adam Goode <agoode@google.com>
Cc: public-audio@w3.org, Chris Wilson <cwilso@google.com>
Ah, you're right, in that case I don't see a reason to duplicate it.

Cheers,
Jussi

On Jun 1, 2012 11:09 PM, "Adam Goode" <agoode@google.com> wrote:
>
> On Fri, Jun 1, 2012 at 4:02 PM, Jussi Kalliokoski
> <jussi.kalliokoski@gmail.com> wrote:
> > The reason I kind of like the idea of having the timestamps specified as
> > DOMHighResTimeStamps is that it will allow the accuracy to live outside
the
> > spec, for example if in the future it somehow becomes desirable to have
more
> > accuracy than double precision, the DOMHighResTimeStamp will probably be
> > updated by then to use a higher precision as well. Although I don't
think a
> > use case for higher resolution than double will come along very soon.
Having
> > the timestamps be related to the creation time of the MIDIAccess is a
very
> > good idea actually, because it makes the problem of accuracy
deterioration a
> > slightly smaller problem. We probably need to introduce some method to
get
> > the current timestamp of the MIDIAccess as well.
> >
>
> If I'm reading the spec correctly, DOMHighResTimeStamp is defined to
> be relative to the start of navigation to the page. I don't think we
> should redefine this, but we could store the creation timestamp into
> each MIDIAccess object if necessary.
>
> http://www.w3.org/TR/2012/CR-hr-time-20120522/#sec-DOMHighResTimeStamp
>
>
> Adam
>
>
> > Cheers,
> > Jussi
> >
> >
> > On Fri, Jun 1, 2012 at 10:39 PM, Chris Wilson <cwilso@google.com> wrote:
> >>
> >> Gah!  yes, sorry, didn't hit reply-all. Only thing in Gmail I'm still
not
> >> quite used to, somehow.
> >>
> >> Yes, I agree that it's not great to have so many different timestamp
> >> formats and reference points.  If the desire is to divorce from
wallclock
> >> time, then I supposed we could do like audioContext does - from when
> >> MIDIAccess is created.  As written in Jussi's last edit, though, it's
> >> "current time" (unfortunately, the definition of what that means (ms
since
> >> UNIX epoch) was removed).  I don't have strong feelings.  I mostly
disliked
> >> DOMHighResTimeStamp because it's one more reference, for what is
essentially
> >> a trivial thing (monotonically increasing, number of milliseconds,
unrelated
> >> to wallclock time), but that spec is really defined for uses relating
to
> >> Performance, so it's confusing to read as a solution for this problem.
 I
> >> think we would need to define our own zero point.
> >>
> >> I like seconds just because I think if it's not integer anyway, it's
> >> easier for humans to think that way, but I don't care that strongly.
 The
> >> newer MIDI interfaces in Windows, I note, use a longlong (64bit int) of
> >> units of 100ns (i.e. tenths of a microsecond, or 0.0001 milliseconds).
 I
> >> think that is kind of confusing, personally.  Seconds are prevalent in
the
> >> Web Audio API, but milliseconds (as ints) are common in other web
> >> programming APIs, so I could be okay with either.
> >>
> >> On Fri, Jun 1, 2012 at 12:17 PM, Adam Goode <agoode@google.com> wrote:
> >>>
> >>> On Fri Jun 01 13:53:52 GMT-400 2012, Chris Wilson <cwilso@google.com>
> >>> wrote:
> >>>>
> >>>> Well there you go - it's been quite a while since I wrote Windows
code.
> >>>>  :)
> >>>>
> >>>>
> >>>> >The point of DOMHighResTimeStamp is that it is divorced from
> >>>> > wallclock time.
> >>>>
> >>>> So is audioContext.currentTime.
> >>>>
> >>>
> >>> Hmmm. It's not great to have so many different timestamp formats and
> >>> reference points. It does make sense for audioContext to have its 0
point at
> >>> its start time. And there is no "start time" for these raw MIDI
events. So
> >>> deferring to page load time seems fine.
> >>>
> >>> But the units are different (seconds in float vs. milliseconds in
> >>> double), and that seems worth addressing.
> >>>
> >>>
> >>> (Did we drop off the public list with this thread?)
> >>>
> >>> Adam
> >>>
> >>>> On Fri, Jun 1, 2012 at 10:49 AM, Adam Goode <agoode@google.com>
wrote:
> >>>>
> >>>> On Fri, Jun 1, 2012 at 12:47 PM, Chris Wilson <cwilso@google.com>
wrote:
> >>>> >
> >>>> > Although I'm not completely opposed to this change, I'd argue
against
> >>>> > the point that millisecond resolution is insufficient.  If using
hardware
> >>>> > MIDI ports, it takes approximately 1/4 of a millisecond to SEND a
single
> >>>> > byte of data - so it will take approximately 3/4 of a millisecond
to simply
> >>>> > transfer the data anyway - and the latency in processing at the
other end is
> >>>> > typically much, much higher than 1ms (I seem to recall around
4-7ms was not
> >>>> > atypical for hardware synths, but can't find my reference ATM).
> >>>> >
> >>>>
> >>>> The issue is more of jitter, not of processing delay. Though 1ms
seems
> >>>> totally sufficient to me, I could imagine issues with the single byte
> >>>> timing code (F8) getting some unwanted jitter. But the real win of
> >>>> this change is monotonicity.
> >>>>
> >>>> >
> >>>> > That said, of course, it's not a bad idea to future-proof better
than
> >>>> > that; many MIDI use cases will never actually see a 5-pin-DIN
cable.
> >>>> >  However,
> >>>> >
> >>>> > 1) I find the usage of DOMHighResTimeStamp very confusing, as it's
> >>>> > deliberately chained to (in terms of "zero" point) to the
Performance
> >>>> > interface.  It doesn't seem to add any value to reference here,
since it's
> >>>> > simply a double; we would still need to provide a way to get
system time in
> >>>> > double units, as I don't think using the PerformanceTiming
interface is the
> >>>> > most intuitive thing to do.  Or suggest that people use Date.now()
(even
> >>>> > though it's millisecond-precision), which is livable, I suppose.
 But we do
> >>>> > need to define that.  I would recommend either a) using a double
for number
> >>>> > of milliseconds, and recommending people use Date.now, or b) (my
preference)
> >>>> > use a double to represent number of seconds, to be uniform with
the Web
> >>>> > Audio API.  I'm ambivalent about whether we use the same
currentTime from
> >>>> > the audioContext as WA or Date.now().
> >>>> >
> >>>>
> >>>> The point of DOMHighResTimeStamp is that it is divorced from
wallclock
> >>>> time. All the MIDI implementations use this kind of time stamp (even
> >>>> Windows, read on).
> >>>>
> >>>>
> >>>> >
> >>>> > 2) I would absolutely recommend that we (similar to
> >>>> > DOMHighResTimeStamp) explicitly state that implementations are
allowed to
> >>>> > have millisecond-only precision in their implementation.  The
underlying
> >>>> > system APIs on Windows are based in milliseconds, for example -
unless
> >>>> > they're building another API, the time stamps on MIM_DATA are in
> >>>> > milliseconds. The underlying API on OSX is a bit harder to
determine
> >>>> > precision, but I think it is higher.
> >>>> >
> >>>>
> >>>> Actually the ONLY part of DirectMusic that is undeprecated (it
> >>>> disappeared briefly in Vista, then was replaced in a service pack) is
> >>>> high resolution monotonic MIDI timestamps:
> >>>>
> >>>> http://msdn.microsoft.com/en-us/library/ee416788(VS.85).aspx#ID4EFEAC
> >>>> http://support.microsoft.com/kb/943253
> >>>>
> >>>>
> >>>> So yes, we can specify that the timestamps might only have ms
> >>>> resolution, but I don't think it's really required.
> >>>>
> >>>>
> >>>> Adam
> >>>>
> >>>> >
> >>>> >
> >>>> >
> >>>> > On Fri, Jun 1, 2012 at 8:48 AM, Jussi Kalliokoski
> >>>> > <jussi.kalliokoski@gmail.com> wrote:
> >>>> >>
> >>>> >> This issue is now pending review per
> >>>> >> https://dvcs.w3.org/hg/audio/rev/b78b7c5e906e .
> >>>> >>
> >>>> >>
> >>>> >> On Fri, Jun 1, 2012 at 6:22 PM, Jussi Kalliokoski
> >>>> >> <jussi.kalliokoski@gmail.com> wrote:
> >>>> >>>
> >>>> >>> Good catch, thank you! As I planned it, the timestamp should have
> >>>> >>> been a floating point value, allowing for sub-millisecond
precision, but
> >>>> >>> actually DOMHighResTimeStamp is actually more fit fore this.
> >>>> >>> I will make the necessary changes to the spec.
> >>>> >>>
> >>>> >>> Cheers,
> >>>> >>> Jussi
> >>>> >>>
> >>>> >>>
> >>>> >>> On Fri, Jun 1, 2012 at 6:16 PM, Audio Working Group Issue Tracker
> >>>> >>> <sysbot+tracker@w3.org> wrote:
> >>>> >>>>
> >>>> >>>> Audio-ISSUE-105 (MIDI timestamp resolution): timestamps in MIDI
> >>>> >>>> should use High Resolution Time [MIDI API]
> >>>> >>>>
> >>>> >>>> http://www.w3.org/2011/audio/track/issues/105
> >>>> >>>>
> >>>> >>>> Raised by: Adam Goode
> >>>> >>>> On product: MIDI API
> >>>> >>>>
> >>>> >>>> The current MIDI API specifies timestamp as a long representing
> >>>> >>>> "milliseconds from the UNIX Epoch".
> >>>> >>>>
> >>>> >>>> For MIDI applications, millisecond resolution is insufficient
and
> >>>> >>>> can cause noticeable jitter.
> >>>> >>>>
> >>>> >>>> Using absolute wallclock time is also problematic, as it is
subject
> >>>> >>>> to system clock skew.
> >>>> >>>>
> >>>> >>>> The MIDI timestamp should use High Resolution Time
> >>>> >>>> (DOMHighResTimeStamp), which solves these problems:
> >>>> >>>>
> >>>> >>>>
http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/HighResolutionTime/Overview.html
> >>>> >>>>
> >>>> >>>>
> >>>> >>>>
> >>>> >>>
> >>>> >>
> >>>> >
> >>>>
> >>>>
> >>
> >
Received on Friday, 1 June 2012 20:12:46 GMT

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