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Re: MIDI Tracks and Sequences (API Proposal)

From: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>
Date: Sat, 15 Sep 2012 22:01:02 +0300
Message-ID: <CAJhzemXMJ8LOOUgHqM+iMiMy7PgyP5Un75UMoYvVcNe2tQtB1A@mail.gmail.com>
To: James Ingram <j.ingram@netcologne.de>
Cc: Chris Wilson <cwilso@google.com>, Joseph Berkovitz <joe@noteflight.com>, public-audio@w3.org
On Sat, Sep 15, 2012 at 3:47 PM, James Ingram <j.ingram@netcologne.de>wrote:

> Hi Chris, all,
>
> (I've replied to Chris's other post separately)
>
> Am 14.09.2012 18:04, schrieb Chris Wilson:
>
>> On Fri, Sep 14, 2012 at 8:52 AM, James Ingram <j.ingram@netcologne.de<mailto:
>> j.ingram@netcologne.de**>> wrote:
>>
>>     Yes, I understand that, but think that the data structures
>>     underlying Tracks and Sequences are so basic that it would be hard
>>     to see them becoming restrictive.
>>
>> The real question would be "what is the value of codifying those data
>> structures into a standard?".
>>
>
> Well, they would not really need to be "coded into the standard", their
> implementation could be hidden behind the relevant parts of the API. To
> recap a little more precisely, I think that the Web MIDI API should include
> support for the following abstractions:
>

To be clear, everything that goes into the API needs to defined in the
standard.


> Sequence: a collection of Tracks
> Track: a collection of MIDIMoments
> MIDIMoment: a collection of MIDIMessages having the same timestamp
>
> I'm not sure, but maybe a MIDIMoment is what the current spec is calling a
> MIDIEvent.
>

Actually the MIDIEvent can contain MIDIMessages with different timestamps.
This is because the underlying API may already use timestamped messages
which lets the implementation give the messages to the API consumer ahead
of time if they're sent ahead of time. Also because the thread might be
blocked while messages were received, so the implementations can group the
messages that gathered during that blocking and send them out in a single
MIDIEvent to avoid overhead. There's no wording of this in the spec yet,
but that's roughly what will be there.


> These are simple, intuitively graspable concepts, which every MIDI
> programmer can understand, and I don't think they are going to change over
> time.
>
> It would be up to the implementation to decide how to implement the
> collections. I could well imagine a Sequence containing a simple array of
> Tracks, and Tracks being kept as sorted, linked lists. MIDIMoments could be
> defined to play "as fast as possible" in order of the message index.
> Implementations are then in control of what "as fast as possible" means
> (apropos "throttling")... Hiding things like that from MIDI programmers is
> a bit of unexpected extra value... :-)
>
> The main reason I want to nail these abstractions down is that I think
> that trying to play Sequences in the user thread is a mistake. Player
> threads really need to sleep between sending messages, and sleeping is
> something the user thread is not allowed to do.
> Also, MIDI programmers should always expect maximum accuracy, and should
> not have to fiddle with variables to get it. Accuracy should be the
> system's problem, not the MIDI programmers'.
>
> A sequence.play() function would initiate one or more worker threads (I'd
> be inclined to have one thread per track), and play the tracks back using
> ordinary sleep() functions. That would be much less hassle, and much more
> accurate, than forcing MIDI programmers to use setInterval() or
> setTimeout() in the user thread.
> The sequence.play() function needs to know exactly how the data structures
> are organised, but the MIDI programmers don't.
>
> Perhaps it would help if I outline what I think ought to be in the API.
> These details would need discussing, of course, but I think the following
> list is pretty complete:
>
> Constructors (empty objects):
> Sequence()
> Track()
> MIDIMoment()
>

MIDIMoment seems like an implementation detail to me, I don't think the API
consumer should have to keep collection of what messages share the same
timestamp.


> Populators:
> sequence.addTrack(track);
> track.addMIDIMoment(**midiMoment)
> midiMoment.appendMIDIMessage(**MIDIMessage) // returns false if the
> timestamp is wrong
>
> Player functions:
> sequence.playSection(**fromTimestamp, toNotIncludingTimestamp);
> sequence.play() // shorthand for sequence.playSection(0, infinity);
> sequence.stop() // stop and rewind to the current fromTimestamp
> sequence.pause() // stops without rewinding
> sequence.resume() // play from the current timestamp up to the current
> toNotIncludingTimestamp
>
> I'd also like to have a way of filtering the tracks, so that not all of
> them are actually played.
> Perhaps like this:
> track.plays(boolean) // default is true
>

I still think it's a pretty bad idea to add this to the API rather than as
a user-side library. The idea can of course be revisited once the current
API is fully defined, specced and hopefully implemented. Or any time,
really, but I very strongly oppose to adding something like this to the API
at this stage.

Cheers,
Jussi
Received on Saturday, 15 September 2012 19:01:29 GMT

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