[gmpi] Re: Topic 6: Time representation
- From: RonKuper@xxxxxxxxxxxx
- To: gmpi@xxxxxxxxxxxxx
- Date: Thu, 1 May 2003 09:28:19 -0400
Trying to rationalize the discussions from yesterday, to see where we are.
We agreed that there needs to be some fundamental unit and data type for
absolute time. (I'll call this data type ABSTIME.) Proposals were to use
either:
- Samples (as in the system audio sample rate)
- "Quanta", ie, an int64 value with a system specified "quanta per second"
to determine the rate
- Seconds (as a double precision float)
A plugin's "process" routine, ie, the function that renders frames of audio,
is supplied 2 ABSTIMEs: the stream time for the current frame, and the UST
(aka "wall time"), for synchronization.
[Brief aside about USTs: I occurred to me that different host platforms
will only be able to update UST values at different resolutions. So we
probably need a "GetUstResolution" API provided by the host to the plugin.]
I think we agreed on at least this much. The other issue discussed was how
musical time information gets passed into the plugin. I'll try to
synthesize a few possible solutions to this point, for the sake of picking
up the discussion again.
First, lets assume we'll define a data type for musical time. I'll call it
MUSTIME. TBD is whether this time uses bars/beats/ticks, floats, or other.
Proposal 1: A plugin's "process" routine is also provided with a MUSTIME
value that provides the current musical stream time for the frame. MUSTIME
is designed (cleverly<g>) to encode tempo information, so some "MUSTIMEs per
ABSTIME" (in this frame) can be computed. Tempo and meter changes are
delivered to the plugin "just-in-time", as separate events.
Proposal 2: The host provides time conversion routines, to go from ABSTIME
to MUSTIME. Implicit in this conversion is the tempo. The host also
provides access to tempo and meter information, if it exists.
Proposal 3: Events that are delivered to the plugin's "process" routine
have unique IDs. The host provides methods to cough up a MUSTIME (and/or
ABSTIME?) timestamp for an event, given its ID.
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