linuxaudio.org

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On Tue, Jun 5, 2012 at 3:44 PM, David Robillard  wrote:
> On Tue, 2012-06-05 at 06:34 -0400, Jeremy Salwen wrote:

quoted text
> > Hi All,

> >

> > From what I can tell, it looks like the LV2 Atom Sequence

> > specification allows you to send events with arbitrary units for the

> > timestamp.

> >

> > I have a few questions about this:

> >

> > 1. How are we to know whether a particular unit uses the double field

> > in the timestamp union, or the uint64_t field in the timestamp union.

>

> Context.  When used as event ports in the run() callback, it is always

> frames, like everything else.

>

> I deliberately avoided complicating it with a bunch of stuff to handle

> beat-based time stamps in this context since this would almost certainly

> be unused (learned that lesson with the event extension).  It can be

> added later if need be.

>

> > 2. The specification says "The unit field is either a URID that

> > described an appropriate time stamp type, or may be 0 where a default

> > stamp type is known."  In what circumstances would the timestamp be

> > known?  When can I expect to see a zero in that field?

>

> See above.

>

> > 3. Are the timestamps absolute times?  Or relative to the previous

> > event?  Or relative to the start of the audio chunk? Does it depend on

> > the units used?

>

> Absolute from the start of the Sequence.

>

> > 4. Why does the documentation show |FRAMES |SUBFRMS| as the timestamp

> > field? From what I can tell, there is no unit which includes frames

> > and subframes subdivided that way, and the sampler example just uses

> > the full 64 bits as a frames field.  Is this just a relic from the old

> > event port documentation? (the diagram seems familiar).

>

> Oops, relic.  I will fix this.

>

> > 5. How are hosts/plugins supposed to deal with the multiplicity of

> > units?  For example, suppose I'm a plugin or host that wants to

> > receive MIDI data. How am I supposed to know what timestamp unit to

> > expect?  Is there a facility for converting between different units

> > automatically?  The only extra information required to convert between

> > any two time units would be the bpm and sample rate.  It would be

> > rather annoying to have to implement a bunch of unit conversion code

> > in every host/plugin you write in order to make sure it can handle any

> > unit which is tossed at it.

>

> Indeed it would be.  Hence why none of this is there, so the extension

> does not imply you are expected to have to deal with any of that.  I

> suppose the fact that it is audio frames in this context needs to be

> more clear.

>

> In short, when used as port buffers, it's always frames, don't worry

> about it.  The potential for other time stamp types is there for other

> cases, since it's possible to use Sequences in other non-port-buffer

> ways (e.g. you could send around events with entire sequences as their

> payloads)

>

> Cheers,

>

> -dr

>

> P.S. lv2-dev is a better place for this sort of thing

>

> Thanks David,

If the timestamps are absolute, are they reset on every call to activate()?
I thought lv2-dev was for developing the lv2 spec and extensions, but my

questions were more about implementation of plugins/hosts.
Jeremy
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On Tue, Jun 5, 2012 at 3:44 PM, David Ro=

billard <d@drobilla.net> wrote:
On Tue, 2012-06-05 at 06:34 -0400, Jeremy Salwen wrote:

quoted text
> Hi All,

>

> From what I can tell, it looks like the LV2 Atom Sequence

> specification allows you to send events with arbitrary units for the

> timestamp.

>

> I have a few questions about this:

>

> 1. How are we to know whether a particular unit uses the double field<=

br>

quoted text
> in the timestamp union, or the uint64_t field in the timestamp union.<=

br>

Context. =A0When used as event ports in the run() callback, it is alw=

ays

frames, like everything else.
I deliberately avoided complicating it with a bunch of stuff to handle

beat-based time stamps in this context since this would almost certainly

be unused (learned that lesson with the event extension). =A0It can be

added later if need be.
> 2. The specification says "The unit field is either a URID that

quoted text
> described an appropriate time stamp type, or may be 0 where a default<=

br>

quoted text
> stamp type is known." =A0In what circumstances would the timestam=

p be

quoted text
> known? =A0When can I expect to see a zero in that field?

See above.
> 3. Are the timestamps absolute times? =A0Or relative to the previous

quoted text
> event? =A0Or relative to the start of the audio chunk? Does it depend =

on

quoted text
> the units used?

Absolute from the start of the Sequence.
> 4. Why does the documentation show |FRAMES |SUBFRMS| as the timestamp<=

br>

quoted text
> field? From what I can tell, there is no unit which includes frames

> and subframes subdivided that way, and the sampler example just uses

> the full 64 bits as a frames field. =A0Is this just a relic from the o=

ld

quoted text
> event port documentation? (the diagram seems familiar).

Oops, relic. =A0I will fix this.
> 5. How are hosts/plugins supposed to deal with the multiplicity of

quoted text
> units? =A0For example, suppose I'm a plugin or host that wants to<=

br>

quoted text
> receive MIDI data. How am I supposed to know what timestamp unit to

> expect? =A0Is there a facility for converting between different units<=

br>

quoted text
> automatically? =A0The only extra information required to convert betwe=

en

quoted text
> any two time units would be the bpm and sample rate. =A0It would be

> rather annoying to have to implement a bunch of unit conversion code

> in every host/plugin you write in order to make sure it can handle any=

> unit which is tossed at it.
Indeed it would be. =A0Hence why none of this is there, so the extens=

ion

does not imply you are expected to have to deal with any of that. =A0I

suppose the fact that it is audio frames in this context needs to be

more clear.
In short, when used as port buffers, it's always frames, don't worr=

y

about it. =A0The potential for other time stamp types is there for other

cases, since it's possible to use Sequences in other non-port-buffer

ways (e.g. you could send around events with entire sequences as their

payloads)
Cheers,
-dr
P.S. lv2-dev is a better place for this sort of thing
Thanks David,If the timestamps are absolute, are=

 they reset on every call to activate()?I thought lv2-dev was for d=

eveloping the lv2 spec and extensions, but my questions were more about imp=

lementation of plugins/hosts.
Jeremy
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