[LAD] CV data protocol in apps.

Simon Jenkins sjenkins at blueyonder.co.uk
Fri Feb 19 14:55:51 UTC 2010

On 19 Feb 2010, at 14:30, Fons Adriaensen wrote:

> On Fri, Feb 19, 2010 at 01:59:34PM +0000, Simon Jenkins wrote:
>> On 18 Feb 2010, at 17:32, alex stone wrote:
>>> So it's feasible to create another type of port,  (CV/Fs), without
>>> crippling something else in jack, or damaging the current API?
>>> If so, surely that would enhance further Jack's capabilities, and open
>>> it up to more options for devs and users alike.
>> A reduced rate CV port doesn't really make much possible
>> that's not already possible with a full buffer of values
>> at the audio sampling rate.
> True. The advantage is that if there is a 'standard' for
> such control signals (e.g. 1/16) it becomes practical to
> store them as well. Of course you could do that at audio
> rate, but just imagine the consequences if you have e.g. 
> at least 4 control signals for each audio channel, as is
> the case in the WFS system here. There is a huge difference
> between having to store 48 audio files of an hour each,
> (25 GB) and 240 of them (125 GB) - in particular if most
> of that storage is wasted anyway. In a mixdown session 
> there can easily be much more than 4 automation tracks
> for each audio one. Reducing the rate at least brings
> this back to manageable levels.

Storage is a good point.

I'd been thinking mainly in terms of something like an (analog-style?) sequencer generating the CV in which case you don't store its outputs, you just set it running again.

But storage is a good point.

>> If a receiving application, for example, wants to update
>> its filter parameters at 1/16th the full sampling rate it
>> is perfectly capable of skipping 16-at-a-time along an
>> audio-rate buffer of values all by itself. Or 8-at-a-time.
>> Or 32 or whatever divisor makes most sense for *that*
>> application, or was configured by the user of that
>> application, or whatever.
>> Meanwhile this same buffer can be routed at the same
>> time to applications that would prefer the full rate data.
> All true, but you are confusing two quite separate issues:
> *internal update rate* and *useful bandwidth*.

I'm not confusing them, I just wasn't considering bandwidth as the variable we were trying to optimise for. Maybe it is though.

> - The internal update rate of e.g. a filter or gain control
>  would always have to be audio rate, to avoid 'zipper' effects.
>  The filter could e.g. use linear interpolation over segments
>  of 16 samples, or 32, or 256. This is an implementation
>  detail of the DSP code.
> - The useful bandwidth of control signals in audio is very
>  low. Even if the internal update rate is audio, there will
>  be no energy in the control signal above a few tens of Hz.
>  If you modulate a filter or gain stage with anything above
>  that bandwidth it is no longer just a filter or gain control
>  - you will be producing quite an obvious effect (e.g. vibrato).
>  That makes sense in synthesisers etc., but in any normal
>  audio processing it's something to be avoided.
> So with the exception of synth modules etc., control signals
> never need to be high rate,

So control signals don't need to be high rate, apart from the exceptions, which do. ;)

> and if they are the DSP code would
> have to filter out the HF parts. Actually 1/16 (3 kHz) would
> be more than sufficient for use as envelopes etc. in a synth
> as well, anything faster would just generate clicks.

If the user sends a 20khz sine wave into an application's "volume" port that's either their mistake, or its exactly what they wanted to do.

> -- 
> FA
> O tu, che porte, correndo si ?
> E guerra e morte !
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