Subject: Re: [linux-audio-dev] Reference amplitudes and LADSPA (again)
From: Steve Harris (S.W.Harris_AT_ecs.soton.ac.uk)
Date: Fri Feb 01 2002 - 22:01:44 EET
On Fri, Feb 01, 2002 at 01:09:19 -0500, Paul Davis wrote:
>
> however, this clipping+mapping not necessarily a good idea. some h/w
> might come with an assumption that a 16 bit signal value less than
> 32767 was 0dB, and that values above this represent +dB values. i
> don't know of any audio interfaces that do this, but most HDR systems
> and digital mixers either work this way by default, or can be set to
> do so.
Yes, it was defined by the SMPTE that way for compatibility with analogue
hardawre. However its not important. 0dB(LADSPA) does not have to relate
in any way to 0dB(SMPTE), though in some cases it will, ie. 0db(LADSPA) =~
+18dB(SMPTE) and 0dB(LADSPA) =~ 0dB(FS), and thats fine.
> its therefore unwise to try to map the -1..+1 range of a LADSPA audio
> value to anything except to pin 1.0f as 0dB, and anything that follows
Unless your soundcard was wired to calibrated, SMPTE compliant AD
converters, but lets forget that for the moment!
> directly from this. having said this, all of my code assumes that 0dB
> represents "full range" for the relevant bit size, and i can't imagine
> people doing anything else for now.
One excpetion is a peak limiter, where you would expectthe input to
be > 0dB and the output to be <= 0dB
> obviously, LADSPA hosts and clients are free to deliver any values
> they want to, as long as they understand the consequences of those
> values falling outside the -1..+1 range.
Yes, but the problem is that some hosts don't understand the consequences,
and nothing the in the LADSPA spec, SDK or example plugins would lead them
to expect those consequences.
I could badger host authours into using 1.0f and keep telling people not
to use applyplugin, but it strikes more as more productive to get it
written into the spec.
- Steve
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