[gmpi] Re: 3.15 MIDI
- From: "Todor J. Fay" <todor@xxxxxxxxxxxxxx>
- To: <gmpi@xxxxxxxxxxxxx>
- Date: Wed, 16 Jun 2004 20:34:44 -0700
I don't know if this is helpful or not:
There is a MIDI standard for volume. It came belatedly in the form of
the DLS standard.
The DLS specification provides a formula for converting a MIDI volume
(or velocity for that matter) into dB:
db = 20 * log10((127/midivolume)^2)
This produces a curve that makes good use of the limited 128 step range.
It's a formula that was agreed upon because quite a few hardware
manufacturers had converged on something close to, if not exactly, this.
In my own work, I use this equation to convert into my internal MIDI
message format, which uses floats for everything. Volumes can be added
and subtracted in dB land, then converted back at the very end if sent
to MIDI equipment.
Todor
-----Original Message-----
From: gmpi-bounce@xxxxxxxxxxxxx [mailto:gmpi-bounce@xxxxxxxxxxxxx] On
Behalf Of Chris Grigg
Sent: Wednesday, June 16, 2004 7:59 PM
To: gmpi@xxxxxxxxxxxxx
Subject: [gmpi] Re: 3.15 MIDI
Jeff said:
>Ron said:
>> I'm not convinced this is achievable. For example, let's consider
the
>> conversion of MIDI volume controller to GMPI. MIDI volume doesn't
have a
>> standard mapping from 0-127 to gain values. So how does this value
get
>> converted within GMPI? How the does the converted value survive the
round
>> trip through a processor that does:
>> MIDI -> GMPI - > [Event Processing] -> GMPI -> MIDI.
>
>All 7 bit controllers get converted like so....
>
>float gmpi_value = (float) midi_cc / 127.0;
>
>.. and converted back like so...
>
>int midi_cc = (int) (gmpi_value * 127.0 + 0.5);
>
>( as usual, i've probley made some terrible coding error, like forgot a
>semicolon, or rounded down instead of up, but I hope you get the gist)
>
>Don't assume the conversion to GMPI is some over-complicated thing,
we're
>not going to convert everything to Angstroms or anything. It's just a
>converion to floating point.
>
>The intention isn't to reinvent the whole thing, only to give
controllers
>more resolution by converting to floating point.
I think what Ron meant is, out of 0-127 MIDI CC values, where's the
nominal 0 dB point? What midiVolNumber-to-dB-curve is used? MIDI
per se doesn't specify either thing, whereas we said GMPI parameters
would use natural/perceptual units, so for volume presumably that
would be expressed as dB. There is no MIDI-spec-governed conversion
between the input and the output, so each plug can decide to do a
significantly different thing, so in fact the host doesn't have
enough information to make the conversion, contradicting your claim
that the conversion is always trivial. You can see the problem.
-- Chris G.
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