antilog

[Fraser, Colin J]

> -----Original Message-----
> From: Jim Patchell [mailto:patchell at silcom.com]
> Sent: 10 August 2000 14:51
> To: synth-diy at node12b53.a2000.nl
> Subject: Re: antilog
> 
>     Depending on what you want to do, in general, a Look Up 
> Table is a lot
> simpler, and faster, to implement in a micro.  One possible 
> solution would be to
> have LUT values for all the semitones, and then linear 
> interpolate between this.

>     Another posible advantage to an LUT is that if you want a 
> pathagorean scale
> or just intonation, all you have to do is change your look up 
> table.  That is
> pretty hard to do with an analog exponential converter (which 
> gives you the
> equally out of tune, tempered, scale).

The advantage of the analogue expo conversion is that you have the same DAC
resolution across the pitch range.
With the expo conversion in software you lose resolution at the low end.
The 12 bit solution works, but only useably over 4 or 5 octaves at most.
'Close enough for rock and roll', or monosynths.

For microtuning, I would use a high res DAC driving an expo convertor, and
build a look up table of offsets for each note in the adjusted scale.
Then you only need to store pitch offsets for each note in any given octave,
rather than frequency offsets for every possible note.

Colin f