[sdiy] Hi / search? / programmable resistor for VCO?

[cs80 at therogoffs.com]

Thanks guys.  Let me make sure I follow on the math.  One semitone is the 12th-root of 2 = 1.059463094359295).    0.5dB -> 1.059253725177289, which is the same to 0.02%!  Cool - I never realized these values were almost the same.  So, the LM1972 would give me almost 100 semitone steps with a total of 1% error over the whole range.   The PGA 2310 ( http://www.ti.com/lit/ds/symlink/pga2310.pdf ) might even be better with 0.5dB steps from +31.5db to -95.5dB, or 127 semitones.    

The catch is that I wouldn’t have any resolution smaller than a semitone.  I guess I could put this in series with another multiplier with a range of around +/-7% to get the extra resolution for tuning and correction.

These could even replace most of the guts of the main, exponential DACs in the Yamaha (with a bit of logic to convert their 3-bit octave + 4-bit note code into a single, 7-bit note code).

Lots to think about - especially with 16 VCOs to deal with and wanting to change as little as possible in the existing electronics.

 David


On Jan 7, 2014, at 12:46 AM, Ullrich Peter <Peter.Ullrich at kapsch.net> wrote:

> Have a look at this one:
> http://www.ti.com/lit/ds/symlink/lm1972.pdf 
> 
> The LM1972 is a digitally controlled 2-channel 78dB audio attenuator fabricated on a CMOS process.
> Each channel has attenuation steps of 0.5dB from 0dB-47.5dB, 1.0dB steps from 48dB-78dB, with a
> mute function attenuating 104dB.
> 
> Ciao
> Peter
> 
> -----Ursprüngliche Nachricht-----
> Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Roman Sowa
> Gesendet: Dienstag, 7. Januar 2014 08:55
> An: synth-diy at dropmix.xs4all.nl
> Betreff: Re: [sdiy] Hi / search? / programmable resistor for VCO?
> 
> try to look for "digital audio attenuators". I'm sure there are models with 0.5dB/step response, which is exactly what you need for a keyboard.
> Well, you need about 0.502dB/semitone but the chips will be much less precise than that anyway.
> Another option is to place at least 12-14 bit DAC in each VCO and control them via look-up table by microcontroller.
> 
> Roman
> 
> W dniu 2014-01-07 05:24, cs80 at therogoffs.com pisze:
>> Hi all.
>> 
>> Thought I was already subscribed here but I guess not.   I have an
>> idea/questions about a digitally controlled resistor for a VCO.
>> 
> 	<snip>
>> 
>> Any ideas on this?  Anyone know of parts like the AD but with exp/log 
>> response - maybe designed for audio use / attenuators?  Any more 
>> knowledgable info on resistor ladders?
>> 
>> Thanks much for any ideas and/or pointers!
>> 
>> Happy Analog New Year!
>> 
>> David
>> 
>> 
>> _______________________________________________ Synth-diy mailing list 
>> Synth-diy at dropmix.xs4all.nl 
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> 
> 
> The information contained in this e-mail message is privileged and confidential and is for the exclusive use of the addressee. The person who receives this message and who is not the addressee, one of his employees or an agent entitled to hand it over to the addressee, is informed that he may not use, disclose or reproduce the contents thereof, and is kindly asked to notify the sender and delete the e-mail immediately.
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy