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Midi 2CV8 Linearity Issue
http://www.paia.com/talk/viewtopic.php?f=8&t=514
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Author:  PAiA-Scott [ Thu Apr 04, 2013 11:08 am ]
Post subject:  9700 MIDI2CV8, IC13

It's good that you have updated the IC13 section of the board. In addition to virtually eliminating the possibility of crosstalk between cv and triggering channels, this will also lighten the load of this MIDI2CV8 module on the wall-mounted transformer, leaving more power available for the other modules in a P9700S.

About the amount of slack in the converter, and the adjustment of the scale trim, while it can be useful measuring the voltage for the setting, it is possible you might still be able to get the trim set so the scaling is within the error range. For the possible 128 MIDI Note numbers, 7 of the 8 bits will be used to make the pitch control voltage (8 bits will give 256 steps and 7, 128). When setting the trim for the v/oct scaling and starting at zero, the first and up to ten octaves will be in one volt increments. This will fit ten octaves into the 128 possible steps. This v/oct, ten octave linear ramp's 0.083 volt steps will have some error range at the upper and lower edges. The middle of a step will be about 0.04 volts between the upper and lower edges. So the trim can be set to set the tilt of the slope about plus or minus 0.04 volts and still work to produce the octave relationships on the exponential-response VCO connected and producing the pitches. So, with some tweaking of the trim between these possible limits, there should be a setting that minimizes error due to the bit-level tolerances. With it in the general range with the multitester reading, connect it to the VCO and nudge the trim a bit one way or the other so the octave relationship works out for various keys pressed on the keyboard controller.

Author:  PAiA-Scott [ Thu Apr 04, 2013 11:27 am ]
Post subject:  Re: Midi 2CV8 Linearity Issue

Hey Brian! OK, I'll try.

As mentioned in the previous post, MIDI Note Messages are just seven bits so an eight be converter does work to make the steps to produce the twelve notes for a keyboard octave (up to ten octaves). When there are more bits used to make the voltage pitches in-between can be produced or even the voltage monitored with any needed correct being made. Silent Way, I think it is, makes a MIDI converter that monitors the pitch of the VCO and changes the DAC generated voltage to correct according to the MIDI Note Number.

Author:  lucidsounds [ Fri Apr 05, 2013 2:04 pm ]
Post subject:  Re: Midi 2CV8 Linearity Issue

Hi Scott et all,

The DAC0800 arrived and I installed it.

It is not perfect but the error is reduced by a factor of about 50%, so before where I saw, say, 0.008V error I now see 0.004V. Together with the VCO tweaks you suggest, I would say it is now within acceptable tolerances. I suspect using a DAC0802 would yield even better results, but I'm happy with it now as it is. It is in tune to my ears, whereas it definitely wasn't before.

Many thanks for your help, I now have an in-tune and very nice sounding modular 9700. :D

Lucid.

Author:  Kyhotay [ Sat Apr 06, 2013 12:43 pm ]
Post subject:  Re: Midi 2CV8 Linearity Issue

Hey Scott,

I haven't had any time to test my system yet but I did look up the Silent Way - Expert Sleeper MIDI converter. It looks like an interesting solution.

So since the topic is various solutions to convert MIDI to CV, I thought I'd add a few things I've picked up on the way.

One solution is Metasonix r-60. http://www.metasonix.com/index.php/r-60 ... -interface It, too, listens and adjusts. The basic unit is more expensive I believe than the Expert Sleeper.

Of course, both of those solutions are Eurorack not Frac. I just received a TipTop Audio Happy Ending Kit what is just a frame, power supply and a "floating" power distribution ribbon cable. It's $150 so a bit more than the PAiA Frac with the 9771-15. It seems to me that PAiA, if they wanted to capture some of the Eurorack market, could easily adapt the frac rack and power supply as a low cost alternative for the Euro market. All of the Euroracks are much more expensive than the TipTop, which itself is a cheap solution. Anyway there's my $0.02 worth of marketing advice.

Two other converters that I know of that are rack "agnostic" would be the MOTU Volta software. It's $200 but you'll also need a MOTU interface with multiple outs. I have their 8-Pre, so unless I added an ADAT to TRS converter, I would only have two CV/Gates. The other possibility is to find an old Encore Expressionist. I found it to be great to set up between V/Oct, Hz/V, positive and S triggers, but it has a poorly designed interface. They are also out of production. I sold mine on eBay for slightly more than what they cost new. Tony at Encore did tell me he is thinking about going back into production, so who knows.

Of course there are other MIDI>CV out there, even Blacet has a frac module. But I have to say that the PAiA 9700 is a way easy to use converter and provides a lot of functionality and the price is great! The only thing I wish it had was rotary switches to change the function and MIDI channel. I had that damned DIP Switch if I want to change its personality!

Author:  ModularMe [ Sat Apr 06, 2013 4:25 pm ]
Post subject:  Re: Midi 2CV8 Linearity Issue

Newb questions on DACs. Is it worthwhile to substitute a DAC0802 for the MC1408L6, just as a general "aftermarket" upgrade? Is the increased accuracy worth the cost of a chip swap?

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