I was taking a look on ebay about roland analog stuff and i came accross an auction of a Roland IR3R05 custom vca/vcf (this is use in the Roland MKS 80 analog synth module and MKS 70 digital analog hybrid synth module).
The seller claims that this chip could be used in a custom design as a moog filter/curtis amp (a 2 in 1 package)
The question is: Does anyone have any idea if and how this could be of any use to the community?
I put this topic in both design and parts sections, because I think it has to do with both subjects.
I relize it is a bit expensive, but woulnâ t it be of very high quality? - And wouldnâ t it simplify the process of building a whole filter module from scratch?
I am not at all sure i am right i just take a rough guessâŚ
I put this topic in both design and parts sections, because I think it has to do with both subjects.
With exactly same content - Itâs still considered as cross posting, and cross posting is still considered harmful as it clutters the forum.. Oh well.
And wouldnâ t it simplify the process of building a whole filter module from Ă scratch?
Using a chip you canât get datasheet for? Youâd have to trace circuit from Roland service manuals..
Since people seem to be replying in this thread, Iâll copy my response from the other thread:
Itâs an interesting idea, but the lack of general availability of the chips might make it of somewhat limited usefulness.
While working on my MBHP SID project (and toying around ideas for my MBHP POKEY and PSG synth projects), I started looking into some hybrid digital/analog synth projects. Currently Iâm considering using a PIC as a digital oscillator, and running it through an analog VCF/VCA.
Using SSM and CEM chips makes designing analog circuitry easier, but thereâs not many of them to go around. Fortunately, there exists the LM13700 OTA (operational transconductance amplifier). There are some simple circuits out there (for example, the 13700 datasheet) that illustrate how to use half of one chip as a VCA. One 13700 can also be used at a 12 dB (2-pole) filter - chain two of them for a 24 dB filter!
If one searches Google for âJuergen Haible Wasp Filter Cloneâ, youâll find a clone of the filter section from the Wasp. Iâm planning to swap out the no-longer-available CA3080s with a 13700 filter section. Hopefully itâll sound cool _and_ have the parts be available from most suppliers.
A MBHP Core with an AOUT module would be great to output envelopes, LFOs, etc. which in turn could be connected to analog VCFs and VCAsâŚ
Since people seem to be replying in this thread, Iâll copy my response from the other thread:
Thank you
âŚI started looking into some hybrid digital/analog synth projects. Ă Currently Iâm considering using a PIC as a digital oscillator, and running it through an analog VCF/VCA.
Iâd like you to take a look at 82C54 timer/counter chip. Itâs predecessor 82C53 was used in many Roland DCO based synths. One chip has three separate 16bit counters, and it has âSquare wave generatorâ mode. So, these can be used to make 3 audible frequencies from one high frequency clock signal.
Whatâs so nice about these chips, is that once you set up a counter - it will keep counting until changes are made in the control register. So it needs very little resources from the controller. And Hey! - Ă this chip is available and probably will be, in near future.
A MBHP Core with an AOUT module would be great to output envelopes, LFOs, etc. which in turn could be connected to analog VCFs and VCAsâŚ
Yes, of course
Bye, Moebius
p.s. I didnât make calculations for this, but I hope that usable ultrasonic frequencies (still near enough half note relationships) are available, so those that need more waveforms (sawtooth waveshaper is in Roland service notes) can make use of a âdigisoundâ type wavetables ;D
Ahh, excellent. Instead of using the 12-bit AOUT (though it would be nice for analog envelopes, etc.), I was thinking of actually using a dedicated PIC for waveform generation - with one âoscillatorâ outputting to an 8-bit resistor ladder, and two sub-oscillators each on a 4- or 6-bit resistor ladder. Grungy!
p.s. I didnât make calculations for this, but I hope that usable ultrasonic frequencies (still near enough half note relationships) are available, so those that need more waveforms (sawtooth waveshaper is in Roland service notes) can make use of a âdigisoundâ type wavetables ;D
Good olâ wavetables. Two weeks ago I added a modular synth to my Things I Wonât Build For Years list, and located the digisound ROM as part of it. :) If the 8254 can run at low-MHz frequencies, it would probably be quite useful as a wavetable driver clock.
I was thinking of actually using a dedicated PIC for waveform generation - with one âoscillatorâ outputting to an 8-bit resistor ladder, and two sub-oscillators each on a 4- or 6-bit resistor ladder. Ă Grungy!
So, You like challenges? Good! But I take Thorstens word, âtrade off, isnât so highâ. I mean: Your time expensive - EPROMs / 8-bit DACs cheap. Grungy - Surey Like, one 27C64 and using 128 bytes per waveform yields 64 waveform locations. Using this type approach, you âd be concentrating more in the âinterestingâ coding/designing challenges (UI, Waveform switching? Envelopes? Modulation Matrix, maybe?), than just hand optimizing assembler code (and headbanginâ) to make it efficient enough to handle waveform generation with external controlâŚ
Two weeks ago I added a modular synth to my Things I Wonât Build For Years list, and located the digisound ROM as part of it. Ă :)
That list has the strangest design I have seen - I seems to work on âRandom In - Random Outâ principle, grows in size continuously, still loses some content now and then - and buffer limits are yet to be discovered ;D
So, You like challenges? Good! But I take Thorstens word, âtrade off, isnât so highâ. I mean: Your time expensive - EPROMs / 8-bit DACs cheap. Grungy - Surey Like, one 27C64 and using 128 bytes per waveform yields 64 waveform locations. Using this type approach, you âd be concentrating more in the âinterestingâ coding/designing challenges (UI, Waveform switching? Envelopes? Modulation Matrix, maybe?), than just hand optimizing assembler code (and headbanginâ) to make it efficient enough to handle waveform generation with external controlâŚ
My plan (one of these years) is to use the PIC as a wavetable oscillator only. The only control would be MIDI In (interrupt-based, very basic support), one analog input to select the waveform, and thatâs it. All envelopes / modulation / etc. will be in hardware. The PIC just serves as a MIDI interpreter and 3-oscillator wavetable in one, reducing chip count.
And if that doesnât work, Iâve got a 2864 EEPROM kicking around that I can use - thatâs the backup plan ;) No doubt by the time I get around to this project, my time will be even more precious.
That list has the strangest design I have seen - I seems to work on âRandom In - Random Outâ principle, grows in size continuously, still loses some content now and then - and buffer limits are yet to be discovered Ă ;D
No kidding! I already had too many MBHP-based projects on my list, and then I decided to use the PIC as an oscillator and/or synth. Then I found the Droid-3 synth project, got some more ideas, and looked at how to do audio filters in software. Learned that itâs not too hard to do it in hardware instead, found out about OTAs, located schematics for simple LFOs and EGs, and then added a minimodular to my list.
