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Midi control for a Digitech Talker Vocoder


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I am looking to add a few simple MIDI controls to a Digitech Talker Vocoder. There are only two functions I am looking to control, one is the bypass switch, the other is the switch that controls which preset is selected. The bypass looks like it could be pretty straightforward, but not certain on the program switching button. The way the pedal works is by default when it's powered on, the first of the six programs is selected named Nuvo. Pressing the program button cycles through the programs in order from left to right until it reaches the end and then repeats back to the first. I haven't opened the pedal up yet to check what the board looks like or what my options might be, also can't seem to find any schematic of the board just yet. 

Link to pedal: http://www.effectsdatabase.com/model/digitech/talker

talker.gif

What might I need to get started with something like this, a core and din? Any insight is appreciated.

 

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Most simple entry point would be the MBHP_CORE_STM32F4 module running with the MIDIbox NG firmware.

No DIN or DOUT module required, you could connect reed relays and buttons directly to J10A/B, configured as digital in- and outputs.

If the vocoder has some kind of "reset" for the program button, it would be easy to setup a .NGR script which issues the reset, and then sends pulses depending on the desired program number. But the documentation doesn't give enough info how a reset could be triggered.

Best Regards, Thorsten.

Edited by TK.
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At page #4 you can see, that BYPASS SELECT and PRESET SELECT are digital inputs with pull-up connected to 5V

If you configure one of the J10 ports as output in open drain mode, it could be directly connected to pin 3 and 4, and Vs (ground) to pin 1 of HDR2

The MIDIbox NG firmware doesn't allow to switch between push-pull mode and open-drain yet, but I planned to provide this option anyhow. :)

Page #6 shows, how the RESET# is generated, actually it's via open drain (resp. open collector) as well. This RESET# goes to U20 where also the buttons are connected. But I fear that it will not only reset the button logic, but also the DSP. Don't know what this means.

You could try this out by (shortly) connecting pin 10 of U20 to ground - does this reset the entire device? Any noticeable effect in the audio path?

Best Regards, Thorsten.

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  • 2 weeks later...

I just had a chance to open this up and take a look inside, I am seeing where HDR2 is, I am attaching a few pictures of the board to get an idea of what it physically looks like as opposed to the schematic (I took pics of both front and back of where this connects).

Also I am seeing the U20 chip and have few questions. I am trying to get a better idea of which is pin 10 on the U20 chip. Any help would be appreciated. One last piece of info, the only soldering gear I own is a 40 watt CSI Soldering Station 1A ( http://www.circuitspecialists.com/csi-station1a.html ) - I haven't soldered anything this small before, but should this iron be able to do this? From what I can tell it should be alright.

 

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Edited by M.U.L.E.
mistake on photo
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From the notch which is marked with a 1, count the pins counter-clockwise. You will notice that the 7th pin is marked with 7, which gives you the confidence that the 10th pin must be the 4th from the left side of the chip, counted downwards.

Best Regards, Thorsten.

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  • 1 month later...
On 11/9/2015 at 3:53 PM, TK. said:

Most simple entry point would be the MBHP_CORE_STM32F4 module running with the MIDIbox NG firmware.

No DIN or DOUT module required, you could connect reed relays and buttons directly to J10A/B, configured as digital in- and outputs.

If the vocoder has some kind of "reset" for the program button, it would be easy to setup a .NGR script which issues the reset, and then sends pulses depending on the desired program number. But the documentation doesn't give enough info how a reset could be triggered.

Best Regards, Thorsten.

Hi Thorsten, I just assembled the MBHP_CORE_STM32F4. I think I missed the portion where you stated I could use reed relays previously, and now am looking in to this. Do you have any recommendations on a relay I could order in the USA or a link to ones that would work for this application? Also for the MIDI control, should I be able to wire a MIDI IN / OUT directly to J11E? I think that was the other portion I was not clear on. I am guessing I would only actually need the MIDI IN for this project, but not clear on how that is done without a MBHP_MIDI_IO board. If you have any tips on that I would appreciate it.

Edited by M.U.L.E.
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  • 2 weeks later...

Before going further, you'll want to check how the reset behaviour impacts the device as Thorsten mentioned above. As all of the reset circuits are linked, applying a signal may interact with other parts with unpredictable results. And looking a bit closer, reset on U20 is an active high signal, so a +5V signal probably has to be applied to your pin 10. Formally this also drives the bases of Q6 and Q7 and activates the two other reset nodes.

One way might be to disconnect that pin 10 out of circuit by cutting the trace (if possible) and using a Core pin to drive reset high or low, but I couldn't tell you how to do that exactly (maybe via a suitable push pull pin + pull down resistor?). 

For MIDI in and out, it's not sufficient to wire directly to J11E because MIDI in should run through an optocoupler. It's quite an easy circuit and could be built on a little bit of perfboard. A ready-made solution is at the midibox-shop but I don't know how much room you've got in your box. Having a MIDI out is useful even just as a pass through port for other devices. You could also go old school Core8 which ends up being smaller and has MIDI in and out, but requires a different approach to programming/updating.

It springs to mind that a more compact MCU breakout board could be quite useful for basic "one time control" applications such as this.

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To the reed relays: I don't know a source in USA - e.g. Mouser and Dikikey have relays for 3.3V (resp. 3V), but they are pretty expensive (ca. 5 USD), there might be better options, e.g. via ebay

Important is the coil voltage - relays for 5V, 12V, 24V or higher won't work, search for 3.3V or 3V

Best Regards, Thorsten.

 

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