Mididac glide …tinkering

[rarecomponent]

I wonder if it’s at all possible to change the characteristics of the MidiDac glide / Portamento.

Ive noticed some other synths have a very “punchy- dynamic” glide sound whereas the MidiDacs is smooth and flows sweetly as opposed to that “in your face” type of glissando such as on disco / funk synths etc.

I posted in the tech dept if changing the C2 cap 1.5uF might alter things but they said it would take more than that. However, I don’t want to mess with the MidiDac as I just don’t have enough tech knowledge. Also mentioned was exponential characteristics and I’m not sure which the MidiDac employs …

If tinkering with the MidiDac is too risky, would I be better off with a separate DIY glide module ?

[Synthbuilder]

I wonder if it’s at all possible to change the characteristics of the MidiDac glide / Portamento.

The midiDAC's glide circuit is based around the TB-303 as well as similar functioning ones in the SH-101 and MC-202. It's a basic exponential glide circuit with pre-loading of the timing capacitor for the ability to turn the glide on and off whether you play legato or staccato.

There are really only two different glide circuits. One, is the exponential one which is based on a simple capacitor being charged by a resistor. This in my view produces the most musical glides and is found on most analogue synthesisers. It's pretty much a one pole analogue filter for the pitch control voltage. The glide starts off quickly and slows as it reaches its destination pitch.

The second is the linear type of glide. This is found on many digitally controlled analogue synths, as well as numerous VST synths, but also classics like the Pro One and most analogue polysynths. The glide moves in a straight line at a fixed rate from one pitch to another. I personally don't like this type of glide mainly because it has a definite end point.

A third type is also available which is a mixture of the two, which uses linear for large jumps but slows, exponential style, as it reaches its destination pitch. I believe the Minimoog does this.

Both the main types of circuits can be used with either V/oct and V/Hz VCOs with each type producing different effects.

Changing the capacitor on either type simply changes the effective range of the glide time. It doesn't affect the shape which is what I think you're after.

Changing an exponential glide circuit to a linear one is not straightforward. Usually a linear one requires a minimum of two op-amps, and we only have one to play with on the midiDAC. I think probably the easiest thing to do is use a separate linear glide module. The VRG will do it but that might be a bit overkill for glide. The EFG's lag generator will also do linear glides, but you can't turn the glide on and off automatically with the legato output from the midiDAC.

Actually, you mention glissando. I tend to think of this as different to portamento in that each note between the start and destination note are played in order and discretely. Standard portamento simply changes the pitch smoothly, swanee whistle style. Discrete glissando is more like what you would get if you tried to slide between notes on a piano or organ. This can be done on a modular using a quantiser between the glide module and the oscillators.

Tony

[rarecomponent]

Many thanks Tony for detailed and very informative reply.

Im in no way going to mess with the MidiDac, I hoped that changing the cap C2 might alter the
characteristics of the slide similar to an EG ie slower or faster but Ive learned now you can’t just do it that simply.

In the meantime Ive found quite a lot of slide / Portamento diy circuits and most are quite simple mainly using a double op amp chip.

I guess the best option is to make one or two up and just experiment. I like the idea of the Minimoog using a more complicated method, I’ll check the schematics. I know AJH do a slide module but it’s expensive.