Q: VCO pitch range

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antto
 
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Q: VCO pitch range

Post by antto »

on the 303, you have 3 octaves (plus C on the next octave) for the pattern, and you can transpose this to one octave up
the range is 3+1 octaves + 1 semitone = 4*12+1 = 49 notes
the same thing can be achieved in the x0xb0x too, that's obvious
however, iirc both sokkos and the adafruit firmware like the idea about adding another octave in the pattern range
..by pressing UP twice (blinking up)
i also wanted to have this feature in n0nx0x, UP+DOWN == double up
this gives a total of 49+12 = 61 notes

but there is a problem, when using this new upper octave and using the transpose feature, some notes get "clipped"
that's because in the firmware, the note value is restricted between 0x00 and 0x3F that gives a total of 64 possible values
but C2 is 0x17 (23) thus, C1 (the lowest note) is 0x0B (11)
so there are about 10 lower pitches that aren't used
there is also a strange comment about this in the original firmware:
/* between 0x0 and 0xA, the VCO voltage pins, so these notes arent really
* 'effective' in that they all sound the same.
*/
i don't understand what this means, it sounds like broken english to me :shock: :?

my idea is, we can extend a propper 1 octave (61 notes) if we shift everything down so that 0x3F is the highest C key (C6) then C1 would be 0x03
and we'll have to retune the VCO (only the offset trimpot, which is the easiest thing, could be done by the TUNE knob too)

any comments?

guest
 
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Re: Q: VCO pitch range

Post by guest »

there is something about the analog hardware
that makes it impossible to play all the way down to 0v cv
im pretty sure its the an6562 that cant go below .5v or so

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antto
 
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Re: Q: VCO pitch range

Post by antto »

i'll do a few tests then

Brassteacher
 
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Re: Q: VCO pitch range

Post by Brassteacher »

guest wrote:there is something about the analog hardware
that makes it impossible to play all the way down to 0v cv
im pretty sure its the an6562 that cant go below .5v or so
This makes perfect sense. I would imagine the only way to guarantee being able to get down to 0V would be to add a negative rail to the power supply. Hmmm... :idea:

"Meditate on this, I will." - Yoda

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antto
 
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Re: Q: VCO pitch range

Post by antto »

uhm, wait
where is the problem?
the CPU sends the pitch voltage out
then, there is a trimpot which "scales" it, and another one that adds offset to it
and also the TUNE knob which also adds offset
and then, the result of all this goes to the VCO, right?

so, the problem is in the CPU output not being precise enough close to 0V ?

guest
 
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Re: Q: VCO pitch range

Post by guest »

no its the opamp between the dac and the vco
it has a limit as to how close to 0v it can get

you can replace it with a different opamp
that can get closer to 0v
they have some that get down to 10mv or so

textile
 
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Re: Q: VCO pitch range

Post by textile »

With the LM358 I can tie the CV direct to 0v and get a solid waveform.

I don't have any an6562 to test against.

Brassteacher
 
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Re: Q: VCO pitch range

Post by Brassteacher »

Does anyone know off the top of their head the frequency for the lowest C on a stock x0x? When I get some spare time (yeah, right), I'll breadboard a TB-303 VCO and play around with this probably crazy idea I have in mind. I'm not wanting to say anything specific at the moment, in case it doesn't work, therefore confirming beyond a shadow of a doubt that I'm an idiot... :mrgreen: If the crazy idea doesn't work, perhaps a small sub-oscillator on a daughtercard will suffice, albeit square wave only. I'm quite enamored with the sub-oscillator on my Roland SH-09, and the way that one is built, tuning is not an issue.

I'm still trying to finish the other x0x I'm building for semiconductor tests. I was all ready to go when I realized I hadn't finished etching the i/o board :? . That's done now, and power supply is all tested, as soon as interconnect cables are finished I'll be ready to roll. VCO only tests will be first...

bcbox
 
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Re: Q: VCO pitch range

Post by bcbox »

32.7Hz

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antto
 
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Re: Q: VCO pitch range

Post by antto »

but with the TUNE knob it can get even lower
i have a recording of a 303 with the oscillator at ~17Hz

Brassteacher
 
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Re: Q: VCO pitch range

Post by Brassteacher »

bcbox wrote:32.7Hz
Thanks! Well, since that's already the lowest "C" on the piano, plus the fact that it's also the lowest "C" in the audible range, I can't imagine going an octave lower than that, to 16.35Hz, unless you were going to reinforce the "C" at 32.7 that is already sounding. Pipe organs that have a rank of 64 foot pipes do this, and the effect is rather dramatic. However, doing that with a x0x, or any other synth really, you would need one HELL of an amp/speaker combination. There is a BIG difference between making a speaker cone simply move at 16Hz, and making it move at 16Hz with a strong enough motor and suspension to control the cone's excursion, in combination with an amp that can generate enough current, with a high enough damping factor to control the motor in such a speaker. Plus, the speaker would have to displace enough air to be heard at that octave with other sounds going on at the same time. A horn-loaded subwoofer comes to mind...

That said, a sub-oscillator may be a more useful alternative. I'm still going to try my little experiment though, because it still may turn out to be useful...

bcbox
 
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Re: Q: VCO pitch range

Post by bcbox »

I can't imagine going an octave lower than that, to 16.35Hz, unless you were going to reinforce the "C" at 32.7
you wouldn't hear that fundamental anyway, only the harmonics, since the the frequency response of the 303 is already -3dB @ 70Hz because of the coupling caps at the input of the VCA.

Brassteacher
 
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Re: Q: VCO pitch range

Post by Brassteacher »

bcbox wrote:
I can't imagine going an octave lower than that, to 16.35Hz, unless you were going to reinforce the "C" at 32.7
you wouldn't hear that fundamental anyway, only the harmonics, since the the frequency response of the 303 is already -3dB @ 70Hz because of the coupling caps at the input of the VCA.
Quite true. No matter what, you wouldn't really hear the fundamental as a tone. As for the roll off frequency, it would get just a little bit of a boost due an interesting phenomenon would happen when the sound finally gets out of the x0x and making the air vibrate through a transducer, assuming a sub-oscillator was being implemented: When a perfectly in tune octave occurs, the resultant tone (sometimes called "beat frequency") just happens to be the exact same pitch, or frequency, as the lowest note in the octave. This has the effect of increasing the amplitude a bit due to resonance, and is usually quite noticeable.

However, if the OP is going to go for the really deep bass effect, There is enough space to replace C20 and C21 with a much larger film cap. A 0.047 or even a 0.1 could easily fit in those spots. Right now, due to staying up way too late last night (working on a x0x, no less), I'm too lazy to do the math to figure the new roll off points with either of those 2 values. Also, C38, C58, and C59 could be replaced with larger. If it has to be electrolytic due to size constraints, I use Nichicon Muse bipolar caps, the ES series (The part number will start with UES). Not only are they high quality, and relatively cheap, they are quite compact enough. A 4.7uf/50V ES Muse will easily fit in the alloted space, and give better bass response and tone all the way around. Probably wouldn't sound "authentic" anymore, but neither do most mods 8) .

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antto
 
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Re: Q: VCO pitch range

Post by antto »

oh noes..
C20 and C21 have a huge effect with the filter
at least that's what i can say from my software synth
changing those has a great impact on the phases
the main output of the filter that goes directly to the VCA (C21) forms a 72Hz HP filter
and the filter itself has a HP filter in the feedback path, which i can't say where it is, there are lots of parts there, but the frequency happens to be also 72Hz (it's an approximitation)
there is an interesting result out of this, if you change the feedback HP - the phase gets ugly, resonance response changes dramatically, and something like a low-frequency oscillation tends to appear under the filter cutoff

in other words - i wouldn't change those values

Brassteacher
 
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Re: Q: VCO pitch range

Post by Brassteacher »

antto wrote:oh noes..
C20 and C21 have a huge effect with the filter
at least that's what i can say from my software synth
changing those has a great impact on the phases
the main output of the filter that goes directly to the VCA (C21) forms a 72Hz HP filter
and the filter itself has a HP filter in the feedback path, which i can't say where it is, there are lots of parts there, but the frequency happens to be also 72Hz (it's an approximitation)
there is an interesting result out of this, if you change the feedback HP - the phase gets ugly, resonance response changes dramatically, and something like a low-frequency oscillation tends to appear under the filter cutoff

in other words - i wouldn't change those values
:oops: That's what I get for reading an edited version of the x0x schematic instead of the TB-303 schematic. I assumed C21 and C22 were the input coupling caps to the VCA, when it actually appears to be C14 and C15. What does the simulated TB-303 do if C14 and C15 become 2.2uf bipolar caps?

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