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DIY Synths General Discussion
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mosc
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PostPosted: Sun Apr 16, 2006 8:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Nice. I'd like to hear how your experiments with the tilt to MIDI interface goes.
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Blue Hell
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PostPosted: Sun Apr 16, 2006 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks cool ! Doing with the tools you describe shows you've got good eyes and hands ! I'm getting too old for that, man I can barely see those 805 "dust grains" even :-)

BTW, what accellerometer did you use ? I'd like to see data sheets of it.

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bigtex



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PostPosted: Sun Apr 16, 2006 10:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
Hey BigTex,

The Korg MS-02 contains both a log amp and an antilog amp. I've got the original schemo around somewhere, but this guy has thrown a lot of work into re-drawing the schemo:

http://www.cykong.com/Synths/Korg%20MS-02/Images/Korg-MS-02-Schematics-Coloured.gif

Cheers,
Scott


*Wow!!*

Those are exactly what I was looking for. Excellent. It's got a log amp and an "antilog" amp, which I'm assuming would be exponential. Now I can go both ways. Thanks!

My next step is to set an amplitude cutoff point where the amp's output goes from linear to logarithmic or exponential. I just need some fast comparators, I guess.
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Macaba



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PostPosted: Mon Apr 17, 2006 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

bigtex wrote:
It's got a log amp and an "antilog" amp, which I'm assuming would be exponential. Now I can go both ways. Thanks!


Nooo! The log amp converts from Linear scaling to Log scaling, and the anti-log converts from Log scaling to Linear scaling.
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Macaba



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PostPosted: Mon Apr 17, 2006 2:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:

BTW, what accellerometer did you use ? I'd like to see data sheets of it.


Datasheet:
http://www.freescale.com/files/sensors/doc/data_sheet/MMA2260D.pdf

Datasheet of accelerometer i'm going to try next:
http://www.freescale.com/files/sensors/doc/data_sheet/MMA7260Q.pdf
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Blue Hell
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PostPosted: Mon Apr 17, 2006 5:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Thx !
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bigtex



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PostPosted: Mon Apr 17, 2006 12:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Macaba wrote:
bigtex wrote:
It's got a log amp and an "antilog" amp, which I'm assuming would be exponential. Now I can go both ways. Thanks!


Nooo! The log amp converts from Linear scaling to Log scaling, and the anti-log converts from Log scaling to Linear scaling.


Actually, I think it does both. If you were to feed a linear signal into the antilog amp, you'd probably get an exponential signal out. That's how it would convert log to linear.

I think.


Here's what that site I referenced before says:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

So I think the "antilog" amp would be an exponential amp. If you were to run the log signal through two of them, you get linear out of the first, and exponential out of the second.

I think.
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zipzap



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PostPosted: Mon Apr 17, 2006 1:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Jürgen Haible, that very extreme diy-guy has done some VCOs with linear detune. Read about it here
http://www.oldcrows.net/%7Ejhaible/jh5/jh5.html
I don´t know, but i think thats got to do with morphing special characteristics, going from log to lin or stuff like that.
When i went to school i should have learned about log and e and r2d2. I didn´t. What´s your idea behind this? whats the purpose?
Is it like some softsynths, where you can shape the responce (slope) of an adsr for example? That´d be cool.
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bigtex



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PostPosted: Mon Apr 17, 2006 1:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've got a few ideas, mostly involving waveshaping and nonlinear sequencing (sequences that change speed across a measure, like a bouncing ball sound, a very IDM kind of sound), but I just want to understand some basic bulding blocks before I go trying to design a circuit and get it all wrong.
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zipzap



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PostPosted: Mon Apr 17, 2006 2:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

sounds interesting. sequencers are some fascination. I´ve been designing a lot around delay circuits, for generating shuffle. That Pingpong thing could be nicely with a lm3914 sequencer i think (other thread).
Abot that amp, do i get this right: You take eg an tiangle wave, put it in the amp and get variable shaping of the rise and fall part, like linear or log curve?
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bigtex



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PostPosted: Mon Apr 17, 2006 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

zipzap wrote:
Abot that amp, do i get this right: You take eg an tiangle wave, put it in the amp and get variable shaping of the rise and fall part, like linear or log curve?


That's right. So your 3914 (or discrete comparator ladder) will have the steps triggered at an accelerating or decelarating rate, depending on the way the triangle wave has been reshaped. Measures will still begin and end in time with other sequences running in parallel (if this is for just one layer of audio) because you can use that same triangle wave, unaffected, to trigger linear sequences.

I think this would sound really cool. You could create a very bizzarely changing syncopation between a regular beat and an accelerating beat, for example.

Of course, this could be done with a sine wave feeding a sequencer with no extra work, but having control over the shape of the sequencer's controlling wave would create some nice possibilities.
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zipzap



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PostPosted: Mon Apr 17, 2006 2:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I think this would sound really cool. You could create a very bizzarely changing syncopation between a regular beat and an accelerating beat

Do you know "Drumming" by steve reich?
He has a group of small stick drums playing a (complex) pattern. One by one the players (those are human musicians) start moving slightly forward or backward. First it sounds like flanging, then eventually it snapps in one microbeat ealier than the rest. Now the next player starts going forward or laid back.
This sounds wicked. We tried this during my studies (with a simple beat), man you need to controll yourself to play like that.
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Uncle Krunkus
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PostPosted: Mon Apr 17, 2006 4:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I might be completely off the track here, but as far as I remember, an exponential function is one which has an exponent, i.e. it is the function of one value raised to the power of the other. The relationship is not linear, as in a graph of it will not be a straight line. It will be a curve. So,
y=x^2
y=x^-2
y=log(x)
are all exponential functions.
So log is just a type of exponential function, of which there are many.

PS I'm not posting this to be a smart arse, I mainly want confirmation that it's right. If it's not I'm willing to learn. Smile

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bigtex



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PostPosted: Mon Apr 17, 2006 5:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

You know what? I'm not exactly sure... I don't really know the exact semantics of math, so I don't know if a log function is considered an exponential function. I don't think so, but I could easily be wrong (It's been a while!).

I guess it doesn't relaly matter what it's called..... just how it works, right? Smile
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Blue Hell
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PostPosted: Mon Apr 17, 2006 5:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
If it's not I'm willing to learn. :)


Exponential functions are of the form

y=p^x

where p is some constant. Thes functions have exponential growth f.i. when p = 2 and x goes from 0 upwards y will be 1, 2, 4 , 8, 16, 32, 64 i..e. it doubles on each iteration.

A function of the form

y=x^p is called a polynomial (function), it has no exponential growth but polynomial growth which for large x is much slower than exponential. Lets take p = 2 again and x as 0, 1, ... We then get for y : 0, 2, 4, 9, 16, 25, 36

No matter what p you choose an exponential function wil outgrow any polynomila functiion for sufficiently large x values. And that's the reason for making a difference between exponential and polynomial growth, it truly are two distinct classes of growth.

Functions of the form

y=logp(x)

are called logarithic, these grow even slower (in the sense that any polynomila will outgrow any logaritmic for sufficiently large x), but they are still unbounded in the sense that any arbitrary (large) value for y can be reached by picking a sufficiently large value for x. Log( 0) is undefined, no matter what p (but for x going to zero from positive y will become smaller and smaller going to minus infinity). I'll not give a range here but your calculator can do that for you.

the growth classes mentioned are not just a curiousity but have important consequences for computability, so called exponentially complex algorithms are practically unsolvable for large input sets. For instance cryptographic strength is based in this principle.

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Uncle Krunkus
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PostPosted: Mon Apr 17, 2006 5:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah,
I was gonna say that! Very Happy

(Thanks Jan)

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PostPosted: Mon Apr 17, 2006 6:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
Yeah,
I was gonna say that! Very Happy

(Thanks Jan)


Had expected you to spot the error I made Very Happy

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Uncle Krunkus
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PostPosted: Tue Apr 18, 2006 12:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Functions of the form

y=logp(x)

are called logarithic


I don't think there is such a word as logarithic.
Or is it that there is no such word as logaritmic?
Either way, one of them (and maybe both) is wrong.

BTW, I wouldn't normally try to pick someone up on their spelling or grammar, especially when english is not their first language.

If you were expecting me to see a hole in your maths, then you've obviously never sat around for days waiting for an overdue baby! Laughing

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PostPosted: Tue Apr 18, 2006 1:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
never sat around for days waiting for an overdue baby! Laughing


I know how that feels. Elise made us wait a couple of weeks. Sort of a sweet limbo. The 3 boys were all on time.
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Blue Hell
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PostPosted: Tue Apr 18, 2006 3:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah a speling error as well :-)

No I've never been waiting like that, must be stressfull I can imagine having everything ready and nothing happens knowing you might have to rush at about any time. Hope it will turn out well !

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Uncle Krunkus
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PostPosted: Tue Apr 18, 2006 5:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Lets take p = 2 again and x as 0, 1, ... We then get for y : 0, 2, 4, 9, 16, 25, 36


No we don't. We get y : 0, 1, 4, 9, 16, 25 etc

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PostPosted: Tue Apr 18, 2006 10:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Very Happy
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zipzap



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PostPosted: Fri Apr 21, 2006 3:12 am    Post subject: Reply with quote  Mark this post and the followings unread

This is another small thing:
I was just playing with my sequencer and a filter. They use different PSUs.
The filter gets +15,0,-15v, the sequencer is running off a small 12v wallplug unit. The Filter can only respond to the sequencer if the two grounds are connected. If i compare the ground level between the two circuits with no connection i mesure about 3-6V. 5-20ma are running (the digital meter is not that fast...). thats 0,12w max.
Is that normal? Is that a problem (besides wasting the current)? Is this what a ground loop is?
The thing is also that i want to use an audio and a digital ground in the future. So then there should be no ground connection between the sequencer and the filter (only in the psu or the wall of my house). Why can´t the VCF respond to just the voltage coming from the seqencer? I mean it´s got it´s own ground to compare the voltage to. I don´t get that.
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zipzap



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PostPosted: Fri Apr 21, 2006 3:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Question 2:
In the shematic down there, can i just add more cv-ins via resistors to T16 (hard to read, it´s the one most bottom-left)?
Or should i use an summing OPamp?
thanks guys


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Uncle Krunkus
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PostPosted: Fri Apr 21, 2006 5:59 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm fairly sure that, to a point, you can just add CVs through resistors to T16. Have a look at the MFOS site and see how Ray adds CVs.
The VCF needs to see voltages which are relative to the 0V they were generated relative to. So you should have a ground connection between the sequencer and the VCF. If you got the ground through the PSU/Power Outlet chain then you would be building ground loops. The digital/audio ground is a per/unit concept designed to give the digital ground a faster short than through the analogue ground, so that the latter doesn't get contaminated. The ground to external units should come from the "star point" which is the junction between digital and analogue ground near the PSU. This sounds kind of weird, I know, because the first thought most people have is that if they are shorted together then there shouldn't be any difference between them. But it is based on the resistance of the actual ground connections (every wire has resistance) and the decoupling of different sections of the circuit, which gives the ground connection the ability to attenuate the high frequency transients on the digital ground. But only if they "see" the digital ground's smoothing caps first. i.e. if there was a faster way to ground through an analogue circuit on the same ground line, then these transients would be redirected through them. Thereby contaminating them with digital "spikes".
Does that make sense?
It's a bit esoteric, I know, but once you get your head around it, it makes sense.

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