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Beginner in need of a few basic answers
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Perejil



Joined: Jan 15, 2009
Posts: 10
Location: London, UK

PostPosted: Thu Jan 15, 2009 1:42 pm    Post subject: Beginner in need of a few basic answers Reply with quote  Mark this post and the followings unread

I am not new to electronics but I am new to synths and I'm looking at building them.

I understand all the circuit diagrams (as I said, i'm not new to electronics) but I don't fully understand how things connect together. I want to keep my first project relatively simple, and I wanted to know, can a VCO go straight to a VCA without worrying about envelopes and filters? or is ADSR an essential part of the system? (I've been looking at the diagrams on www.musicfromouterspace.com). I realise it would be less sophisticated but would it at least make music?

My other question is, what is the function of the control voltage on the VCA? does it control the amplitude in the way it controls frequency in the VCO or is it something else?

Many thanks, and I apologise for being so uninformed.
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DrJustice



Joined: Sep 13, 2004
Posts: 2114
Location: Morokulien
Audio files: 4

PostPosted: Thu Jan 15, 2009 4:15 pm    Post subject: Re: Beginner in need of a few basic answers Reply with quote  Mark this post and the followings unread

Hi Perejil,

welcome to electro-music.com!

Perejil wrote:
...can a VCO go straight to a VCA without worrying about envelopes and filters?

Yes, it can. Although you would get a rather static sound. The frequency of the VCO can usuallty be changed by applying a control voltage on its frequency control output. A typical VCO will also have a control for its basic frequency and perhaps a switch for selecting different waveforms.

Quote:

or is ADSR an essential part of the system?

ADSR (or similar) envelope generators are widely used in most types of synthesizers, and synthesists generally regard them as essential to the synthesis of musical sounds. As you probably know, they generate a changing voltage over time. This is very useful for controlling such things as frequency, timbre and amplitude.

Quote:

My other question is, what is the function of the control voltage on the VCA? does it control the amplitude in the way it controls frequency in the VCO or is it something else?

The control voltage on a VCA is an input that controls the amplitude of the VCA, hence the name Voltage Controlled Amplifier. The output of, e.g. an ADSR, is often connected to the VCA control voltage input, so that the amplitude will change over time according to the ADSR settings. An ADSR usually have an input to trigger the envelope so it (re)starts from the Attack portion

A very basic synthesizer could look like this:
Code:

         VCO                VCA
     +---------+        +-----------+
 >---|freq  out|------->|in      out|------> output
     |         |        |           |
     |         |    +-->|amplitude  |
     +---------+    |   +-----------+
                    |
        ADSR        |
     +---------+    |
 >---|trig  out|----+
     |         |
     |         |
     +---------+


In this diagram the "freq", "amplitude" and "trig" connections are all CV inputs that provides Voltage Control of some aspect of the different modules. The "in" and "out" connections make up the audio signal flow. The "freq" input will typically be connected to the output of a keyboard, which generates a voltage corresponding to the pressed key. The "trig" input would then be connected to the trigger output of the keyboard, which generates a positive voltage when a key is pressed. This illustrates the basic concept of a Voltage Controlled synthesizer. Of course, the frequency and trigger control voltages could come from something other than a keyboard, but keyboards are the most widely used type of controller.

The distinction between control voltages and signals is useful to begin with. However, when one gets into playing around with more adventurous synthesis, connecting signal outputs to control inputs is a big part of the game. E.g. FM synthesis is based on connecting the output of one oscillator to the frequency control input of another oscillator, such that the frequency changes very rapidly and you get a completely new sound.

DJ
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Perejil



Joined: Jan 15, 2009
Posts: 10
Location: London, UK

PostPosted: Thu Jan 15, 2009 5:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you, this has been very helpful. I have thought of a couple more things I'm not sure of:

The vco I've been looking at has a separate output for each waveform - if I wanted to combine more than one waveform would they require separate amplifiers or can they be combined through one?

Also, what is the actual musical difference (if there is one) between linear response and exponential response VCAs? I have found graphs but they didn't explain what it means for the sound, or the input method.

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"In the context of Philip, hair is dissonant." - An explanation of Schenkerian theory by Dr. J. P. E. Harper-Scott
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DrJustice



Joined: Sep 13, 2004
Posts: 2114
Location: Morokulien
Audio files: 4

PostPosted: Thu Jan 15, 2009 6:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Perejil wrote:
The vco I've been looking at has a separate output for each waveform - if I wanted to combine more than one waveform would they require separate amplifiers or can they be combined through one?

In most cases you can simply mix the waveforms by summing the outputs through series resistors of, say 1 kOhm, without any buffering amplifiers. In the specific VCO that you linked to, there are already 1kOhm resistors in series with each waveform output, so you can just connect the outputs together. Many, if not most, synth modules are made in this way so that mixing outputs does not require additional circuitry. You can also hook up some potentiometers if you want variable mixers :
Code:

                +------------------+----> mix output
                |                  |
  in1 >----+    |  in 2 >----+     |
           |    |            |     |
        R1 #    |          R2 #    |
           #<---+             #<---+
           #                  #     
           |                  |     
  gnd >----+------------------+----------> gnd     



Quote:
Also, what is the actual musical difference (if there is one) between linear response and exponential response VCAs? I have found graphs but they didn't explain what it means for the sound, or the input method.

A VCA with an exponential input will give a volume change that may sound more 'natural' when fed with the linear output from an ADSR. But there are no rules here, use what sounds best to you in any given situation.

DJ
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Sound



Joined: Jun 06, 2006
Posts: 842

Audio files: 1

PostPosted: Fri Jan 16, 2009 6:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hello Perejil, I recommend you to read "Electronic Music- Systems, Techniques and Controls " by "Allen Strange.

Regards.
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