LF411 Op-Amp dilemma

Jim Shearer jim at base.demon.co.uk
Thu Jan 30 00:34:09 CET 1997

 In advance, apologies if this is seen 'late' in the discussion
as my service provider is a little slow at the moment :-) 

Brendan Heading <lists at heading.demon.co.uk> writes

------------- SNIP ---------------
>>> Some confusion about string synths <<<

Hi Brendan,

      Most string synths used a simple divider system run from >ONE<
fixed frequency  oscillator running at some 100's of Khz. (ARP's Quadra,
and I think, the Omni, use a simple 1-transistor/inductor osc)
   This >fixed frequency< signal fed a "Top Octave Generator" chip which
divided the signal into 12 frequencies, these corresponding to the 12
notes of the highest octave on the keyboard. As octaves are related by
factors of 2, it is then simply a matter of dividing each of these notes
by 2,4,8,16.... for as many octaves you want. Simple logic flip-flop's
will do this and so keep down the cost.
    The advantage of this system is reliable tuning and "tracking"
(doesn't go out of tune the further up/down the keyboard you play) and
full polyphony. The down-side is that it is relativly limited in
flexibility (compared to polyphonic synth's a la Prophet 5, OB8 etc)

-------------------- Tone Generation Scheme ------------------

      Fixed   _____________Top Octave
    Oscillator              Generator
                     ___________|______ _ _ __
                     |   |   |   |   | ......|
                     A5  A#5 B5  C5  C#5.....G#5
                     |  .....................|
                   div2 -A4                div2 -G#4
                     |  .....................|
                   div2 -A3                div2 -G#3
                     |  .....................|
                   div2 -A2                div2 -G#2
                     |  .....................|
                  etc etc                 etc etc

---------------------- Note Keying Scheme -----------------------

  +V (+12v)             A4 signal
      |      key A4      |
      |       _|_      __|__ gate
      |-------   ------|    |
      |                |____|
      |                  |________ A4 when key pressed _____
      |                                                     |
      |                 B4 signal                           |
      |      key B4      |                                  |
      |       _|_      __|__ gate                           |---> O/P
      |-------   ------|    |                               |
      |                |____|                               |
      |                  |________ B4 when key pressed _____|
      |                                                     |
      |                                                     |
     \/                                                    \/
   further                                               further
    keys                                                  keys
   These notes are then switched to the o/p circuits, either by the key
contacts directly (noisy), or indirectly via an electronic switch.
This is the technique used by the TDA1008.(Note- the amplitude of the
TDA1008's o/p is dependent upon the voltage level applied from the keys,
hence by using an appropriate R/C circuit, a simple but effective
envelope can be generated for each note individually! This is, in my
opinion, what gives the Crumar Performer it's wonderfully rich string
sound -chords swelling up whilst others fading, unlike the ARP Omni,
which rudely cut off a fading chord when a new chord is played.
   By the way, some designs use a simple narrow-range VCO for the Fixed
Oscillator so it can be modulated for vibrato effects.
   As you can see from above, the keys on the keyboard merely 'gate'
existing generated signals, in such a scheme, it makes no sense to talk
of 'controlling the oscillator' with them.
   Hope this clears things up Brendan.
Jim Shearer

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