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Re: [cosmacelf] Re: Interfacing the1802 and AY-3-8910A

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  • David W. Schultz
    ... Two of the do not substitute transistors are in Figure 5. This circuit is in effect, a VCO. Digital signals drive the bases of Q1-Q3 and adjust the
    Message 1 of 25 , May 17, 2013
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      On 05/14/2013 04:45 PM, Lee Hart wrote:
      > He put "do not substitute" on several of the transistors. One is a JFET,
      > and one is a unijunction. I'm not sure what's special about the others.
      > I just know they are not common generic part numbers.
      >

      Two of the "do not substitute" transistors are in Figure 5.

      This circuit is in effect, a VCO. Digital signals drive the bases of
      Q1-Q3 and adjust the collector current of Q4. (A three bit DAC) That
      current charges C5 which with Q5 (a unijunction transistor) forms a
      relaxation oscillator. The frequency is proportional to the collector
      current in Q4.

      But by using some different parts you can get the same thing only better.

      Instead of a hacked together 3 bit DAC use a DAC08. This is a basic 8
      bit DAC with current output. That output is perfect for charging a
      capacitor connected to a 555 except for one little detail: the DAC
      output is a current sink and you need a current source. Nothing a three
      transistor current mirror can't fix. Then use an opamp in place of that
      other do not substitute transistor. (That part has a higher than usual
      gain which minimizes loading on the capacitor voltage. Opamps are even
      better at that.)

      One downside to this is that the DAC limits you to a 256:1 range of
      frequencies. But that isn't all bad. The following voice selector is
      just a set of simple filters that are frequency dependent.


      --
      David W. Schultz
      http://home.earthlink.net/~david.schultz
      Returned for Regrooving
    • /M
      Or how about, but that may be a not that simple maybe, settuing up a table of reasonable weighted valuee (logarythmic), and having the 1802 send those values,
      Message 2 of 25 , May 18, 2013
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        Or how about, but that may be a not that simple maybe, settuing up a table of  reasonable weighted valuee (logarythmic), and having the 1802 send those values, randomly to the DAC that in turn feeds a VCO ... or better still have a VCO (1V/octave) that recieves random values from the computer. Then you can have the CPU send out values to a VCA...
         
         
        HEY, lets rethink somewhat here. I saw in eithe PE or RE a Chord Generator based on a top octave generator, whose OPs were mixed up to generate chords. These in turn were switched by random circuits. It could be possible to combine the two circuits with the 1802 as "dispatcher".
         
        Magnus
         
        ----- Original Message -----
        Sent: Friday, May 17, 2013 11:34 PM
        Subject: Re: [cosmacelf] Re: Interfacing the1802 and AY-3-8910A

         

        On 05/14/2013 04:45 PM, Lee Hart wrote:
        > He put "do not substitute" on several of the transistors. One is a JFET,
        > and one is a unijunction. I'm not sure what's special about the others.
        > I just know they are not common generic part numbers.
        >

        Two of the "do not substitute" transistors are in Figure 5.

        This circuit is in effect, a VCO. Digital signals drive the bases of
        Q1-Q3 and adjust the collector current of Q4. (A three bit DAC) That
        current charges C5 which with Q5 (a unijunction transistor) forms a
        relaxation oscillator. The frequency is proportional to the collector
        current in Q4.

        But by using some different parts you can get the same thing only better.

        Instead of a hacked together 3 bit DAC use a DAC08. This is a basic 8
        bit DAC with current output. That output is perfect for charging a
        capacitor connected to a 555 except for one little detail: the DAC
        output is a current sink and you need a current source. Nothing a three
        transistor current mirror can't fix. Then use an opamp in place of that
        other do not substitute transistor. (That part has a higher than usual
        gain which minimizes loading on the capacitor voltage. Opamps are even
        better at that.)

        One downside to this is that the DAC limits you to a 256:1 range of
        frequencies. But that isn't all bad. The following voice selector is
        just a set of simple filters that are frequency dependent.

        --
        David W. Schultz
        http://home.earthlink.net/~david.schultz
        Returned for Regrooving

      • Lee Hart
        ... I agree. I like the basic concept. It is an example of extreme simplicity performing a surprisingly complex function. I would just do it with newer and
        Message 3 of 25 , May 18, 2013
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          On 5/17/2013 4:34 PM, David W. Schultz wrote:
          > This circuit is in effect, a VCO. Digital signals drive the bases of
          > Q1-Q3 and adjust the collector current of Q4. (A three bit DAC)...
          > But by using some different parts you can get the same thing only better.

          I agree. I like the basic concept. It is an example of extreme
          simplicity performing a surprisingly complex function. I would just do
          it with newer and easier to get parts. If anything, you could use even
          fewer parts to do the same thing.

          --
          Anyone can make the simple complicated. Creativity is making the
          complicated simple. -- Charles Mingus
          --
          Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
        • Lee Hart
          ... A top octave generator is just a counter. The 1802 itself can do the same thing with a little software. Long ago I got interested in driving AC motors for
          Message 4 of 25 , May 18, 2013
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            On 5/18/2013 2:16 AM, /M wrote:
            > Or how about, setting up a table of reasonable weighted values...
            > have the 1802 send those values...

            A top octave generator is just a counter. The 1802 itself can do the
            same thing with a little software.

            Long ago I got interested in driving AC motors for electric cars. I put
            a sinewave lookup table in an EPROM. The 1802 had a counter wired as a
            divide-by-N, where N was loaded from an output port. Each time the
            counter reached 0, its carry did a DMA-OUT cycle. Thus, the 1802 could
            output the waveform in the table under software control at a wide
            variety of rates with minimal software overhead.

            Something like this could output any arbitrary waveform from a table in
            memory.

            --
            The reasonable man adapts himself to the world. The unreasonable man
            persists in trying to adapt the world to himself. Therefore all progress
            depends on the unreasonable man. -- George Bernard Shaw
            --
            Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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