More on Crosspoints.

[Paul Maddox]

Dear all,


  One of the problems I am facing is the fact that the cross point switch
needs a high'ish on
resistance. I shall try and explain.

  Consider the matrix patch bay of the EMS VCS3.

  Adding CVs that are postive is done with diodes (as in the VC3). However,
if you want
to add several AC voltages you cant use diodes.
  Another problem is that you need a high'ish on resistance so that when you
sum say 4 CVs together you can use the virtual earth of an opamp to prevent
any reverse feedback.
If you had no on resistance then there is a problem if a CV is connected to
several pointsa and other CV is connected to a same point that some of the
voltages can go 'back' through the switchs and affect the wrong output.

  This isnt very clear, its difficult to put into words..

1>   ooooxooo
2>   oxooxooo
3>   oxooooxo
4>   ooooxooo


  Ok in the above example imagine that 1,2,3,4 are CVs going INTO the matrix
and that the
X's represent an ON point.

  If the switchs had no on resistance then the last point on CV3 would be
affected by all the
other CVs as at one point they are connected to each other. The other
problem would be that the CVs comming into the matrix would be fighting each
other and that if one input was at say 5v and the other at -5v then you
would be shorting stuff out.

  Does this help?

  Paul Maddox
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