Scott Newell <PICLIST@MITVMA.MIT.EDU> wrote:

> > One relay would switch from "off"
> > to "on", and all the lower-order relays would switch from "on" to
> > "off".  Since they all switched at slightly different rates,
> > there would sometimes be NO attenuation for a few milliseconds,
> > and you'd hear loud pops from the speakers.
>
> Sound to me like it needed a PIC to sequence the relay break before
> make order (or make before break).  Could you not mute (or switch in
> the maximum attenuation) first, then move all the other relays to
> the new setting, and then unmute?

    Scott:

    That was the first thing WE thought of, too... It helped, but it
    didn't COMPLETELY solve the problem, for two reasons:

    1.  Particularly at high volume levels, ANY sharp change in the
        speaker volume (either up OR down) was intrusive and
        annoying.

    2.  I was controlling the relays through a series of relay
        drivers that worked like serial shift registers.  Because it
        took a fairly long time to send out the serial bit stream
        that controlled the relays, the user could actually turn the
        knob faster than the relays could follow it.

        When the knob was turned quickly, there were large
        discontinuous jumps in the output volume, as the relays
        skipped steps trying to keep up with the knob.  This was bad
        enough, but when those big steps were interleaved with short
        periods of silence, the "machine-gun" effect was VERY
        annoying.

    We considered a method that involved separate (i.e., not part of
    our attenuation ladder) voltage dividers that were controlled by
    single relays which could be switched in while the ladder was
    muted and being adjusted.  Those separate dividers would only
    switch in at the power-of-2 steps.

    Unfortunately, the complexity of that solution -- and the
    limitations it presented in terms of the balance control -- made
    it impractical.

> [snipped discussion of Dallas digital pots]
> > However, there were three big problems with them:
>
> What about maximum input level?  Aren't they picky about high level
> input signals that go beyond the power rails, or have they fixed
> that?  I guess you could attenuate the input, at some cost in noise.

    Thankfully, that wasn't an issue in our design.

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
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