On Wed, 2005-06-29 at 11:56 +0800, Chen Xiao Fan wrote:

> Looking forward to see your post on the FilterPulse(). Thanks really
> a lot for your detailed explanation.

Well, I was hoping that others would've chimed in by now in the other
thread you started on how to filter your pulses. But the reason they
probably haven't is because without more information about your system,
there's no way to design a filter. So stating the obvious, a filter must
separate the noise from the signal. This means you have to know the
characteristics of each.

You've briefly alluded to a few noise sources: fluorescent lights, 50/60
Hz stuff, and neighboring sensors. You really haven't described the signal
other than to say that it consist of pulses and that you'd like to detect
them fast as possible. I can infer from your earlier posts that a 1 mS
signal detection time is acceptable, but that you really want something
like 400 or 500 uS. I have no idea what your detectors are attempting to
detect. I mean, I know they're proximity detectors, but are you wanting to
detect bullets coming from the muzzle of a rifle or count Furbies on a
conveyor belt? These two 'signals' have distinctly different
characteristics, and I'm sure your marketing department wants you to be
able to detect both! :)

Before pretending that I can propose a filter, I think you'll find that
the two techniques of pulse rejection mention so far (varying the pulse
timing and measuring the pulse width) can go a long way in rejecting
noise. While you agree in principle that these are good ideas, you've
suggested that amplifier-introduced distortions limit their usefulness.
I'd suggest fixing the amplifier! For example, have you ever investigated
a cascading logarithmic amplifier? I think you could construct a 3 stage
transistor amplifier following a simple preamp. The output of each stage
can be monitored with PIC I/O pins. This essential becomes a 3-bit
logarithmic ADC with a dynamic range of something like G^3 where G is the
gain of each stage:

 signal --> preamp --> LinAmp --+--> LinAmp --+--> LinAmp --+--> ...
                                |             |             |
                              I/O 0         I/O 1         I/O 2

The 'LinAmp' is a transistor-based linear amplifier that is *designed* to
saturate. I won't elaborate much more because a) I'm not expert in this
area and b) I'm sure there are other physical constraints in your system.
I can say that a single BJT, 4 resistors, and 2 capacitors can make a
decent single stage. Also, another subtle feature is that the I/O pins can
also be controlled outputs from the PIC. This could be used to create some
form of active noise cancellation (e.g. 50/60 Hz noise could be rejected
by adding an active source that's out of phase).

But sticking with the amplifier you have already, the filter you proposed
before seems reasonable enough. Here's  a variation that is fast:

    MOVLW   1
    BTFSS   PULSE_PORT,PULSE_PIN
     MOVLW  -1
    ADDWF   FilteredPulse,W
    BTFSC   FilteredPulse,3
     COMF   FilteredPulse,W
    ANDLW   0b00000111
    MOVWF   FilteredPulse

Here's the C-version that may be easier to follow:

    if (bPulse) {
      // Pulse is high
      if (++FilteredPulse > 8)
        FilteredPulse = 7;
    } else {
      // Pulse is low
      if (--FilteredPulse < 0)
        FilteredPulse = 0;
    }

The filtered signal can be bit 2 of the filtered pulse:

    BCF   Signal,0
    BTFSC FilteredPulse,2
     BSF  Signal,0

Scott
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