[sdiy] Tuning indicator

[Don Tillman]

   > Date: Thu, 14 Nov 2002 02:19:55 -0800 (PST)
   > From: Tim Ressel <madhun2001 at yahoo.com>
   > 
   > How do you build an indicator that shows if one input frequency
   > is higher or lower than another frequency input? Kinda like a
   > guitar tuner. Would a phase-frequency comparator a la PLLs work I
   > wonder?

Here's how I would do it...  (and most of you know that I almost never
do things the normal way).

I wouldn't use PLLs because we care about frequency here, not phase. 

I would go for a "strobe" display, a modern version of the old
Peterson/Stroboconn tuners.  It's easy to view and it's easy for
the eye follow the slightest frequency drift.

I would use a 10-segment bar graph LED display.  Given a reference
freqeuency clock 10 times the tuning frequency I would throw it
through 5 flip flops wired for a circular phase shifting output:

  0 0 0 0 0
  1 0 0 0 0 
  1 1 0 0 0
  1 1 1 0 0 
  1 1 1 1 0 
  1 1 1 1 1 
  0 1 1 1 1 
  0 0 1 1 1 
  0 0 0 1 1 
  0 0 0 0 1

Drive the cathode ends of the LEDs with inverted and non-inverted
versions of the five flops.  Drive the anode with the audio input.

I'm surprised nobody makes tuners like this.  It would be so much
better than those cheezy digital tuners.  Sure it'd be expensive,
you'd need one of these displays for each of the 12 chromatic notes,
but it would be so nice to use.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com