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From: roskos@IDA.ORG (Eric Roskos)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re: Anomaly in A&A AM WEFAX Demodulator's ADC
Keywords: ADC0804
Message-ID: <1989Oct25.170818.1007@IDA.ORG>
Date: 25 Oct 89 17:08:18 GMT
References: <1989Oct24.135458.28372@IDA.ORG>
Organization: IDA, Alexandria, VA
Lines: 56

The story continues ...

Last night I modified my previously unmodified A&A board to bring the
/RD and /WR lines from the A/D converter to the set of pins on the edge
of the demodulator board.

After doing this, and building the logic on the 8-bit port board to generate
the /RD and /WR pulses when the port was accessed, I connected it all
together, and found that the problem still exists!  So it is not due to
incomplete conversions being read from the board.  It appears simply to
be a problem with the demodulator board itself... possibly that the
A/D converter that is used is too slow to convert a particular value of the
demodulated signal before it changes to a new value, so the ADC gets partway
through the conversion, finds the value has changed significantly, and
gets 0's for the low-order bits as a result.

The distribution of values out of the A/D converter is interesting... 
the values with low-order bits of zero seem to "borrow" from neighboring
values, and the height of the spikes is in proportion to the number of
low-order bits that are zero.  For example, a portion of the curve looks
something like this:


	--------
	--------  <- these 3 show what the frequency counts "should" be
	--------
  6FH	--
  70H	-----------------
  71H	---
  72H 	---------
  73H	-----
  74H 	------------
  75H	------

I did some testing with artificially-generated files to be sure it's not the
histogram-producing software (it's not); and with random white noise (it gave
a plausible distribution of the white noise, but with the "borrowing" anomaly
as shown above).

At this point I don't have a good idea as to what might be causing it. 
It doesn't adversely affect the WEFAX pictures you get if you only use
4-bit data, and in fact the pictures are very good considering the price
of the demodulator (both the AM and FM demodulators purchased together
cost around $85), but it means you can't do enhancements of low-contrast
areas of the image, or other types of image enhancements that rely on
having data at a higher grey-scale resolution than you are able to
display. 

It looks like some artifact of how the ADC works...  I have an ADC0820
which I am thinking of trying to see if it works better (the A/D
converter on the A&A board is an ADC0804), since the ADC0820 has a
built-in sample and hold, and can convert at around 20,000
samples/second. 

-- 
Eric Roskos (roskos@CS.IDA.ORG or Roskos@DOCKMASTER.NCSC.MIL)