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    Date: Mon, 20 Dec 1999  09:45:22
    From: "Nikolai Golovchenko" 
      To: "pic microcontroller discussion list" 
 Subject: Re: Digital sampling
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-----Original Message-----
From: Jinx 
To: PICLIST@MITVMA.MIT.EDU 
Date: Monday, December 20, 1999 3:41 AM
Subject: Digital sampling


>I saw a mention of PICs with < > and = functions. I'm not familiar
>with the particular one, 766 was it ?, but I plan to use PICs other
>than the F84 next year. I've a specific app that I want to know
>whether there's a PIC to suit.
>
>I'm using a sampling chip that employs CVSD ADC, Continuously
>Variable Slope Delta-modulation. What it does is sample the signal
>and compare it to the last sample. If higher it produces a "1" data
>bit, if lower a "0" data bit. When the signal is flat-line it alternates
>ie puts out a 55 or AA byte. This would appear to be an efficient
>way of sampling, as only the direction of the wave is noted, not its
>absolute value, it works very well anyway. Sampling rate is >60kHz
>and 10-bit resolution.
>
>The question I have is ; is there a PIC that has ADC and comparison
>functions that will emulate this sampling chip at speed ? From the
>limited reads I've had of ADC PICs, it looks as though 20us is about
>the max. The primary reason for asking is that the sampler is now out
>of production and I'm relying on stocks to see me through.


There is a PIC with comparators - 16c62x series. I strongly doubt whether it
can perform CVSD modulation faster than 20 uS, even at 20 Mhz clock.
Besides, bit rate in delta modulation is several times higher then the
signal bandwidth. For voice (300 - 3400 Hz) a 16 kbit/s rate provides good
enough quality. That is 16000/3400=4.7 ratio of bandwidth. Resolution also
depends on signal frequency.
AFAIK, CVSD was developed for voice application. So, the goal of this
modulation is to significantly decrease bit rate without loosing the
possibilty of voice recognition. It can not be used as a normal ADC, because
CVSD's output is just bitstream.

If you want ADC faster than 20 us and 10 bit resolution, there is only one
way - use some external ADC chip.

BTW, see www.mxcom.com  or
http://www.cmlmicro.co.uk/Products/Datasheets/fx609ds.pdf for CVSD codec.

>
>Alternatively, can anyone suggest a way of using discretes to do the
>same thing ? It has to be done at least > 50kHz to maintain signal
>integrity. I want to to rationalise the whole circuit using 1 or 2 PICs,
>rather than the sampler/PIC combination there now. If two PICs are
>needed that's OK, perhaps one sampling, the other supervising. An
>external ADC for a PIC to read is acceptable. Speed and resolution
>are important, I don't especially care how it's done.
>
>Jinx
>
>

Merry Christmas!
_

Nikolai Golovchenko, Electrical Engineering Student
National Mining University of Ukraine www.nmuu.dp.ua
Dnepropetrovsk, Ukraine
E-mail: golovchenko@mail.ru