# repeated data conversions

Discussion in 'Electronic Basics' started by Jon Slaughter, Jul 6, 2007.

1. ### Jon SlaughterGuest

I was wondering just how bad data is corrupted through repeated conversions.
I have this idea to test it and was wondering how hard it would be to set
up.

Essentially you have a ADC, DAC, buffer, and control system.

you initially setup the buffer with the required digital data.

Then the buffer sends the data into the DAC and the DAC feeds the ADC which
then goes back into the buffer. The control system handles all the necessary
stuff and counts each time a full conversion of the buffer has happened.

diagrammatically,

^ /
\ /
\ /
\ /
\ /
\ V
Buffer

After the nth conversion(of the buffer) you can then compare the buffer with
the original data and see just how different it really is.

I'm wondering if such a system could be designed to handle virtually and DAC

Any ideas?

Thanks,
Jon

2. ### Stanislaw FlattoGuest

"Information theory" says that every conversion gives you error of the
least significant unit of measurement.
No arguments on this! How many repeats of it will give you recognisable
error from original depends on your setup.
One way is to define the ending error and let the system repeat those
cycles, counting them, till it gets there.

Have fun

Stanislaw.

3. ### Jon SlaughterGuest

Well, I don't know if thats information theory but the error is obviously in
the least significant bits because that is how the devices work. The issue
is how the error propagates. Also there is a difference between the
mathematical representation of the error and what actually happens. i.e.,
just cause you assume it is guassian distributed doesn't mean it actually is
and I'm interested in what really happens rather than what happens
mathematically(not that it can't be described mathematically but its much
more complex that I care to deal with).

Well, I want to actually see what happens... ideally I would take a snapshot
after each step and I could then have a time series of how the error
propagates.

The reason I'm interested in this is because I have some things I'd like to
do but they require going in and out of the digital domain many times and
I'm not sure the signal will ultimately be affected to due the conversion
process. For example, if the signal is dithered then how will that affect
the outcome? with errors from the conversion process creep into significant
bits?

Jon

4. ### Stanislaw FlattoGuest

Your guess is as good as mine, the converter cannot close one eye and/or
tilt its head to "better" read the displayed value. It will simple guess
depending of the size of the known to it smallest digit. For this
accuracy the converter IS built, you want better accuracy use more
smaller units in conversion or use statistics. Nothing is absolute.
"Significant" means what percentage of the whole? On how many digital
digits on what accuracy of the analog conversion against what reference.
When looking for creeping errors it is good to have _very_ reliably
performing setup.

Good luck

Stanislaw.

5. ### John O'FlahertyGuest

If you wanted to model the basic parameters (# bits, AWGN or other
noise) of the system, you could probably model it in Simulink pretty
easily. If you actually want to test physical parts, you could still
use Matlab/Simulink to provide the original data, and to recirculate
the data in digital form using the data acquisition toolbox, putting
the current digital state into successive dimensions of a variable.
Then you could plot the whole thing pretty easily. You'd just need to
attach a supported digital I/O to the computer. Try a new ADC/DAC
pair? Just hook them up and press run again.

6. ### Rich GriseGuest

.
When I was a kid, we used to do this with a tape loop. We'd say something,
and it would echo, and echo, and echo, getting noisier, and noisier, and
noisier until it was like listening to ocean waves.

As far as your scheme, the only errors will be in the analog part.

Have Fun!
Rich