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Discussion Groups | Comp.DSP | Question about Sigma-Delta conversion/decimation


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Question about Sigma-Delta conversion/decimation - Kiran - 2007-06-12 11:35:00

Hi all,
  My question is related to sigma-delta conversion.  I understand the
analog side of this, i.e. the modulator, and have developed a matlab
simulation which gives me 1's and -1's output for any input signal.
Essentially, this is the high data rate 1 bit output of the sigma
delta modulator.
  Now, I need help understanding the decimation process.
Conceptually, I understand what decimation is (low pass filtering and
then downsampling).  However, how does this yield a multibit output?
i.e., how do the high rate of 1 bit outputs turn into a multi-bit
output?

thanks for your help!

"Roughly one-third of the American diet comes from food-"

Holden, Science October 2006 issue


Re: Question about Sigma-Delta conversion/decimation - Jerry Avins - 2007-06-12 12:53:00

Kiran wrote:
> Hi all,
>   My question is related to sigma-delta conversion.  I understand the
> analog side of this, i.e. the modulator, and have developed a matlab
> simulation which gives me 1's and -1's output for any input signal.
> Essentially, this is the high data rate 1 bit output of the sigma
> delta modulator.
>   Now, I need help understanding the decimation process.
> Conceptually, I understand what decimation is (low pass filtering and
> then downsampling).  However, how does this yield a multibit output?
> i.e., how do the high rate of 1 bit outputs turn into a multi-bit
> output?

Let's stick with analog, since you have no trouble with that. Imagine 
your high-speed stream of 1s and -1s fed into an analog low-pass filter, 
and consider the output. There will clearly be a continuous signal that 
could in some cases exceed +/-1 by a little. It will in fact be a pretty 
good reconstruction of the original signal.

Now feed the 1s and -1s into a digital filter instead and decimate its 
output. That is a sampled version of the analog filter's output.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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