On Sep 16, 2:21 am, "doh ..." <so...@nospam.com> wrote:
> The high data rate bit stream created by a one bit DSM can be filtered, decimated
> and turned into a wider bit width with a simple counter.  This illustrates, I think, all
> the principles about which you are confused.

i think the OP needs to understand that when decimating, the word
width *must* increase (from 1 bit wide), just so that there is no loss
of information.

> Just count the number of "1" bits over a fixed period of time.  The period
> of time represents the output frequency, ...

i would not express it this way.  the DSM stream of zeros and ones
(i'll call a[n] where "n" is discrete time) can be thought of as
representing this signal (after biasing and scaling):

    x[n] = -2*a[n] + 1

since a[n] is always 0 or 1, x[n] is always -1 or 1.  another way to
look at it, which is useful for applying autocorrelation or cross-
correlation operations:

    x[n] = (-1)^a[n]

comes out to be the same relationship but the latter means that when
multiplying two signals, you just add the exponents and since the
exponents are 0 and 1, your addition can be modulo 2 addition (a.k.a.
"exclusive-OR") and then all sorts of logical theorems are available
to you.

to the OP: you said you were satisfied with the sigma-delta theory.
what about the gain of the quantizer in the closed loop system?  do
you understand what that is about and what you need to do to model the
system.

r b-j

The high data rate bit stream created by a one bit DSM can be filtered,
decimated
and turned into a wider bit width with a simple counter.  This illustrates,
I think, all the principles
about which you are confused.

Just count the number of "1" bits over a fixed period of time.  The period
of time represents the output frequency, and the
number of bits counted represent the data value for that sample.  The filter
is convolution with a simple rectangular window, the counter
width is the bit width, and the ratio of the bit rate to the counter period
is the decimation rate.

Better filters will yield better results (the rectangular window is pretty
sucky)

Bear in mind that multi-bit DSM outputs are also possible, so just sum the
up for the period.  Same thing..

Hope that helps


"HardySpicer" <gyansorova@gmail.com> wrote in message
news:1189631543.939690.61160@k79g2000hse.googlegroups.com...
> I am happy with everything in a SD modulator except the decimation
> part. You filter then decimate (down-convert the sample rate) and I
> can see how this gives a bit stream. How to get the other bits though
> for say a 16 bit word? Or does it not matter ie the bitstream can be
> taken into a processor in serial form. I always imagine an ADC output
> as say a 12 bit or more word and not a bitstream which is read into
> the processor. Am I missing something here?
>
> Hardy
>

Hi,

the sigma delta -MODULATOR- gives a high-rate stream of ones and zero. 
It's like a pulse-width modulated version of the signal.
The output stage of the -CONVERTER- is a counter, that averages the bits
(or numbers) over a longer time. 

For example, the first four paragraphs in
http://en.wikipedia.org/wiki/Sigma-delta_modulation discuss this in more
detail.

-mn

On Sep 13, 9:45 am, Randy Yates <ya...@ieee.org> wrote:
> HardySpicer <gyansor...@gmail.com> writes:
> > I am happy with everything in a SD modulator except the decimation
> > part. You filter then decimate (down-convert the sample rate) and I
> > can see how this gives a bit stream. How to get the other bits though
> > for say a 16 bit word? Or does it not matter ie the bitstream can be
> > taken into a processor in serial form. I always imagine an ADC output
> > as say a 12 bit or more word and not a bitstream which is read into
> > the processor. Am I missing something here?
>
> You seem to be confusing transport mechanism with the digital signal.
> For example, assume you have a 16-bit 48000 Hz digital signal. That
> can be transported 16 bits at a time at 48000 transports per second,
> or 8 bits at a time at 96000 transports per seond, or one bit at a
> time at 768000 transports per second.
>
> No matter how you transport it, it's still a 16-bit 48000 digital signal.
>
> In your situation you can either transport the delta sigma modulator
> output or the decimated filter output. The transport mechanism is
> irrelevent. The digital signal, however, will be different between
> those two cases.
> --
> %  Randy Yates                  % "Bird, on the wing,
> %% Fuquay-Varina, NC            %   goes floating by
> %%% 919-577-9882                %   but there's a teardrop in his eye..."
> %%%% <ya...@ieee.org>           % 'One Summer Dream', *Face The Music*, ELOhttp://home.earthlink.net/~yatescr

ok so you are saying that the port of the computer only has a digital
in so to speak like a serial connection?

Hardy

HardySpicer <gyansorova@gmail.com> writes:

> I am happy with everything in a SD modulator except the decimation
> part. You filter then decimate (down-convert the sample rate) and I
> can see how this gives a bit stream. How to get the other bits though
> for say a 16 bit word? Or does it not matter ie the bitstream can be
> taken into a processor in serial form. I always imagine an ADC output
> as say a 12 bit or more word and not a bitstream which is read into
> the processor. Am I missing something here?

You seem to be confusing transport mechanism with the digital signal.
For example, assume you have a 16-bit 48000 Hz digital signal. That
can be transported 16 bits at a time at 48000 transports per second,
or 8 bits at a time at 96000 transports per seond, or one bit at a
time at 768000 transports per second.

No matter how you transport it, it's still a 16-bit 48000 digital signal. 

In your situation you can either transport the delta sigma modulator
output or the decimated filter output. The transport mechanism is
irrelevent. The digital signal, however, will be different between
those two cases.
-- 
%  Randy Yates                  % "Bird, on the wing,
%% Fuquay-Varina, NC            %   goes floating by
%%% 919-577-9882                %   but there's a teardrop in his eye..."
%%%% <yates@ieee.org>           % 'One Summer Dream', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

I am happy with everything in a SD modulator except the decimation
part. You filter then decimate (down-convert the sample rate) and I
can see how this gives a bit stream. How to get the other bits though
for say a 16 bit word? Or does it not matter ie the bitstream can be
taken into a processor in serial form. I always imagine an ADC output
as say a 12 bit or more word and not a bitstream which is read into
the processor. Am I missing something here?

Hardy