# DSP question , cascading functions

R

#### RealInfo

Jan 1, 1970
0
Hi all
I hope I came to the right group

My question is general DSP question .

In case I need 2 DSP functions to be done on the same
analog signal one after another, for example a low pass filter firs function
and on the result a noise reduction function .

My question is : can the 2nd function be done on the digital
byte stream of the 1st DSP hardware block , or the 1st output should be filtered and the 2nd DSP function will be done on the
analog signal derive from 1 DSP ?

Thanks
Elico

U

#### Uwe Hercksen

Jan 1, 1970
0
RealInfo said:
In case I need 2 DSP functions to be done on the same
analog signal one after another, for example a low pass filter firs function
and on the result a noise reduction function .

My question is : can the 2nd function be done on the digital
byte stream of the 1st DSP hardware block , or the 1st output should be filtered and the 2nd DSP function will be done on the
analog signal derive from 1 DSP ?

Hello,

that is a problem of the sampling frequency and the time needed to
compute the first and the second function. If the sampling requency is
40 kHz, one sample must be computed in less than 25 µs. If both
functions are done in less than 10 µs for each, it is possible within
one DSP, if less than 20 µs you will need two DSPs.

Bye

R

#### Robert Macy

Jan 1, 1970
0
Hello,

that is a problem of the sampling frequency and the time needed to
compute the first and the second function. If the sampling requency is
40 kHz, one sample must be computed in less than 25 µs. If both
functions are done in less than 10 µs for each, it is possible within
one DSP, if less than 20 µs you will need two DSPs.

Bye

can it not be 'pipelined' and simply delayed? The first function is a
simple low pass, of sorts. Memory would only take enough memory
samples to represent the low frequency cutoff, right?

J

#### josephkk

Jan 1, 1970
0
If it's a uP, you don't gain any compute power from pipelining. If it's done in
an FPGA, you can pipeline many, many clocks. One can also clock the early stages
of a lowpass filter at a higher rate than later ones.

Come to think of it, one can do that in software, too: run the first stage or so
more often than later ones.

At any rate (pun!) there's no need for filtering between DSP functions; just
stack them up.

Just the same, the only real question is can you get all the tasks done on
time. This is a simple hard real time thing. OTOH the comm time to hand
it off to a second DSP for the next stage may kill the timing.

?-)

T
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