Decimation

On Apr 6, 10:02=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On 04/05/2011 11:27 PM, Muhammed Shafi wrote:
>
> > Hi All,
>
> > =A0 First of all let me thank you all for your valuable solutions. If I
> > go for Upsampling( Interpolation) i will have to fit some data based
> > on the previous data . Am i right?
>
> Yes, kind of. =A0Strictly speaking, interpolation means that you're takin=
g
> data both before and after the point that you're interpolating, which
> means that you have to allow some delay into your resampling process.
> Taking data that is strictly before the point you're generating is
> extrapolation (i.e. prediction), and is much more subject to corruption
> by noise.
>
> > =A0 Since the ADC is sampling some valuable medical diagnostic data(ECG=
),
> > i guess its not a good practise to do upsampling.
>
> Maybe. =A0If it's so dang valuable, why can't you just sample the ADC at
> the right frequency?
>
> I wouldn't expect ECG data to have much frequency content above a few
> hundred Hz. =A0If this is so, then you should be able to resample it
> fairly cleanly.
>
> Finally -- what are you going to _do_ with it at your 16kHz? =A0If it's
> some analysis, then perhaps that analysis will work fine after
> resampling, even if the result wouldn't look perfect to a cardiologist.
>
> You should find out what the analysis is, and whether it will tolerate
> resampled data. =A0You should also find out what the characteristics of
> the signal you're playing with -- if it's already way oversampled, then
> doing a bit of interpolation on it will probably not hurt at all.
>
> > =A0 Is there anyway to do with just downsampling or do I need to sample
> > at even higher frequencies for downsampling to be applied?
>
> Yes. =A0In fact, you've already been told -- sample rate conversion, whic=
h
> uses polyphase filtering. =A0There's not need to actually upsample then
> downsample -- polyphase filtering gives you the result of that
> upsample/downsample combination, without forcing you to actually do all
> the work.
>
> > Another option is to use 200 Khz CNV frequency and do the sampling for
> > just 192 KHz(192k samples) and for the rest 8 Khz send some junk
> > control word to the ADC in the TX_interrupt handler. Once it's
> > elapsed(8k samples), again come back to the previous control words. Is
> > this feasible?
>
> If you're planning on doing what I think you are, it would mess up the
> signal worse than sample rate conversion.
>
> > Can we keep some kind of count in the interrupt
> > handler(ISR) that lets us know that 192k and 8k samples have been
> > sampled?
>
> > My CNV is 200 Khz and the SCLK is 8 Mhz.
>
> --
>
> Tim Wescott
> Wescott Design Serviceshttp://www.wescottdesign.com
>
> Do you need to implement control loops in software?
> "Applied Control Theory for Embedded Systems" was written for you.
> See details athttp://www.wescottdesign.com/actfes/actfes.html


Thanks a lot  for the support that i have got in this forum. It was
really helpful and a good learning experience for me.
I have decided to sample only 10 channels of ADC since the Blackfin
can exactly generate the CNV pulse of 160 kHz so that i can get the
exact 16kHz sampling frequency per channel.

@Tim Wescott : Yes, I will do the analysis as you have told. I am just
a beginner so , i dint feel that i had to go deep into that part
mostly because i thought i won't understand those things.

Regards
Shafi