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From: Tim Wescott on 11 Apr 2010 17:33
crasic wrote:
>> General tip: if you want to get the result you want, you should put a
>> question in a comprehensible form.
>>
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultant
>> http://www.abvolt.com
>>
>
>
>> General tip: if you want to get the result you want, you should put a
>> question in a comprehensible form.
>>
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultant
>> http://www.abvolt.com
>
>
> Sorry, I'm in physics and not DSP or Engineering, so I apologize if the
> question was unclear.
>
> I need to implement a tight tracking filter that works in near real time.
> Currently I have a scheme that uses a Sinc window bandwidth filter with
> filter coefficients updated every 100mS or so by adjusting the center
> frequency using an FFT.
>
> The question is basically if this is a workable solution for a tracking
> filter.

That will work, but it is a lot of unnecessary work.

Make your filter from a prototype at DC, and turn it into your desired
bandpass filter by multiplying it by a sine wave at the desired band
pass frequency. This gets you the same effect, without having to
recompute the FFT all the time.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: crasic on 11 Apr 2010 17:46

>
>That will work, but it is a lot of unnecessary work.
>
>Make your filter from a prototype at DC, and turn it into your desired
>bandpass filter by multiplying it by a sine wave at the desired band
>pass frequency. This gets you the same effect, without having to
>recompute the FFT all the time.
>
>--
>Tim Wescott
>Control system and signal processing consulting
>www.wescottdesign.com
>

The only trouble is that the desired bandpass frequency is determined from
the input signal (which is why we use the FFT, to find the frequency with
max amplitude). And this frequency is constantly drifting.
From: Tim Wescott on 11 Apr 2010 21:37
crasic wrote:

(missing context restored)
crasic wrote:
>> General tip: if you want to get the result you want, you should put
a question in a comprehensible form.
>>
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultant
>> http://www.abvolt.com
>>
>
>
>> General tip: if you want to get the result you want, you should put
a question in a comprehensible form.
>>
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultant
>> http://www.abvolt.com
>
>
> Sorry, I'm in physics and not DSP or Engineering, so I apologize if the
> question was unclear.
>
> I need to implement a tight tracking filter that works in near real time.
> Currently I have a scheme that uses a Sinc window bandwidth filter with
> filter coefficients updated every 100mS or so by adjusting the center
> frequency using an FFT.
> The question is basically if this is a workable solution for a tracking
> filter.
(end missing context)

>> That will work, but it is a lot of unnecessary work.
>>
>> Make your filter from a prototype at DC, and turn it into your desired
>> bandpass filter by multiplying it by a sine wave at the desired band
>> pass frequency. This gets you the same effect, without having to
>> recompute the FFT all the time.
>>
>
> The only trouble is that the desired bandpass frequency is determined from
> the input signal (which is why we use the FFT, to find the frequency with
> max amplitude). And this frequency is constantly drifting.

Ah -- I thought you were synthesizing the filter using the FFT.

Why not use a phase locked loop to track the signal, then either count
the transitions on the phase-locked signal, or if you do it digitally
just look at the frequency command to the NCO?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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