Hello, All I want to design a digital filter with the cutoff frequency 20Hz. The sampling frequency of my acquiring system is very high, up to 8192 Hz. The important requirement is the delay introduced by the filter should be as small as possible. The filter will be used online, and therefore I don't want to use the inverse model of the filter to cancel the delay. I tried the elliptic filter, and the delay is about 20 to 30 ms. Too large for me. What type of filters can I use? Thank you for your help

--- Quote Start --- I don't want to use the inverse model of the filter to cancel the delay. --- Quote End --- I assume, you can't cancel the delay even if you want. Group delay is a characteristic parameter of a filter, it can't be changed at will. You get the lowest with a low order Bessel filter. But it depends on your filter characteristic requirements.

I'm not an expert in digital filtering but I believe a real filter with cutoff frequency F always introduces an intrinsic delay which can't be lower than 1/(6.28 F). Maybe I'm wrong; this is only something I (confusely) remember from university times, back in another millenium... Cris

Yes, 1/(6.28 F) is the delay of a first order filter, 8 ms in this case.

Many thanks to both of you Sure, filters always introduce delay, but sometimes the delay can be cancelled in offline data processing. Here I just hope to design a filter with small delay. As suggested by FvM, I tried Bessel. The results show the delay is a little smaller than that of Elliptic filters. Maybe 20ms is smallest delay of a direct-designed filter in this case.

you might try asking on comp.dsp

how to design a low pass digital filter with smallest delay

26 Replies

Altera_Forum
Honored Contributor
15 years ago
In my last post, I wrote the transition band is 8Hz, from 20Hz to 28Hz
Altera_Forum
Honored Contributor
15 years ago
i understand, this is why i said designing a filter with a transition band of 1/1000 of the sample rate is not a simple task
Altera_Forum
Honored Contributor
15 years ago
It simply involves a considerable filter order. To reduce the resource consumption, one would prefer a multi-rate design usually
Altera_Forum
Honored Contributor
15 years ago
in this case you can do a down sample by 256 (try a whole series of half band filters too). i designed a filter with a transition band of 1/1000 of the sample rate (1 dB in band ripple, 100 dB stop band attenuation) and couldn't decimate (the passband was just over the quarter sample rate). it was a challenge
Altera_Forum
Honored Contributor
15 years ago
If you indeed wanted to go for decimation then a decimation polyphase structure will reduce multipliers by decimation factor. Using cascaded halfband is an overkill.

However, I feel an IIR filter is best option provided phase linearity is not a problem and provided the maximum delay in the band of interest is kept minimum.
Altera_Forum
Honored Contributor
15 years ago
Many thanks to everyone!

Now I am using two filters in parallel. It is not so bad and I have to accept the delay.

Thank you again.

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how to design a low pass digital filter with smallest delay

26 Replies

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