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Old 25th February 2006, 12:59 AM   #1
PatPet is offline PatPet  Hong Kong
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Default FIR Digital Filter

Recently I read quite a number of articles on digital filters, especially FIR ones. In a FIR filter, there are taps in which digital signals are delayed for certain period of time before they come out multiplied by a (unique?) coefficient and are summed as a single output. The result is there are new interpolated samples which smooth out the staircases and thus the analog filters can be of lower orders.

This is how a common digital filter works. Usually the filter chips has limited internal accumulator length which may be a problem. And usually there are stages of taps. For example, 2X filter followed by 2X filter followed by 2X filter. This according to Ryohei Kusunoki will cause audible accumulated delay.

Is it possible to design a single stage FIR digital filter that doesnt sum the output, and in other words the individual lines from the coefficient mulitipliers are fed to individual DAC chips?
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Old 25th February 2006, 06:41 AM   #2
wa2ise is offline wa2ise  United States
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Quote:
This according to Ryohei Kusunoki will cause audible accumulated delay
If both channels of the audio are filtered the same way, that won't matter. Sure, there's delay, but a few fractions of a second are of no consequence. As for truncation effects, a good filter design maintains all the bits, and rounds things back to 16 bits (or more if the DAC chip can accept them) just before they hit the DAC chip.

If the coefficients are carefully selected, one can build filters without having to use real multipliers. Simple combinations of powers of 2 are easily done with shifts and adds.
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Old 25th February 2006, 08:38 AM   #3
4real is offline 4real  Netherlands
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Default Re: FIR Digital Filter

Quote:
Originally posted by PatPet
This is how a common digital filter works. Usually the filter chips has limited internal accumulator length which may be a problem. And usually there are stages of taps. For example, 2X filter followed by 2X filter followed by 2X filter. This according to Ryohei Kusunoki will cause audible accumulated delay.
Yes, there is a small delay, but since FIR filters are usually 100% phase linear, you will not hear it, no matter how often you do the filter.
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Old 25th February 2006, 09:02 AM   #4
PatPet is offline PatPet  Hong Kong
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My intention of the approach to not summing the taps is to let the DACs run at 1fs which means the advantage of jitter immunity of non-os dacs is not taken away by oversampling.

For the delays, i think one stage filtering is the best but needs lots of processing power.
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Old 25th February 2006, 10:10 AM   #5
4real is offline 4real  Netherlands
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Originally posted by PatPet
My intention of the approach to not summing the taps is to let the DACs run at 1fs which means the advantage of jitter immunity of non-os dacs is not taken away by oversampling.
Why would 1x fs have less jitter than a Xx fs DAC? Besides, it will not work! You can't filter digitally wil a 1x fs DAC, since the output of the filter is still 1x fs, the filter is of no use.

You need Xx fs to gain bandwidth, and then filter it down with the FIR filter.

Quote:
For the delays, i think one stage filtering is the best but needs lots of processing power.
Not more processing power, but more memory. Also, this is were oversampling comes in handy, since at higher frequency, you'll need less tabs for the same result as a comparable filter at low frequency.
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Old 25th February 2006, 11:08 AM   #6
PatPet is offline PatPet  Hong Kong
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Why would 1x fs have less jitter than a Xx fs DAC? Besides, it will not work! You can't filter digitally wil a 1x fs DAC, since the output of the filter is still 1x fs, the filter is of no use.
The output of an ordinary digital filter is the summed signal from all taps.
What I'm interested is if we dont sum the taps and feed the DACs with individual signals from the taps and sum finally at the outputs of the DACs, we'll get an analog signal that looks like it has been oversampled. If a DAC is fed with 1Fs signal, it is less sensitive than it will be if fed with oversampled signal.
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Old 25th February 2006, 11:30 AM   #7
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A multistage oversampling filter will have at least one stage with well over 100 taps, a single stage filter would have considerably more. How much do PCM1704's cost in 100 off quantities?
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Old 25th February 2006, 11:53 AM   #8
PatPet is offline PatPet  Hong Kong
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A multistage oversampling filter will have at least one stage with well over 100 taps, a single stage filter would have considerably more. How much do PCM1704's cost in 100 off quantities?

1. What if I dont use all taps?
2. What if I sum up to get 8 individual lines?
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Old 25th February 2006, 12:22 PM   #9
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You decide the type of filter you want and the performance you want it to meet and the taps are determined by that. If you don't use all the taps you don't get the performance you want.
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Old 25th February 2006, 12:57 PM   #10
4real is offline 4real  Netherlands
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Again: you cannot FIR filter a non oversampled signal.

Well, you van of course, but not for this application. The whole point of this working is using oversampling.
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