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Old 26th February 2006, 03:51 AM   #21
tmblack is offline tmblack  United Kingdom
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Its very easy to compute a FIR LPF up to half the sampling frequency.
Then convolve the filter coeff. with NOS signal.

Tom

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Old 26th February 2006, 06:46 AM   #22
rfbrw is offline rfbrw
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Back to PatPet. So you want to build a standard oversampling digital filter with the exception that the summing of the taps is done externally.
As an example I'll use the first stage of the SM5847. The first stage is a 2x filter. It has 169 taps. You could fold the filter but that would be cheating, so 338 dacs it is. And then there is the physical implementation of the filter. Still think this is a viable idea ?
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Old 26th February 2006, 06:58 AM   #23
PatPet is offline PatPet  Hong Kong
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Back to PatPet. So you want to build a standard oversampling digital filter with the exception that the summing of the taps is done externally.
As an example I'll use the first stage of the SM5847. The first stage is a 2x filter. It has 169 taps. You could fold the filter but that would be cheating, so 338 dacs it is. And then there is the physical implementation of the filter. Still think this is a viable idea ?
For this number of taps, I barely think the impulse response will look good.

But I tried some FIR filter designers and found anything more than 3 taps will start resulting in pre-echo and post echo, which I think is no good at all. And for 3 taps we get 4x OS.

Maybe after that I play with linear interpolation and get more staircases on a 4X waveform. And hopefully the ultrasonics decay much faster.

And with 3 taps, the digital filter wont have good specs. But I'll trade something for good impulse response.
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Old 26th February 2006, 10:41 AM   #24
4real is offline 4real  Netherlands
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Quote:
Originally posted by tmblack
Its very easy to compute a FIR LPF up to half the sampling frequency.
Then convolve the filter coeff. with NOS signal.
Sure it's easy, but what is the point in doing this? It won't even be a lowpass, since you don't have any audio data after 1/2 x fs anyway

Quote:
Originally posted by PatPet
But I tried some FIR filter designers and found anything more than 3 taps will start resulting in pre-echo and post echo, which I think is no good at all. And for 3 taps we get 4x OS.
Who sais it's audible? And in what freuquency range? With three tabs, I don't think you'll have an acceptable passband ripple. No way that's going to sound any good. You'll also need a speed enough stopband of course. The ringing will occur arround the crossover freqeuncy, and therefore with oversampling, you'll not hear any of it.

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Maybe after that I play with linear interpolation and get more staircases on a 4X waveform. And hopefully the ultrasonics decay much faster.
Effectively, the FIR filter will do the non linear interpolation for you

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And with 3 taps, the digital filter wont have good specs. But I'll trade something for good impulse response.
Just forget it, it's no good.
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Old 26th February 2006, 12:11 PM   #25
PatPet is offline PatPet  Hong Kong
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Quote:
Quote:
quote:
Originally posted by PatPet
But I tried some FIR filter designers and found anything more than 3 taps will start resulting in pre-echo and post echo, which I think is no good at all. And for 3 taps we get 4x OS.
Who sais it's audible? And in what freuquency range? With three tabs, I don't think you'll have an acceptable passband ripple. No way that's going to sound any good. You'll also need a speed enough stopband of course. The ringing will occur arround the crossover freqeuncy, and therefore with oversampling, you'll not hear any of it.
Impulse Response

I think one of the offenders in OS is the pre-echo in the time domain and the other is jitter sensitivity.

I start digging myself into IIRs. Post echo is less offensive.
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Old 26th February 2006, 12:46 PM   #26
4real is offline 4real  Netherlands
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Quote:
Originally posted by PatPet
I think one of the offenders in OS is the pre-echo in the time domain and the other is jitter sensitivity.
That's why the crossover is at such a high freqency where you don't hear it anyway. If you think it is audible, point me to a single decent oversamping DAC that has this problem. Good luck!

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I start digging myself into IIRs. Post echo is less offensive.
Yes, but they are not phase linear, specially if you want a brick wall filter. Besides, there is still no use of such a filter if you don't oversample!

So explain exactly what do you intend to do with such a filter?
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Old 26th February 2006, 01:37 PM   #27
PatPet is offline PatPet  Hong Kong
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Quote:
Originally posted by 4real


That's why the crossover is at such a high freqency where you don't hear it anyway. If you think it is audible, point me to a single decent oversamping DAC that has this problem. Good luck!



Yes, but they are not phase linear, specially if you want a brick wall filter. Besides, there is still no use of such a filter if you don't oversample!

So explain exactly what do you intend to do with such a filter?
Okay I will answer this later. For the time being I find this quite interesting

http://www.mlssa.com/pdf/Upsampling-theory-rev-2.pdf
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Old 26th February 2006, 01:51 PM   #28
4real is offline 4real  Netherlands
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Quote:
Originally posted by PatPet
http://www.mlssa.com/pdf/Upsampling-theory-rev-2.pdf
Upsampling != oversampling, the fact that the article does not seem to know the difference is not really good.

I will never claim that upsampling would have any positive effect on sound, oversampling just makes stuff a bit easyer. But the filtering just won't work with a non oversampling DAC, not FIR, not IIR.
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Old 26th February 2006, 09:19 PM   #29
tmblack is offline tmblack  United Kingdom
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Sure it's easy, but what is the point in doing this? It won't even be a lowpass, since you don't have any audio data after 1/2 x fs anyway
No you have multifple images up to fs and beyond.

Maybe you should take a course or 2 in DSP.

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Old 27th February 2006, 08:06 AM   #30
tubee is offline tubee  Netherlands
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one could just build a filter using many DACs being fed digital signals with appropriate delays applied to the inputs.
I am that "one" at this moment, with 4 dacs. But Wadia and an other UK manufacturer used it also a long time ago.

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And select difering sized summing resistors to create the tap weighting
Wa2ise, what do you exactly mean with this?
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