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Old 30th June 2004, 01:09 PM   #1
Rocky is offline Rocky  Norway
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Question Newbie on digital filtering... some questions!

I am curious about benefits of digital filtering versus active analog filters (those op-amp-based like 24dB/oct LR).

My analog signal chain is of very high quality with compression midrange horn (TAD) and low-watt SET amplifier, but I have need for equalizing, and since my source is digital music (CD) I am thinking it would be better to approach this in the digital domain than battling it with active LR notch filters, to keep my analog chain the highest end possible. Afterall, digital is digital, with the flexibility and limitations attached..

But I am a newbie to digital filters... Will the filtering be possible to do by software on a PC, using a quality CDROM as digital source, or do I need a dedicated DSP development board? Will digital signal processing on a PC degrade the audio signals in any way (theoretically)? How is a PC suited as a digital source for a quality DAC? And how do I output digital signals to the DAC?
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Old 30th June 2004, 02:38 PM   #2
Rocky is offline Rocky  Norway
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Just ran across a webpage of the sort of system I had in mind.. http://home.pacbell.net/donwm/

Any comments?
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Old 30th June 2004, 02:40 PM   #3
MWP is offline MWP  Australia
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LOTS of info on this topic in here:
PC music players

But to answer your questions.
Filtering can be done on a PC quite easily... but the resulting quality you get really depends on how much time you can give to setting the system up.
This is because the top-quality software is quite difficult to setup, while the average quality software is easy to setup.

The average PC is able to do high quality filtering without the need for external DSP hardware.

Theoretically, digital processing should give FAR superior results than any analog filtering.
This is simply because digital xover filters can give very sharp & accurate roll-off frequencies while having 0 phase shift (well FIR filters can... IIR isnt as accurate).

The best way to setup a PC for filtering is somthing like:
TRANSPORT -> SPDIF -> PC -> SPDIF -> DAC -> AMPS

But setting up this way is quite difficult to the requirements of the PCs soundcard (=expensive).
Remember if you have say 3way L&R speakers you want to xover using the PC, youll need a soundcard with at least 3 SPDIF outputs.

You could play off your PCs CDROM drive, filter, then play out of a soundcards SPDIF outputs (again, you need a soundcard with multiple SPDIF outputs = expensive).

Anything i missed?
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Old 30th June 2004, 02:46 PM   #4
Rocky is offline Rocky  Norway
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Quote:
Originally posted by MWP
Anything i missed?
I dunno But you gave me a good start and now I have something to think about.. Think I'll start by loooking up the term "SPDIF"
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Old 30th June 2004, 03:23 PM   #5
Rocky is offline Rocky  Norway
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I would need 2x SPDIF for 2-way output, right? And with an external CD transport I would need a third for input? So am I looking for a soundcard with 3x SPDIF i/o's? Or are they i/o at all?

Would there be any suitable sound card for PCMCIA/USB2/Firewire/MiniPCI on a laptop? I have a stationary PC with Linux RedHat IIRC, but it's stored in my mothers garage at the moment. It has a large and dominating presence, so if I could get away with using my IBM ThinkPad X24 laptop and external CD transport, I'd be happy to spend the extra bucks..

Recommended software? BruteFIR (linux only?), AxioXO? Can any of these handle SPDIFs in and out through a sound card?

Also; Is there any measurement/mic hardware available for laptop computers (or USB,Firewire, etc...), that I could use to calibrate my filters?
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Old 30th June 2004, 04:17 PM   #6
ojg is offline ojg
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For stereo SPDIF in/out, have a look at the M-Audio Firewire Audiophile or Firewire 410.

For 3*stereo SPDIF, things will get expensive very fast. You may need to look at a card with ADAT interface and an external ADAT to SPDIF/AES-EBU box.

http://www.staudio.com/products/dsp24adat.html
http://www.alesis.com/products/ai4/
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Old 6th July 2004, 12:59 AM   #7
Rocky is offline Rocky  Norway
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If I buy/DIY some USB-SPDIF converters, is there any way I can play a wav file and filter it digitally into 2 USB-SPDIFs?
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