A brickwall is a theoretical concept. It means that it will pass everything up to a certain frequency and then without a transitionband, pass absolutely nothing. (or the inverse if a highpass) Its filter slope is infinitly steep. Its phase is perfect.
So the only things you will find are at best approximations. In general, the higher order filters with a steep slope have more phase deviation and ripple in the passband. Unless you go digital, but then you would not be on this part of the forum.
Why do you need it for? That way we could give you some ideas of what to look for and what to avoid.
So the only things you will find are at best approximations. In general, the higher order filters with a steep slope have more phase deviation and ripple in the passband. Unless you go digital, but then you would not be on this part of the forum.
Why do you need it for? That way we could give you some ideas of what to look for and what to avoid.
Look for the filter design program on the TI website.
You'll be able to design 4-pole and 8-pole filters that will approximate a brickwall, and will also give you the phase response accross the frequency band. There was a link to the page on the forums, dang if I can find it. I have the file on my machine, but it's about 4 MB so I can't attach it here.
You'll be able to design 4-pole and 8-pole filters that will approximate a brickwall, and will also give you the phase response accross the frequency band. There was a link to the page on the forums, dang if I can find it. I have the file on my machine, but it's about 4 MB so I can't attach it here.
Attachments
Low-pass Filterdesign Program
Hi e96mlo,
Links to the FilterPro lowpass filterdesign program can be found in this link. The program also gives a picture of the amplitude and the phase vs frequency.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=6901&highlight=filterpro
A 8th order Chebyshev will be pretty close to a "brickwall" filter.
<B><I>Don't throw any bricks Fred!!!</B></I> (My avatar will protect me)
Hi e96mlo,
Links to the FilterPro lowpass filterdesign program can be found in this link. The program also gives a picture of the amplitude and the phase vs frequency.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=6901&highlight=filterpro
A 8th order Chebyshev will be pretty close to a "brickwall" filter.
<B><I>Don't throw any bricks Fred!!!</B></I>
(My avatar will protect me)
e96mlo said:
Hi!
My old Denon DCD-1000 had a 7th order LC-filter (20 kHz) and my other one DCD-1500 had a 9th order LC-filter at 35 kHz. Neither the DCD-1000 nor the DCD-1500 sounded very well. Was the LC-filters the cause? I don't know.
But what are going to use this filter for? It's much better to use a digital filter if it's possible.
I don't know about a schematic that's already made for you - normally, you just have to pull out the 'ol TI-85 and make some calculations for some standard opamp filters. And because no design equation actually works in real life filter applications, you get to build it and test it.
But to make it easy on you, TI does have an application program for designing filters... not the greatest in my opinion though. Filter Wiz has a freeware version of their program and I like that much better than the TI program.
And to make it even easier... Linear Technology makes a variety of filter ICs already predesigned. The LTC1562 seems to have some good possibilities for an antialiasing filter or to rid your system of high frequency (above A-weighted values) noise in a post-DAC filter. My team looked into the LTC filters for our senior design project (DD, DPL, DTS, and stereo digital decoder) as additional antialiasing filters prior to our Burr Brown ADCs, but as it turns out the LTC filters are waaaaay to expensive for any realistic production cost. But for a DIYer, they're pretty fun to play with. And we really didn't need antialiasing filter because the BB ADC had one built in. I did some distortion analysis on Audio Precision with and without a modular filter I made with BB OPA2134 opamps, and it made no major improvement in distortion... so we just omitted the extra filter circuitry.
Hope this helps,
But to make it easy on you, TI does have an application program for designing filters... not the greatest in my opinion though. Filter Wiz has a freeware version of their program and I like that much better than the TI program.
And to make it even easier... Linear Technology makes a variety of filter ICs already predesigned. The LTC1562 seems to have some good possibilities for an antialiasing filter or to rid your system of high frequency (above A-weighted values) noise in a post-DAC filter. My team looked into the LTC filters for our senior design project (DD, DPL, DTS, and stereo digital decoder) as additional antialiasing filters prior to our Burr Brown ADCs, but as it turns out the LTC filters are waaaaay to expensive for any realistic production cost. But for a DIYer, they're pretty fun to play with. And we really didn't need antialiasing filter because the BB ADC had one built in. I did some distortion analysis on Audio Precision with and without a modular filter I made with BB OPA2134 opamps, and it made no major improvement in distortion... so we just omitted the extra filter circuitry.
Hope this helps,
The so called very steep "brickwall" filters ( 100 dB was not an exception ) are supposed to be THE reason that the first cdplayers sounded so bad. From experience I can tell I never heard a cdplayer with a brickwall filter that sounded even good or satisfying. Phase behaviour was terrible too. But the so-feared RF did not pass through the interlinks !
Digital filtering wasn't invented for nothing. Cheaper than the tuned coils in de brickwall filters, occupying less space on the PCB and no necessity for manually tuning the coils. Better phase behaviour because of the simple mostly Bessel filtering that could take place now.
But with oversampling/digital filtering jitter-sensitivity has become more prominent than before.
Since I play around with non os DAC's I slowly get the feeling ( not more than that ! ) that because the jitter-sensitivity is lower sound immediately is better also. I don't have the sophisticated equipment to measure this, that's why I assume this. Even listening without any filtering ( don't do this at home ! ) is bearable too. Why did the engineers fear the very small RF output so much ? Soundquality really is better without steep filtering with some DAC's. Not all of course.
Sidenote: when I compare old 4 Fs cdplayers that are well modified with Bitsream/Mash/1 bit/high Fs cd-players and DAC's they win quite a few times in terms of soundquality and imaging. I know this is comparing pears with apples but it is fun to do
If you want to try these filters ( old thread only just reopened as I saw after posting ) the cheap way I recommend to wreck a very old cdplayer ( with 16 bit Linear written on the front ). Just cut the board where the filter with the surrounding circuitry is. You'll have a tuned brickwall filter with I/V convertors that works well enough for testing. A bit better opamps won't be a luxury since most of them used NJM4558's.
The LTC1562, which seems a very interesting chip, may be more expensive but it is far more flexible if you plan to roll your own brickwall filter.
Digital filtering wasn't invented for nothing. Cheaper than the tuned coils in de brickwall filters, occupying less space on the PCB and no necessity for manually tuning the coils. Better phase behaviour because of the simple mostly Bessel filtering that could take place now.
But with oversampling/digital filtering jitter-sensitivity has become more prominent than before.
Since I play around with non os DAC's I slowly get the feeling ( not more than that ! ) that because the jitter-sensitivity is lower sound immediately is better also. I don't have the sophisticated equipment to measure this, that's why I assume this. Even listening without any filtering ( don't do this at home ! ) is bearable too. Why did the engineers fear the very small RF output so much ? Soundquality really is better without steep filtering with some DAC's. Not all of course.
Sidenote: when I compare old 4 Fs cdplayers that are well modified with Bitsream/Mash/1 bit/high Fs cd-players and DAC's they win quite a few times in terms of soundquality and imaging. I know this is comparing pears with apples but it is fun to do
If you want to try these filters ( old thread only just reopened as I saw after posting ) the cheap way I recommend to wreck a very old cdplayer ( with 16 bit Linear written on the front ). Just cut the board where the filter with the surrounding circuitry is. You'll have a tuned brickwall filter with I/V convertors that works well enough for testing. A bit better opamps won't be a luxury since most of them used NJM4558's.
The LTC1562, which seems a very interesting chip, may be more expensive but it is far more flexible if you plan to roll your own brickwall filter.
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