asymmetric apodizing filter

The word "apodization" literally means "cutting off the foot" but in signal processing & in optics it is a technical term that means adjusting signal using a windowing function so that the resulting adjusted signal shows some favourable properties such as xdB attenuation at a certain frequency, xKHz or xMHz bandwidth, etc.
Apodization has been used in optics long before it became "cool" to use in audio.
here is an interesting paper by Ayre on their use of a minimum phase filter which they have settled now after having used apodization filters in their prev (2nd) generation.
http://www.ayre.com/pdf/Ayre_MP_White_Paper.pdf

it is my understanding that 'minimum phase' & apodization filters are 2 slightly different things - minimum phase filters eliminate pre-ringing (you know what this means now that you've read Ayre's paper) at the cost of different delays at different frequencies. This means that minimum phase filters are NOT linear phase filters i.e. they do introduce phase distortion but the designers argue that the phase distortion is at high freq & is minimal & is often not of any consequence. Apodization filters reduce the amount of pre-ringing & post-ringing but do not eliminate them. This means that apodization filters are linear phase filters i.e. constant group delay or same delay thru the filter for all frequencies.

here is another nice blog post detailing the 5 (digital) filter choices from a Wolfson DAC WM8741: http://hifiduino.blogspot.com/2009/05/wm8741-digital-filters.html

hope that this helps....

Bombaywalla,
I know it wasn't meant for me, but thats the glory of this forum. I recently bought a dual mono WM8741 DAC. This filter info is very informative...
Thank you,
Tim

Welcome Timlub - glad that you found the info useful. yeah, that's the beauty of forums - the replies are there for all to read & learn & comment.

One source of information on the topic is Technical Paper 3. HD Audio: 'Ringing' why it is undesirable and how to address it on the AMR website.

Dealer disclosure.

the ps audio pwd and pwd2 have both minimum phase filter and asymmetric apodizing filter along with other combos. i found them very helpful/noticeable with some recordings and not so much with others. am yet to determine any pattern or why. i'm thinking that's why ps audio gives you 4 choices along with an auto mode.

some info towards the bottom of this write-up

http://www.psaudio.com/products/audio/media-players/perfectwave-dac/

cheers

Great read guys. MAny thanks

Hi Glai - I recently uploaded an apodizing filter into my Meridian dac/preamp. To my ears, it is a remarkable improvement.

FWIW, I did NOT upload Meridian's proprietary apodizing filter, although I had the option to do so. Instead, I uploaded an apodizing filter created by an independent programmer.

My decision to install a non-official filter was based on multiple reports from Meridian owners who were less than overwhelmed by Meridian's proprietary filter. They felt that Meridian's filter removed some of the "air" in the recording. I'm just reporting something second hand, so keep that in mind.

I mention this because it highlights the fact that there appears to be quite a bit of variation among apodizing filters. I think it would be interesting if a manufacturer offered multiple apodizing filters in a single product. Maybe that will happen at some point.

Bryon

We can talk about various advantages and disadvantages but it all comes down to your own ears. Some won't like different filters because it's just not what they've become conditioned to. Thirty years of "brickwalls" sets a de facto standard of sorts.

Newer DAC chips, like some of the Wolframs and ESS Sabres, have multiple filter choices on board but the surrounding circuitry and implementation can make a large difference.

Related topics, in a more analog context, might come from terms like "group delay" and "Bessel filters".