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Old 14th February 2015, 07:42 PM   #1
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Default Filter brewing for the Soekris R2R

Rather than clogging up the main vendor thread with posts and discussion about alternative filters, it seems more useful to corral all the information into one spot.


My intention is to put together a collection of basic filters starting with:
  • Linear-phase anti-aliasing
  • Minimum-phase anti-aliasing
  • Linear-phase soft roll-off
  • Minimum-phase soft roll-off.
At the moment I'm planning on working on 44.1khz versions of the filters as these influence the sound more than at higher sampling rates. I'll look at doing higher sample rates when I'm happy with the 44.1 files.

I'll post up response curves measured directly from the filter parameters I post up, and ask anyone else contributing filters to do the same, to make comparisons more straight forward.

If anyone wants to contribute analog measurements of the filters they are welcome to do so.

cheers
Paul


**********
Quote:
Originally Posted by soekris View Post
If people want to start then I now have the tools on our website:

http://www.soekris.dk/1021filt.txt -- ascii file with all filters
http://www.soekris.dk/1021filt.skr -- binary file for downloading onto a dam1021
http://www.soekris.dk/MKROM.EXE -- 32 bit dos utility to convert txt file to skr file, usage "mkrom 1021filt"

Please note that messing with IIR filters without knowing what you're doing can result in oscillations which will blow your speakers and/or ears.

****************
Background Reading
****************
****************
Test Tracks
****************
DPA Microphones - Miking A Grand Piano
Promotional CD illustrating techniques for miking a grand piano using their product range.
The was given away on request by DPA Microphones. This was released in 2007/8 and is long out of print.
Files are 44.1/16bit in .flac format and the zip archive is 270Mb.

As this is promotional material for DPA Microphones I believe there should be no issues sharing/spreading the word about DPA Microphones.

https://www.dropbox.com/s/223h1yuvlp...AC441.zip?dl=0

The SACD/CD contains 7 performances, with 8 different mic configurations.

1-8 Fallin' Leaves
9-16 Evening Song
17-24 Slowly Drifting
25-32 One for the New Ones
33-40 The Dat I Met Mr. Hayden
41-48 La Gazzellina
49-56 Minature XIV

The mic configurations are in the same order for each performance
1) 4041-SP, AB Stereo
2) 4006-TL, AB Stereo
3) 4052, In magnets on frame
4) 4011-TL, Bass/mid-treble positioned
5) 4021, ORTF Stereo
6) 4021, In magnets on frame
7) 4061, Hung from lid
8) 4061, In magnets on frame

The booklet that came with the disc can be download from the DPA site, but is mainly a discussion of miking techniques used.
http://www.dpamicrophones.com/en/Dow...grandpiano.pdf

Last edited by spzzzzkt; 22nd February 2015 at 08:02 PM. Reason: Add links.
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Old 14th February 2015, 07:54 PM   #2
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Default Minimum-phase soft roll-off

To start the thread rolling, this filter is a second attempt at the Ayre Listen filter.
Unlike the MixPhase1 filter posted to the vendor thread it is pure minimum-phase.

I discovered the "trick" to minimising ringing is to stack filters in rePhase. By placing filters at approximately 2kHz intervals it's possible to create curve that replicates the desired behaviour using gentle filter slopes.

The filter impulse response is now very close to the 1 cycle of post ringing described in the Ayre MP white paper.
Attached Images
File Type: png MinPhaseSlowRev1_FR.png (29.9 KB, 1346 views)
File Type: png MinPhaseSlowrev1_IR.png (28.7 KB, 1332 views)
Attached Files
File Type: zip 1021MinPhaseSlowrev1.skr.zip (3.6 KB, 77 views)

Last edited by spzzzzkt; 14th February 2015 at 08:07 PM.
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Old 14th February 2015, 08:14 PM   #3
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Hoping to get my DAC up and running so I can give your work a try.

I knew I could trust you when you said you use PUBLIC IMAGE/Album as one of your reference recordings.

It can tell one many useful things plus it is a great record.

Are you able to enjoy the latest one?
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Old 15th February 2015, 12:17 AM   #4
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Quote:
Originally Posted by rickmcinnis View Post
Hoping to get my DAC up and running so I can give your work a try.

I knew I could trust you when you said you use PUBLIC IMAGE/Album as one of your reference recordings.

It can tell one many useful things plus it is a great record.

Are you able to enjoy the latest one?
I haven't had a chance to listen to Album with the new filter. I'll have a better chance to listen tomorrow, but at the moment I'm playing Bill Laswell's "Means of Deliverance" (solo acoustic bass), and my initial impression is this is a clear improvement over the MixPhase1 filter.

cheers
Paul
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Old 15th February 2015, 12:22 AM   #5
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Paul--thank you for doing this (both the thread organization and the filter experiments). I'm still piecing together parts of my board, but am really looking forward to the filters that will be available. I've been using Audircana+ for awhile, and the filter adjustments in that have almost put my turntable up in the attic.
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Old 15th February 2015, 01:33 AM   #6
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I meant the latest PIL record though the question very well could have been a double entendre ...
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Old 15th February 2015, 01:58 AM   #7
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Quote:
Originally Posted by rickmcinnis View Post
I meant the latest PIL record though the question very well could have been a double entendre ...
I was tossing up which interpretation to answer....

No, I haven't.
I have to admit I was a Bill Laswell completist for a while, which is one of the main reasons I own Album.
That said, it's an incredible lineup playing on that recording.
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Old 15th February 2015, 02:16 AM   #8
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Even if one is not a PIL fan that record is easy to like.

I really like them, especially the second record.

One great thing about Lydon is he is always changing so when ALBUM was a departure one went along with it.

Thanks for your explorations.

Went to an electronics store and bought the wrong gender RS232 cable so tomorrow I get to hear the DAC with correct firmware and your filter.
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Old 15th February 2015, 02:28 AM   #9
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Default "NOS" filter

Not too much you can say about this really.

All sampling rates have a single filter tap set to 1.000 which effectively means they are a bypass.

44.1 and 48khz = x64 up-sampling
88.2 and 96kHz = x48 up-sampling
176.4 and 192 = x16 up-sampling

I haven't adjusted the gain, so you'll need to turn it up a bit.

No graphs, as it's dead flat response, and the IR is a single value.

All that remains is Søren's final FIR on the x8 upsampling, which I don't plan to touch at this point as it filters out content above 220Khz.
Attached Files
File Type: zip 1021NOS.skr.zip (850 Bytes, 62 views)

Last edited by spzzzzkt; 15th February 2015 at 02:46 AM.
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Old 15th February 2015, 03:06 AM   #10
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Default Linear-phase Slow Roll-off

This is a similar filter to the Ayre Linear-Phase "Listen" setting.

As with the minimum phase filter, the main aim was to get as little ringing as possible.
The stacked filter technique seems to help in achieving this goal.
Attached Images
File Type: png LPSRO441_FR.png (31.7 KB, 1260 views)
File Type: png LPSRO441_IR.png (33.5 KB, 1245 views)
Attached Files
File Type: zip 1021LPSRO441.skr.zip (3.6 KB, 53 views)
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