DSP Tower of Babel

Bennett Prescott

Bennett Prescott

Just This Guy, You Know?
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Jan 10, 2011
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www.bennettprescott.com
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I am going to write an article about different DSP platforms, in an attempt to have a semi-definitive source to point people to the next time I get asked why those Driverack settings sound so funny in the KT processor.

I would be touched if y'all would do me an enormous favor and help me get measurements from different processing platforms. I have designed an imaginary loudspeaker preset that I think will stress the differences in their filter definitions. If anyone thinks they have better ideas, speak now or forever hold your peace. I am trying to make it somewhat realistic while fitting within the filter capabilities every processor is likely to have. Any reasonably mainstream processor welcome. These exact frequencies and bandwidths may not be available (on purpose), please get as close as possible.

If you would like to help, please program these filters into whatever DSP you have at hand:

High Pass - Bessel - 97Hz 18db/octave
Bell - 280Hz - 2 octaves/0.66 Q -4.5dB
Bell - 176Hz - 0.6 octaves/2.39 Q +5dB
Bell - 6.35kHz - 1/8 octave/11.54Q -8dB
Shelf - 8kHz - 1.6 octave/0.86Q/6dB +5.5dB

Measure with Smaart, using the standard internal pink noise source as a reference. Use 16 averages, 48k sample rate, and 24bit, no smoothing, and set delay compensation using the auto delay finder. Please take a measurement with a 16K FFT and one with MTW (or FPPO for Smaart 5-6). Please send me a Smaart (7 preferable) reference file for each of the two measurements, and if you know how to send an impulse response that might be helpful as well.

Please label your measurements and email them to me, don't post them here... I need to know which processor which measurement came from, but I don't want everyone to know. Anyone who wants to go even further and put in bandwidth instead of Q and take another measurement of that would receive another free Internet.

bennettprescott at gmail dot com

Thanks in advance to those of you with a little time to waste and some measurement chops!
 
Re: Pro Audio Tower of Babel

I've got measurements from two different processors already, and thought everyone might find this interesting. This processor natively uses Q to define filter bandwidth. The green trace is what these settings ''should'' look like. The blue trace is what happens when you plug bandwidth numbers into a Q device, as the two definitions are inverse of each other all the filters become much wider, although apparently this processor doesn't let you have 11+ octave filters.

attachment.php
 
Re: DSP Tower of Babel

I have a connection in a good MI/Prosound level retailer if there are any holes in what people can measure for you I could see if they have a floor model to test.

In addition, if anyone has a DSP/system processor in the DC area that they want to check with Bennett's method, let me know.
 
Re: Pro Audio Tower of Babel

Ryan Lantzy wrote on Tue said:
Bennett,

Were the bandwidth numbers first converted to Q before plugging them in?
Click to expand...

Ryan,

I did the conversion above, so that people can use ''my'' numbers. From the results so far they are very close, far closer than the differences between devices themselves. The picture above is an example of doing it way wrong, obviously.

One interesting result came from a forum reader who sent me traces from the same device in BW & Q that were essentially identical... turns out, the device will take either!
 
Re: DSP Tower of Babel

Arthur Skudra wrote on Wed said:
Can you give us an updated list of what DSP's you already have this data for so we're not duplicating efforts here?
Click to expand...

I have:

XTA DP6i
EV Netmax
EV DC One
EV DX46
Sabine Navigator
dbx DriveRack 4800
dbx DriveRack 260
VMB ARK-48
KT DN9848
KT DN7454
Digisynthetic DMX16
Mini DSP
Symetrix SymNet 8x8
K&F CD24
Dolby Lake Processor
Studer Vista (desk)

But I would appreciate duplicates to weed out errors, funny DSP math, old firmware, etc.
 
Re: DSP Tower of Babel

Bennett Prescott wrote on Wed said:
Arthur Skudra wrote on Wed said:
Can you give us an updated list of what DSP's you already have this data for so we're not duplicating efforts here?
Click to expand...

I have:



XTA DP6i

EV Netmax

Sabine Navigator



But I would appreciate duplicates to weed out errors, funny DSP math, old firmware, etc.
Click to expand...

I should be able to get you Biamp Audia-Danley DSLP48-maybe QSC (if it comes in-in time for me to play with it before it is installed) possibly DBX260 for now.
 
Re: DSP Tower of Babel

Some more results in. Not only are the magnitude results interesting, the phase results are totally wild. Have two more results that need some processing, but you can see the general trend. Only two processors so far that are way off, but the majority are far enough off that it would definitely be audible.

attachment.php


Magnitude only to help to see dB offset differences.
attachment.php


These have been scaled down by the forum software, click the attachment links below to see them full size.
 
Re: DSP Tower of Babel

Bennett Prescott wrote on Tue said:
Some more results in. Not only are the magnitude results interesting, the phase results are totally wild. Have two more results that need some processing, but you can see the general trend. Only two processors so far that are way off, but the majority are far enough off that it would definitely be audible.
0


Magnitude only to help to see dB offset differences.
0


These have been scaled down by the forum software, click the attachment links below to see them full size.
Click to expand...

With the phase responses as they are-that could also throw off the delay time between passbands.

What I also find interesting is the ''quick little flip'' at the very top end of the response. Some going up-some going down.

Are those real? or an artifact of the measurement interface used?

I thought that the ''experts'' in audio said that all of these type issues were going to be ''fixed'' when we went to digital.

Looks like it got worse if you ask me. Now we don't know what we are dealing with.
icon_lol.gif
 
Re: DSP Tower of Babel

Ivan Beaver wrote on Tue said:
I thought that the ''experts'' in audio said that all of these type issues were going to be ''fixed'' when we went to digital.



Looks like it got worse if you ask me. Now we don't know what we are dealing with.
icon_lol.gif
Click to expand...



No kidding.

I have been well aware of lack of consistency between DSP's, but seeing the documented results so far is eye opening.



 
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