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post #1 of 11 Old 08-08-2014, 02:31 AM - Thread Starter
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miniDSP OpenDRC-DI

New toy inserted.

This one:

http://www.minidsp.com/products/open...ies/opendrc-di

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equitech 1.5RQ > digits > miniDSP > behringer > benchmark > krell pre and monoblocks > reQuest
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post #2 of 11 Old 08-08-2014, 04:56 AM - Thread Starter
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Here's a graph of the phase (delay?) by frequency for my system at the sweet spot.



It looks like the lows become progressively late, though from 800hz on up the output is already phase coherent.

This will be the first thing I experiment with, using the tool rePhase.

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post #3 of 11 Old 08-08-2014, 08:14 PM
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Ideally the phase would be a linear line, which in turns leads to constant group delay. That means all frequencies are delayed equally, no time distortion.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #4 of 11 Old 08-09-2014, 01:17 AM - Thread Starter
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First try at phase correction gives (using the same scale as original above):



Group Delay is decreased, but not so flat, so have to think about that part a little bit more.


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post #5 of 11 Old 08-12-2014, 01:08 PM - Thread Starter
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Well, having played for a while, a little surprise.

Most of my investigation so far is with phase correction. To do that the DSP delays or advances frequency bands in time relative to each other, but does not aterially change the frequency response.

So, by doing that, waves are relocated in time, and when summed with the non-relocated waves, produces a clip. The results are a bit unpredictable when trying to guess how much attenuation is enough to prevent clips.

I've had to attenuate the input to the DSP by 9dB to prevent clippage showing up at the input to the Behringer. That may or may not be enough for all source material.

I've thought to use white noise input at 99% full scale, but the no-clip input setting determined with that source was about -3dB.

Unfortunately, the miniDSP hardware does not have a level indication, and the software plugin has what appears to be a very rudimentary level meter.

I'm liking the unit though, overall.

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post #6 of 11 Old 08-13-2014, 11:42 PM - Thread Starter
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Weekend Update...


I've brutally twisted the phase into alignment from about 700hz on down.

Increasing torque was applied to the lower frequencies, correcting a maximum of 1477 degrees of retardation at 30hz.

The bottom end is all cleaned up.


Any questions?


Yes, why didn't the Group Delay flatten out more?

Yes, uh, well, the, uh... Are there any other questions?

no.

Post adjourned.

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post #7 of 11 Old 08-16-2014, 09:47 AM - Thread Starter
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Effect of time displacement on REW's logarthmic sweep frequency response

Very small to none.

(Yes, I know I have a big suckout in the lows due to the room)


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post #8 of 11 Old 08-16-2014, 09:58 AM - Thread Starter
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Group Delay reduction - green trace is with the time filter, red is without


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post #9 of 11 Old 08-16-2014, 12:09 PM - Thread Starter
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Since the miniDSP and FIR filter is messing with my signal, let's look at the output of the preamp.

(This measurement includes a 25 foot RCA to 1/8" miniplug from preamp to PC onboard sound device, and any defects caused by them.)




Blue is without the filter, and green is with. Basically no difference in frequency response is seen, results within .1dB or so. (Flat dark blue line is just the measurement cursor)






The FIR filter is changing phase, so here is that measurement at the preamp:

Blue is the preamp output without the filter, green is preamp output with the phase correction filter applied.







And finally, for right now...

Top green is the in-room measurement of phase with the FIR filter, red is the unfiltered room measurement of phase, and the lower green is the phase correction applied at the preamp by the filter (as measured at the preamp out).

This shows how the FIR filter creates* a reasonable inverse to the phase error detected in the room, and applied that to the signal, and corrected the sound in the room.




*rePhase is a manual tool, so the inverse isn't perfect, I have to move a lot of sliders and mess with things a while to have gotten it that close

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post #10 of 11 Old 08-21-2014, 11:35 AM - Thread Starter
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A tip on using rePhase:

At first I used a lot of travel on a lot of sliders to get that major phase/delay out of the low end.

Then experimented on another page of the tool.

Filters Linearization tab, Minimum-phase Filters Phase Linearization

One setting there can twist the big curve in the phase nearly flat.

Then, go to the Paragraphic Phase Eq sliders to clean up, after the major demolition work is done.

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post #11 of 11 Old Today, 12:38 PM - Thread Starter
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Here are some current measurements.

Red = no DSP

Green = FIR filters applied

1/3 octave smoothing applied
Attached Images
File Type: png spl.png (76.0 KB, 2 views)
File Type: png phase.png (63.9 KB, 1 views)
File Type: png gd.png (63.8 KB, 1 views)
File Type: png impulse.png (66.9 KB, 1 views)
File Type: png step.png (69.2 KB, 1 views)

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equitech 1.5RQ > digits > miniDSP > behringer > benchmark > krell pre and monoblocks > reQuest
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