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Old 11th November 2010, 02:14 AM   #61
minidsp is offline minidsp  Hong Kong
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Quote:
Originally Posted by cuibono View Post
I've noticed in my system (where I do digital attenuation), it requires a lot of reduction. -40dB-ish. I've also measured an increase in THD with more and more attenuation. Maybe gainphile is really cranking it down?
Hello,

Following the question of Dave, we're also interested to hear how you're reaching higher levels of THD when using the attenuation.. It just doesn't line up to our measurements on the miniDSP and technically I'm not sure how I could explain it. (though doesn't mean it doesn't exist as we certainly don't pretend speaking the truth... :-)

With any attenuation you perform, you'll get your signal close to the noise floor. i.e. making your SNR worse. As you may know, SNR specs is a different one than THD specs though... Now by my standards a 40dB attenuation is a LOT and you should be conscious that on ANY DSP platform, it's not a good idea to work that way. Digital gain is great to tune for minor tweaks, but with a grain of salt for what I explained above (SNR).

Now attenuation is required in a lot of cases (e.g. HF doesn't require as much amp power than Mids or LF) but instead of attenuating in digital domain so much, best is to match up your amplifier volume. i.e. keep what we'd call unity gain (feed a strong signal at input, strong signal at output to maximize the ADC/DAC stage) THEN attenuate at the amplifier itself. (with the volume). You'll see that your system will sound better.

This technique/procedure is what has been done in the ProA/V for many years and what we'd call "gain structure". Without proper gain structure, you don't optimize the full potential of your DSP system.

Hoping this explanation makes some sense, we're still interested in seeing these THD graphs/test conditions if any... It may bring up more food for thought on what is happening.

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Last edited by minidsp; 11th November 2010 at 02:26 AM.
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Old 12th November 2010, 06:40 AM   #62
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I recently ordered a mini-dsp and am eager to get it!

One question/observation though. It seems Bob McCarthy defines an all-pass filter differently than is being discussed here (see "Sound Systems: Design and Optimization" page 26). According to him, an all-pass lets us set arbitrary ranges of the frequency response and tune the delay within each band.

It seems the biquads allow one to define a function that determines the range and amplitude over that range, but I don't see any variable to set the delay of each individual biquad filter in the mini-dsp. I'm wondering what the mini-dsp people have to say about this??

Having a roughly flat phase response across the entire speaker would be a "nice thing to have." I'm assuming it is more important for line-arrays in pro audio where multiple speakers need to work in unison to send the same signal, or in subwoofers when various subwoofers are working together to produce a signal (and can have dramatically different phase response depending on if they are ported, sealed, etc.)

Having total control of the phase could also be useful to cancel out noises coming from behind the speaker to reduce unwanted room colorations? This would require a second driver behind the primary driver to control the beam, I think. It seems like the pro-audio people use phase to do things like this, from what I can understand from their books.

Last edited by bluewhale; 12th November 2010 at 06:44 AM.
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Old 13th November 2010, 04:14 PM   #63
jackies is offline jackies  United States
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Hi gainphile!
That's a lot of cool OB projects you have built!
Now, I wanted to ask, what did you think about sound quality, when you switched from analog crossovers to digital? Notice, it's not about flexibility, ease of use, etc.
I have a DCX2496 and honestly, I think it sounds like crap, I much prefer the sound of regular crossovers with caps and coils. Granted, I only tried it with regular, boxed speakers. But I think digital processing just sounds bad, really bad...
Now I'm thinking to attempt something OB and was thinking to build some active crossovers with opamps. Or maybe it's worth to try and mod the DCX with transformers and all? I imagine an opamp based crossover would still sound better. There's also PLL crossovers of course with tube/fet buffers...
Just wanted to hear your opinion, as a person who tried both digital and analog active crossovers...
Thanks!

Last edited by jackies; 13th November 2010 at 04:29 PM.
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Old 13th November 2010, 07:37 PM   #64
jackies is offline jackies  United States
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Hah, just noticed that it's in miniDSP manufacturer's forum, probably not the best place for digital crossover bashing, my apologies...
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Old 13th November 2010, 08:43 PM   #65
Davey is offline Davey  United States
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Regarding the distortion claim: Here are some distortion measurements performed on my MiniDSP unit. The "loopback" plot is a baseline measurement showing the residual distortion of my testing system. The other plots should be self-explanatory. (You folks can draw your own conclusions from the results.)

Cheers,

Dave.
Attached Images
File Type: png loopback.png (144.9 KB, 1015 views)
File Type: png 0db.png (133.2 KB, 968 views)
File Type: png -10db.png (144.0 KB, 958 views)
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Old 13th November 2010, 08:44 PM   #66
Davey is offline Davey  United States
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-20db and -30db attenuation.

Dave.
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File Type: png -20db.png (151.7 KB, 937 views)
File Type: png -30db.png (150.9 KB, 928 views)
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Old 13th November 2010, 10:57 PM   #67
cuibono is offline cuibono  United States
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Sorry! I'm not using miniDSP! The THD increase I was referring to was seen in SoundEasy's signal generator. Davey, thanks for showing all these measurements - I'm probably going to be buying a couple of miniDSPs, so all this is nice to see.
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Old 4th December 2010, 01:58 PM   #68
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Hi, Davey or sdsnyc.

Have you made the settings for Pluto 2.1
I have tried Davey´s settings for Plutto 2.0,but would like to try settings for 2.1. too

Bernt "Båndsei"
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Old 4th December 2010, 08:16 PM   #69
sdsnyc is offline sdsnyc  United States
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See attached. Note this is completely untested. I won't have the time to finish this project until after the New Year.
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Old 4th December 2010, 09:38 PM   #70
sdsnyc is offline sdsnyc  United States
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