US-144 Calibration Question... - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-07-09, 10:48 AM
HTS Senior Moderator

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Location: Katy, Texas
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Re: US-144 Calibration Question...

This mixer has a USB output for the computer. It is the soundcard! IOW, it does what your Xenyx/soundcard set-up does, all in one unit instead of two...

Regards,
Wayne

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Old 01-07-09, 10:57 AM
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brucek

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Re: US-144 Calibration Question...

Quote:
This mixer has a USB output for the computer. It is the soundcard!
Oh OK, I see... huh, it looks just like the XENYX so I assumed it was the same....
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Old 01-07-09, 11:58 AM
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Re: US-144 Calibration Question...

Soo... can an acceptable soundcard calibration be had between the balanced output and the mic input?

Regards,
Wayne

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Old 01-07-09, 12:32 PM Thread Starter
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etc6849

Join Date: Jan 2009
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Posts: 67
Re: US-144 Calibration Question...

Yep, that's me. Good to hear from you again. I think you were the one over there that told me about REW and I followed the links in your signature :-)

I am wondering about your statement of shifting the corner frequency. I think you have a good point.

Gain wise, you are just shifting the curve downward right by adding a padding circuit? This should not shift the corner frequency to the left or right since ideally I am adding only resistance.

My understanding is one should look at the gain from the output of the sound card, the gain from the voltage divider (padding circuit plus some cabling) and the gain from the soundcards pre amp. If all are in dB, you can add. If they are in voltage (vout/vin) you would multiply them. Since, with the voltage divider, I'm adding what should ideally be a constant impedance over all frequencies (no reactance, only resistance) I shouldn't see a corner shift left or right; only up or down. Now, if I open up the soundcard and modify the resistors connected to the op amps inside, yes this would shift the corner frequency. I wish I had a an adapter to connect this special padding circuit from the output to the input to verify this though cause you have an interesting point.

Bruce: Thanks for the results. I soldered the padding circuit up last night. I didn't use a metal box to shield the dividing circuit though, but just soldered and used shrink wrap to prevent ground to signal contact. I might use some foil and solder this to the ground later on. I don't know how that would work though as I've never tried soldering foil; I don't think it's worth finding a metal box for yet. My soundcard and mic are arriving today so I will post my results soon.

Quote:
DrWho wrote: View Post
Hey etc....you're from the Klipsch forum right?

I think you might be way overthinking the issue....the US-144 has analog volume controls for the outputs and inputs. Just turn down the lineout. The problem with an L-Pad is that it changes your effective output/input impedance in terms of where the LF corner ends up.

Also, when driving a balanced input with an unbalanced output, you will lose 6dB since you're tying the balance leg to ground with the adapater. I should have the opportunity to compare the line input to the XLR input, but I'd wager that the difference is trivial. It should be interesting to compare notes though.

The last set of measurements I did, I just used a loopback from the left output into the left unbalanced input. The ECM8000 was hooked up to the right channel xlr input. My measurements were very comparable to my M-Audio box that I had the opportunity to compare against the Klipsch anechoic chamber setup...that was within half a dB from 100Hz to 10kHz. Within a dB from 20Hz to 100Hz, and within 2dB above 10kHz.

The only source of frequency non-linearities in these devices is the digital anti-aliasing filters in the DACs/ADCs and then the series dc blocking caps. I suppose it's due process to ensure the XLR inputs have the same LF corner frequency as the line inputs, but I'd be surprised if it was very different. It would be pointless for them to not be using the same front-end for both the balanced and unbalanced inputs.

Ok, so I just looked at the spec sheet...unbalanced input impedance is 10k. Balanced input impedance is 2.4k. If the unbalanced input has a corner frequency of 3Hz, then the balanced input will have a corner at 13Hz. I just measured the corner of the line input at 3Hz... I'll bust out an adaptor tomorrow evening and actually measure the LF corner of the balanced input. Anyone wanna take some bets?
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Old 01-07-09, 12:53 PM
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brucek

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Re: US-144 Calibration Question...

Quote:
Soo... can an acceptable soundcard calibration be had between the balanced output and the mic input?
Of course. You would use a partial H-Pad. The partial would be the best in this case. Simply take the values you would use for the simple divider (such as 3.3K and 100ohms for ~ -30dB) and divide the series resistor in half (i.e. 1650 ohms. Standard value of 1800 would be fine or go down to 1600).

The two 1800 ohm resistors would attach from the balanced line-out XLR pin 2 and 3 (or equivalent 1/4" phone), and then the other resistor of 100 ohms is soldered across the 1800 ohm resistor ends. Then feed the new balanced output from the 100 ohm resistor to the pin 2 and 3 of the mic-in XLR.

I could draw a picture, but I'm sure you get it......

Quote:
This should not shift the corner frequency to the left or right since ideally I am adding only resistance.
It won't, you'll be fine.

Quote:
I didn't use a metal box to shield the dividing circuit though
Not a problem, just keep the leads short and cover it with some foil or whatever. It'll be fine. I had 3 foot clip leads and it was sitting on my computer and mine worked fine....

brucek
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Old 01-07-09, 10:11 PM Thread Starter
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etc6849

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Re: US-144 Calibration Question...

I've attached the measured file for one of my speakers. I had no idea how bad it would look. Is there something wrong with my setup? I plotted this after calibrating the ECM8000, adding loopback cable on the left output/input. I had the mic input on the card set on max, line out on max. I let REW do 4 iterations with 512k length.

I guess I really didn't realize there were 40 dB swings in my speaker's output. Is this normal or am I missing something? The green is without audyssey multiEQ, the blue is with Audyssey. The speakers are Klipsch RF-83's.

PS: Is there a best way to point the mic? Right now I'm pointing it straight at the speaker near the tweeters height, horizontal with the floor.
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Old 01-07-09, 10:31 PM Thread Starter
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etc6849

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Re: US-144 Calibration Question...

Here's what I get for the soundcard with the padded loopback.
Attached Thumbnails

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Old 01-07-09, 11:24 PM
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jason

Join Date: Jan 2007
Location: Australia
Posts: 193
Re: US-144 Calibration Question...

the ECM 8000 mic should point up towards the ceiling at ear hight when seated, if you are checking the room response, it is designed to graze like the Audyssey mic....

Cheers..
Old 01-08-09, 03:29 AM
REW Author

John

Join Date: Apr 2006
Location: UK
Posts: 6,305
Re: US-144 Calibration Question...

The wild swings at higher frequencies are normal and caused by comb filtering, reflections from room surfaces partially cancelling the direct signal from the speaker at frequencies where the difference in arrival times corresponds to 180 degrees of phase shift. To see the underlying response apply some smoothing to the trace.

You will generally get perfectly good results with a single 256k sweep, multiple longer sweeps are useful in environments with higher background noise or if you need a very high signal-to-noise ratio for some post processing of the impulse response.
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Old 01-08-09, 08:33 AM
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brucek

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Re: US-144 Calibration Question...

Quote:
Is there something wrong with my setup?
No, it's your plot setup that needs to be changed.
For subwoofers, always use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 200Hz) using the Graph Limits button in the top right corner of REW.
For full range, use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - upper limits you desire, i.e. 20KHZ - certainly no higher than your soundcard can extend).
For full range, enable smoothing to eliminate the comb filtering. Use a 1/3 octave smoothing.

brucek
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