Home Theater Forum and Systems banner

1 - 9 of 9 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter #1 (Edited)
Hi All

I am using REW with USBPre (first one they made)

Connections
Reference Mic: XLR Input 1
Loopback: 1/4 TRS Input 2 to 1/4 Output
Output: 1/8 TRS To PA

1st Measurement: With Loopback Connection, Not Set As Timing Ref
2nd Measurement: No Loopback
3rd Measurement: Loopback Set As Timing Reference (Preferences, Analysis, Set Loopback As Timing Ref)


It seems like the measurement with no loopback is somewhat like what I'm looking for, but I'm getting a message saying:
"Very Low Signal On Measured, Highest level measured input is -1dB (-87dBFS) This is much lower than it should be. Check the input ch selection, connections, input volume, and output volume/mute. The highest level should be above -35dBFS"

I did calibrate my SPL meter, but something seems weird.

Anyone have any insight :rubeyes:
 

Attachments

·
Premium Member
Joined
·
1,562 Posts
Possibly there is confusion regarding setup connections/process?

Assuming the REW default channel (channel 2) was calibrated properly with a loopback cable then:
> Remove the loopback cable and place the mic into channel 2 input.
> Use channel 2 output for the REW signal to the sound system. [Channel 1 input is not used for normal sound measurements. Channel 1 may be used for loopback timing if you have a need for that feature. Most users don't need it.]
> The internal REW SPL meter is then calibrated to match an external hand held SLM reading as explained in the setup instructions. [The USBPre channel 2 input gain is adjusted again in this process to provide reasonable headroom for sound measurements. If the channel 2 input gain is then ever changed, the REW meter calibration should be redone.]
> Ready for sound measurements.
 

·
Registered
Joined
·
6 Posts
Discussion Starter #4 (Edited)
Yes~

Confusion on setup!

Do I have to have an external SPL meter? I have tried calibrating REW's SPL Meter and it doesn't work, always give me a negative number.

First, I am not sure what driver I should use. ASIO or JAVA.

Here are pics with ASIO & JAVA options.
 

·
Registered
Joined
·
6 Posts
Discussion Starter #7
USB Pre doesn't have Ch 1 or Ch 2 as outputs, only stereo outs (1/8, 1/4, RCA L&R).

I am using the 1/8 out to mixer and 1/4 out for loopback to Ch 2 input.
 

·
Premium Member
Joined
·
1,562 Posts
There are several possible setups - too many to cover them all so I will offer one good option to get started on audio measurements:
> The left line output is also known as channel 1 and the right line output is also known as channel 2.
> Use the ASIO Drivers and REW settings exactly as shown in Post 5. [Channel 1 input and output are selected for measurements. Channel 2 input and output is selected as timing reference. This is the reverse of the REW defaults but that is no problem.]
> Just deleted the current calibration file unless you are sure it was properly done using channel 1. [The USBPre does not really need a calibration file as it is flat within in the audio range.]
> Plug the mic into mic 1 input and turn on 48V PH. Set channel 1 gain to the 3:00 O'clock position
> Plug a 1/4" TS to RCA adaptor into the left line output (channel 1). Connect an RCA cable from that adaptor to the stereo receiver or AVR left channel RCA input.
> Set both REW and the USBPre to 44.1k sample rate.
> Set the USBPre to activate Mic 1 input. Leave the Mix set to PC as it is now shown.
> Channel 2 is not normally connected or used for normal REW frequency response measurements.
> Set the receiver's input selector to the input that the cable is plugged into and also select stereo mode.
> Open the REW SPL meter for calibration and select 'use REW speaker cal.' You should now hear the pink noise sound from the left speaker. Set the receiver volume to a strong sound level but not uncomfortable and enter the calibration level as 75 dB. The REW meter will report a 'headroom' level that results. That headroom is a function of the USBPre mic 1 input gain we set (3:00). If the headroom is in the range of 95-110 dB that is good for most work. If it is too low or too high you can adjust the USBPre gain accordingly and do the REW meter calibration again.
> Done - ready for measurements. We are setup correctly to measure the left channel so do that, then just move the RCA cable to the right input channel on the receiver for a measurement of the right speaker.

The actual REW meter is not calibrated accurately to 75 DB as we did not use a an external meter. We just estimated it. This is no issue however as most all the work we do relies on relative levels. That is, we want a relatively smooth SPL across the frequency range. It does not matter if we do that work at 75 dB or some other reasonable level the results will be the same.

I do not have any experience with the USBPre so there may be other setup requirements with it, but these are the basic ones. Hopefully with this info and a review of the REW help information you will now get good measurements.

I do not recommend using the USB Pre headphone output as shown in the pictures even though it may work okay. There may be some risk of overload of the receiver input if the gain is too high?
 
1 - 9 of 9 Posts
Top