REW Cabling and Connection Basics

[brucek]

REW Cabling and Connection Basics


This thread is a quick reference that covers the basics of connecting cables and SPL meters (or microphones) to Room EQ Wizard software (a.k.a. REW) in order to take audio measurements of any home theater, stereo system, or studio.

Additional details and the use of Room EQ Wizard software (REW) are available in the REW HELP FILES.




REW connections using a Radio Shack or Galaxy CM-140 (recommended) SPL meter.


Below is a functional connection block diagram when using an SPL meter (Radio Shack, Galaxy CM-140 or equivalent) with REW. More cable details and pictures are outlined below the diagram. Please Read.


Connection diagram using an SPL meter (Galaxy CM-140, Radio Shack or equivalent):



Every SPL meter or microphone requires a calibration file to be loaded into REW (unless it's perfectly flat or perfectly tracks a C-Weight curve over the frequencies you're interested in testing). Most SPL meters do not track their C-Weight curves at low frequencies, so a cal file is required. Radio Shack meters with a cal file are acceptable for subwoofer testing and up to approximately 3KHz. For full range measurements, a better SPL is required (such as the Galaxy CM-140).

We do offer generic calibration files that are suitable for home use on the Downloads Page for the Behringer ECM8000 microphone, Dayton EMM-6 microphone, Galaxy CM-140 SPL meter, Old Analog Radio Shack SPL Meter Model 33-2050 as well as the two newer Analog and Digital Radio Shack SPL Meters - Model 33-4050 and Model 33-2055. The generic files are fine for home use. For more accuracy it is advisable to obtain a professional calibration for your own microphone.

While it's acceptable to connect your PC (and the REW program) to your equalizer (BFD) and subwoofer directly, it is preferred to connect the PC soundcard through your receiver or processor, so that you have bass management (crossover) available and a convenient method of controlling the level of the signal from the PC. Additionally, it makes the integration of the subwoofer and mains much easier as it relates to level and phase. A subwoofer is certainly not required to use REW. REW can be used for stereo systems or home studios to assist in room treatment, speaker positioning, and equalization.

Remember that all testing with REW through the receiver is done with the receiver in regular stereo mode (not direct) with all effects and soundfields shut off, and with the crossover (if used) set to the frequency you'd normally use with your system (i.e. 80Hz). Usually testing with REW starts with the sub only measurements and the mains disconnected. Then, once the sub response is where you like it (after equalization, treatment or placement changes), add the mains speakers and measure the same as you did with the sub alone. This is the time to adjust for the best transition at the crossover area between the sub and mains. Usually the sub phase control is useful to help get a smooth transition.



Important notes:

You will require a soundcard with a line-in and a line-out jack.

Both the Radio Shack and Galaxy CM-140 output a mono line-level signal. (note, we recommend the Galaxy meter for full range measurements. The Radio Shack meter is not suitable for full range, although it can be used up to ~3000Hz with no problems for testing subwoofers. Here's a fairly good article on the Radio Shack meters).

A microphone input (mic-in) on a soundcard is not suitable (many laptops only have mic-in).



LAPTOP Note:
If your laptop is limited to a mic-in port with no line-in, then you'll require an external USB soundcard. There are basically two flavors of external USB soundcards.

One type contain an internal microphone preamp(s) with phantom power for condenser mics (such as the ECM8000 or EMM-6). The popular manufacturers of these type are Tascam, EM-U, and M-Audio. The connection diagram for these soundcards is shown at the bottom of the page, but be sure to read through this entire article first.

The second type do not contain a microphone preamp, and have analog line-in and line-out only. The interconnects for this type are the same as for an internal PC soundcard.

Most brands of external soundcards work fine with REW. Many members are using the Creative SoundBlaster Live 24bit external card. It's inexpensive and the REW HELP files have a section on setting up the Creative SoundBlaster Live 24bit external card.
In the earlier days of VISTA, there were reports of some soundcards not working well. The drivers for VISTA are now readily available and there shouldn't be any problems. I personally use VISTA 32 on my laptop and use a Turtle Beach Audio Advantage SRM Soundcard. It works very well with VISTA and REW. Here's a post on my experience with it and how to set it up.



The jacks on most soundcards are stereo, so you require a stereo plug for both the line-in and the line-out connections - no exceptions when the plug is stereo. A mono plug will simply not work in a stereo jack. (there are a few external soundcards that use separate left and right line-in and line-out jacks, such as Creative MP3+ and some M-Audio products for example - with these you will use a mono plug into a mono jack).

An 3.5 mm Stereo Plug To Dual RCA Jack Adapter is usually required to separate out the left and right channels of both the line-in and the line-out jacks of the soundcard, although in lieu of these adapters, you may use cables that have a stereo 3.5 mm plug on one end and a pair of RCA connectors on the other end to facilitate the left and right channel breakout.

The left channels of the soundcard are normally not used and are simply never connected. To compensate for any weakness in the soundcards response, we create a calibration file using the right channel of the soundcard, by use of a loopback cable and a special one-time-only routine that saves that file into REW. That loopback is removed after running that calibration routine and then the right channel is used for all testing.

Alternatively, for more advanced users, you may loopback connect the left channel line-out of the soundcard to the left channel line-in of the soundcard, and use the right channel for measuring and this will serve as the soundcard calibration in lieu of a calibration file. To use this special feature, you must check the 'Use Left Channel as Calibration Reference' in the REW Settings page. This method is not recommended. The standard cal file method is recommended. It's simpler, uses less cables and so less chance of problems, offers a bit better S/N ratio, and provides vetting of your setup and connections. The Left channel calibrate method is best used once you've become familiar with REW and want to test System Delays. Other than that, stick to the standard soundcard calibration file method.

The SPL meter connects to the line-in right channel of the soundcard.

The line-out right channel of the soundcard connects to the receivers (or processors) AUX or CD input. It's a good idea to use a Y-splitter at the receiver to feed both the left and right channels, so both mains speakers may be checked along with the subwoofer. Mains and subwoofer are the only speakers checked with REW, and the receiver must be operated in stereo mode at all times during REW tests.


A 3.5 mm Stereo Plug To Dual RCA Jack Adapter shown below is required to plug into both the line-in and line-out stereo jacks of the soundcard.

This separates out the left and right channels and makes them available to use with standard RCA cables. Two of these adapters would be required (one for line-in and one for line-out).

Normally the left channel of the soundcard isn't used. The right channel is first calibrated using a simple loopback cable, then the loopback is removed and that right channel is now used for the microphone and output to the sound system to be measured.





Some people don't like these adapters because they're heavy, and put a strain on the soundcards rather poor 3.5 mm jacks once RCA cables are plugged into them.


A preferred solution is to use a ready made adapter cable such as the ones shown below. They will put much less strain on the soundcard plugs.
Again, two of these cable adapters (shown below) would be required.

The two cables (3.5 mm plug end) would connect into the line-in and line-out stereo jacks of the soundcard. Then use a suitable RCA inline connector is used to loop the right channels together so that a soundcard calibration can be performed. Then the loopback is removed and not used again.

Once the soundcard calibration is complete the right channel RCA cable of the line-out would go to your receiver and the right channel RCA cable of the line-in would connect to the Radio Shack meter. For a Galaxy meter you would need a RCA Jack To 3.5mm Mono Plug Adapter at the meter itself to convert from RCA to 3.5mm plug.


You may also require a couple standard single RCA cables to extend the distance required to get to your listening position and to your receiver equipment. RCA inline connectors would again be needed to connect your standard RCA cables.

Any suitable adapter cable can be used depending on your situation. Below are a couple types.

3.5mm Stereo Male To RCA Male Cable 1.5 ft. adapter cable.



3.5mm Stereo Male To RCA Female Adapter Cable




There are many alternatives to the cables shown, including making your own. These are just suggestions, but any cables/adapters that you choose would be fine as long as they accomplish the job as described.







REW connections using an ECM8000 microphone and XENYX802 preamp (recommended combo)


Connecting a Behringer preamp and ECM8000 microphone to use REW is similar to connecting an SPL meter since the output for both is a line-level signal that feeds the right channel of the line-in of the soundcard.

In the case of the preamp you have convenient adjustment of the line level signal. The ECM8000 microphone is attached to the preamp either directly or with a balanced XLR cable. This is an advantage for longer runs. (A picture of the standard dial settings for the 802 preamp is shown a few pictures below).


Connection diagram using ECM8000 mic (or equivalent) and XENYX 802 (or equivalent) preamp:


The output of the XENYX 802 preamp is an unbalanced line level from a 1/4" TS phone jack. An RCA Jack To 1/4" Mono TS Plug Adapter is required (as shown below).

RCA to 1/4" mono TS adapter




This adapter allows the use of the standard 3.5mm Stereo Male To RCA Cable 1.5 ft. splitter and RCA cable arrangement (as outlined in the SPL meter section above) to be used.

The ECM8000 microphone can plug directly into the XENYX802 preamp (as shown in the picture below), but is best used with an XLR microphone cable and tripod.


TYPICAL dial setup of XENYZ 802 preamp.


Standard XLR microphone cable ends.







REW connections using an ECM8000 microphone and an external USB soundcard with integral microphone preamp with phantom power


External soundcards that offer an internal preamp with phantom power preclude the user from purchasing an external mic preamp. This simplifies the connections and offers easier portability of the equipment. To accomplish a soundcard calibration, a loopback is connected from a line-out to line-in and then removed once the calibration file is created and saved. Experience has shown that the mic stage response anomalies are small and that response drop-off is mainly attributed to the line stages.






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As another convenience option, you may add a connection from the PC (REW) to the equalizer (BFD) using a midi interface to quickly download filters instead of manually entering the REW recommendations into the BFD front panel.

You will require a USB to Midi interface cable that will connect from the PC's USB port to the BFD MIDI-IN port. The cable most used is the

Edirol UM-1EX

Amazon

.

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brucek