REW Cabling and Connection Basics

[brucek]

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 in a home theater to help get the best response of the system.

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

Also remember that 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). We do offer generic calibration files that are suitable for home use on the Download page for the ECM8000 and the Galaxy CM-140 and the Old Analog Radio Shack Meter Model 33-2050.cal and the two newer Analog and Digital Radio Shack Meters - Model 33-4050 and Model 33-2055.

While it's acceptable to connect your PC (and the REW program) to your equalizer (BFD) and subwoofer directly, it is preferred to connect through your receiver, 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.

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 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 is useful to help get a smooth transition.

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

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. 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).

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

Many members are using the Creative SoundBlaster Live 24bit external card with XP operating system. It's inexpensive and the REW HELP files have a section on setting up the Creative SoundBlaster Live 24bit external card.

VISTA USERS NOTE:There have been many reports of the SoundBlaster not working well with VISTA. I personally use VISTA 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.
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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, 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. I much prefer to use the soundcard cal file method. It's simpler, uses less cables and so less chance of problems, and offers a bit better S/N ratio. The Left channel calibrate method is best used once you've become familiar with REW and want to test System Delays.

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.

Below is a functional connection block diagram when using a Radio Shack or Galaxy CM-140 meter with REW. More cable details and pictures are outlined below the diagram.


Connection diagram using Galaxy or Radio Shack SPL meter:
REW interconnection RS meter1.jpg

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.

Parts Express 090-294L stereo mini to RCA adapter.jpg



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


A preferred solution is to use a ready made splitter cable such as the one shown below. They will put much less strain on the soundcard plugs.
Two of these cables (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 an RCA inline connector to loop the left channels together.
Then 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 would 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.

3.5mm Stereo Male To RCA Cable 1.5 ft. splitter cable.
Parts Express 181-590L stereo to RCA cable.jpg



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.


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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 and XENYX 802 preamp:
REW interconnection ECM mic.jpg



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

RCA to 1/4" mono TS adapter
Parts Express 090-285L RCA to phone adapter.jpg



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.
802.jpg

Standard XLR microphone cable ends.
Parts Express 184-240L XLR cable.jpg



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I include below the REW HELP file pictorial of a typical REW setup using a Radio Shack meter (as a convenience).

The receiver and BFD connections are shown with the standard RCA Jack To 1/4" Mono Plug Adapter used at the BFD input and output.

REW HELP FILE DIAGRAM.jpg

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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.

brucek