# Brucek: MobilePre Hookup Question



## jmalto (Jan 26, 2008)

Bruce,

I know you are pretty familiar with the mobilepre and I was wondering if you could help me with the initial loopback test. I have read so many methods that my head is about to explode :gah:

Would this be the correct method for making the calibration file? 

1.) Connect channel 1 out to channel 2 in, using the 1/4" connections for the loopback

2.) Use a mono 1/4" to RCA adaptor to get from the MobilePre 1/4" output to the input of the processor. 

3.) Use "Y" adaptor to feed both channels of the processor with the mono out from the MobilePre

Is there any advantage to splitting the signal to use 2 RCA inputs vs just using one? Which line-out would I use (1 or 2) once I make the calibration file? I would assume CH1 but that is the L-channel and from what I've read you do not use that one.

Thanks for the help in advance!

-Jonathan


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## brucek (Apr 11, 2006)

> I know you are pretty familiar with the mobilepre


Not in the least. I simply downloaded the manual and read it.



> Would this be the correct method for making the calibration file?


No, the soundcard calibration is carried out with a single loopback cable installed on the channel you want to calibrate. So, assuming the Right channel corresponds to Channel 2 on the soundcard, then install a 1/4" TS phone cable from line-out 2 to line-in 2 and do your calibration. Do a quick measure once the soundcard cal is saved in REW to be sure you get a flat line measurement result, then remove that loopback cable.

Now, use that calibrated channel for REW measurements. Channel 1 will never be used.

When you feed the channel 2 signal to your receiver (after the soundcard cal is complete), then you can use a splitter at the receiver to feed the left and right CD or AUX input.

brucek


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## jmalto (Jan 26, 2008)

Thanks Bruce, I really can't tell you how much I appreciate the response. I did read the manual several times and how you said to calibrate it is how I did it. I found a post over at HTguide from another user that was preaching to calibrate the mobilepre the above method I posted so it confused me.

I was also confused about splitting the signal and if it was really necessary or not. Is there any benefit to splitting it or not? 

Thank you again!


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## brucek (Apr 11, 2006)

> Is there any benefit to splitting it or not?


It allows you to check both mains speakers at once.

brucek


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## jmalto (Jan 26, 2008)

brucek said:


> It allows you to check both mains speakers at once.
> 
> brucek


Ah, makes sense. So to check just the center channel I would just send a mono signal non-split to my receiver and turn off the mains correct?


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## brucek (Apr 11, 2006)

> So to check just the center channel


No, you can't check the center channel or any other speakers than the mains and sub. The REW signal is mono, and so, the soundfields to direct signals to other than the mains or sub will not do their job.

If you want to check other speakers, then you need to disconnect a main and substitute the center speaker for example.


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## jmalto (Jan 26, 2008)

Bruce: I got a good calibration file tonight (slight dip in the upper range but acceptable) and the output is -8db with the right speaker calibrated right at -8 db as well. When I create the cal file it says I have about 5db in the headroom. Everything is in the acceptable range according to the REW guide and I calibrated my MIC to my SPL meter. When I run a measurement it is telling me there is a low signal level detected -60 or -59db. I am also getting the same when I go to "check levels." I am running my Onkyo 805 receiver at -20 DB (0 is reference) and I am scared to go much louder because it seems pretty loud when running the sweep.

I am running a 1/4" stereo plug at the Mobilepre line-out that I calibrated to red/white rca ends and have it going into a stereo y-cable at the receiver to split the signal.

Can you offer some guidance as to what the issue could be? I can take some pictures if needed.

Thanks.


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## brucek (Apr 11, 2006)

> it is telling me there is a low signal level detected -60 or -59db. I am also getting the same when I go to "check levels


Well, we know the line-out and line-in and level adjustments work fine, since the soundcard cal is good. You removed the loopback and hopefully are using that channel for the hookup from the mic and to the receiver.
We know that the line-out to the receiver is fine, since you hear the signal.
So, all that's left is the SPL meter and the cable from the meter to the line-in. Check the meter by plugging it into the receivers CD in, and talk into it - it's a microphone.



> I am running my Onkyo 805 receiver at -20 DB (0 is reference) and I am scared to go much louder because it seems pretty loud when running the sweep.


Well, the level you are suppose to set, is 75dBSPL at the listening position, using the 80dB dial position on the RS meter (if that's the type you have). If you're checking the subwoofer, disconnect the mains, and be sure to use the Subwoofer test signal. 75dB is not loud...

brucek


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## jmalto (Jan 26, 2008)

Almost there bruce. I had the gain knob on the mobilepre set to around 7:00 and that was the problem. If I bump it up to around the 12:00 position the levels check out to be ok around -12db (I assume this is acceptable) with a headroom of around 1.1db. 

I have 2 more questions and I can stop bugging you 

1.) I do have a Radio shack meter and I have it setup on my tripod and I put it on 80 and upped my receiver's volume until it read 80db on the RS meter. I had my ECM8000 directly beside the RS meter and dialed that into the REW. Does that mean from now on when I do my measurements I should use -21.5db on my receiver which is where the RS meter registered 80db?

2.) In order to get an acceptable calibration file I have to keep the gain knob on the mobilepre set on channel 2 (what I am using for my line-in, line-out, and MIC) around the 7:00-8:00 position or I get clipping. In order to get a valid sweep measurement I have to up the gain on the mobilepre to around the 12:00 position or it tells me my levels are way too low. When I up the gain knob during the calibration portion it changes the right channel's volume meter. Is it going to interfere with the calibration in any way if I up the gain knob from what I did the calibration with? It would seem you would have to use all the exact settings for measurements sweeps with how you created the initial calibration.

When I go to my windows/mobilepre settings there is no way to change just the MIC strength, it is grayed out in my windows control panel which I assume is because I am not using a standard microphone that goes into a 1/8" jack. I can up the output and input but they are set on max already.

Can't thank you enough for the help. I think I am making this out harder then it needs to be but I just want to make sure before I start testing.

-Jonathan


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## brucek (Apr 11, 2006)

> 1.) I do have a Radio shack meter and I have it setup on my tripod and I put it on 80 and upped my receiver's volume until it read 80db on the RS meter. I had my ECM8000 directly beside the RS meter and dialed that into the REW. Does that mean from now on when I do my measurements I should use -21.5db on my receiver which is where the RS meter registered 80db?


No, every time you use REW, you have to go through the Check Levels routine, etc, and set the level at the ECM8000 mic area with the RS meter. It only takes a second.

BTW, set the level to 75dBSPL at the listening position - not 80dBSPL.



> Is that going to interfere with the calibration in any way since it was calibrated for the 7:00 position on the gain knob or does it not matter?


Matters not. The soundcard calibration routine was carried out to get the saved soundcard cal file. Once that's saved, that's it. Now all the levels are set with the mic plugged into the mic-in, with the line-out to the receiver, and with the levels set using the Check Levels routine.



> When I go to my windows/mobilepre settings there is no way to change just the MIC strength, it is grayed out in my windows control panel


Yeah, it looks like the Mobile pre doesn't have a mic level adjust. It simply has a single level knob that controls the digital output level to the PC. That should suffice. As long as REW see a decent input level, you're fine.

brucek


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## jmalto (Jan 26, 2008)

Perfect, I am good to go now  I think these questions may help others that bought the mobilepre because they seem common across other forums and here time to time. 

Graphs to follow tomorrow and you da man Bruce!


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## brucek (Apr 11, 2006)

Good.....

Oh yeah, in one of my posts above, I didn't remember that you had an ECM and so I said, '_So, all that's left is the SPL meter and the cable from the meter to the line-in. Check the meter by plugging it into the receivers CD in, and talk into it - it's a microphone._. 

Of course, that doesn't apply here because you're using an ECM. The ECM plugs into the mic-in and the SPL meter is only used to set a rough 75dB near the ECM when setting up the Check Levels.... but you knew that. 

brucek


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## jmalto (Jan 26, 2008)

Alright Bruce.. After testing today I have another question :coocoo:

So I have 4 home theater chairs, 2 on a back platform riser and 2 in the front on carpet but they are all centered in spaced accordingly. My question is in order to get readings from all 4 chairs I have to adjust the gain on the mobilepre's channel 2 input (what control the mic volume) and the headroom is different each time I measure. If I do not adjust the gain then it either says the db is too low or the signal is clipping.

My question is isn't that going to cause inaccurate results DB/SPL wise? If the gain is not consistent across the board for each seat then how do you know which seat may be in a null without testing influnces? It would seem to me by calibrating each seat to 75db inside REW from my RS meter reading that would solve this problem but apparently it does not. If I change the sweep level from -8db to -12db it will still complain about clipping at times so I wasn't sure if I should go much lower/higher in the given circumstance.

Here is the graph I came up with for all 4 chairs with 1/3 smoothing enabled (not sure if I needed to enable this or not because the graph was extremely hard to read without it.) Something doesn't seem right to me because I know for a fact the red line has a huge null under 50hz based on you can barely feel the bass in that seat.


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## brucek (Apr 11, 2006)

> If I do not adjust the gain then it either says the db is too low or the signal is clipping.


Yep, very time you move the mic, you'll need to run Check Levels routine, especially if it clips the signal.. It only takes a second.

The objective is to take a reading, and get a relative response plot. If it happens to be at 72dB or 79dB, it matters not. That doesn't change the response for that position. If there's a null, then it will show a null.

If you want to compare responses against another for 'absolute' level, then you would need to be as accurate as possible in the Check Levels routine to set it to 75dBSPL. Even then, realize if the new position you move to has a huge peak within the bandwidth of the pink noise test signal, then the 75dB will be different. The best method of checking different positions is to concentrate on the relative response rather than the absolute level.

Take your multiple readings and examine them on the All Measured tab. 

Your biggest problem is that you are using a poor scale. Use the standard of vertical = 45dB-105dB and a horizontal of 15Hz-whatever you want.

Then pick a spot on the various measured lines and adjust all the lines to overlay at that frequency (i.e. 1000Hz), using the Trace Offset feature. You have to select each line and adjust the Trace to overlay them. Now you can see where the nulls and peaks are in relation to other lines.

brucek


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## jmalto (Jan 26, 2008)

Bruce: When you say "run check levels routine" you mean just to verify the signal is not too weak or too strong to clip? I wouldn't be changing any actual settings within the application other then the gain on my mobilepre correct?

Does it matter what the headroom is for each seat (is there a certain DB +- I should try to stay within for each seating position?)

I will be ready to write a "Room EQ Wizard Setup for Complete Morons" article pretty soon


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## brucek (Apr 11, 2006)

> When you say "run check levels routine" you mean just to verify the signal is not too weak or too strong to clip?


Yes, whenever you move the mic, the level that it receives can change, and so the REW input level would be different (higher or lower). It only takes a second to run the Check level routine and the Calibrate routine to ensure it's still all OK.



> Does it matter what the headroom is for each seat


Absolutely not. It only matters if it clips, or it's too low. So, if it is, you have to run the Check Level routine and Calibrate to 75dB.

Again, the concern is that the *relative* level is fine, not the *absolute* level.

brucek


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## jmalto (Jan 26, 2008)

Man, every time I think I have this figured out something new pops up. I did exactly what you said, read the help file in the little box explaining to calibrate levels and I put my Onkyo receiver on -10db (0 is reference) and I used the sweep level DB to calibrate the signal to 75db on my RS meter which ends up being -22db on the sweep adjustment. I calibrated my mic so it is -24db (a value recommended in the help file) and everything looks great the output number on top is -9.7 with the bottom -22 and the right channel is between -13 and -11 on top and -23 on the bottom. The LEFT channel is measuring -68 on top and -81 on bottom (is this normal since I have the signal split so I can measure both my mains, or should it be the same output as my right channel?) 

I calibrated my spl using the meter to 75db and run a measurement and it runs out of headroom after about 2 seconds into the signal. I went back and adjusted all the values to meet what the help file said with the MIC and there is no way to stay within the set parameters it wants me to in order to get a valid sweep measurement (I have to turn the gain knob down on my mobilepre almost all the way) 

When I go to Make a Measurement>Check Levels it comes back Level ok -16.9DB and then it runs the sweep and goes 0.0 out of headroom. 

What am I doing wrong Bruce


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## jmalto (Jan 26, 2008)

Ah, I think I may of figured it out. I was calibrating the MIC based on the number on the bottom, not the top.. I think that is the problem.

Question still stands on the right channel, should it be registering louder during the check levels or no?


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## brucek (Apr 11, 2006)

> the little box explaining to calibrate levels and I put my Onkyo receiver on -10db (0 is reference) and I used the sweep level DB to calibrate the signal to 75db on my RS meter which ends up being -22db on the sweep adjustment.


No, it doesn't really say that. It says to set the sweep level at -12dB and adjust your receivers volume control to set the level to 75dBSPL at the listening position. 

The sweep level is not touched unless by some strange situation that you couldn't turn your receiver volume control up high enough to get a 75dBSPL reading - would never occur. 

Since you're measuring full range, and not just a sub, be sure to use the Mains speaker test signal and not the Sub signal.

Once the 75dBSPL is reached, adjust (in your case), the level knob on the soundcard to be about -12dB to -18dB on the input VU meter.

Then run the Calibrate routine.

Measure with a sweep from 0-20KHz (if you want full range).

brucek


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## jmalto (Jan 26, 2008)

brucek said:


> No, it doesn't really say that. It says to set the sweep level at -12dB and adjust your receivers volume control to set the level to 75dBSPL at the listening position.
> 
> The sweep level is not touched unless by some strange situation that you couldn't turn your receiver volume control up high enough to get a 75dBSPL reading - would never occur.
> 
> ...


:hissyfit:

Ok, sweep level -12db I used my onkyo to get it to 75db no problem, that is not an issue at all. I am definitely using the main speaker test signal and I am watching the VU Meter for the Right Channel. The top number is registering -12db and dipping down into the 13's at time. No problem there, everything SHOULD be good based on what you are telling me and the help file. I run the "check levels" it tells me it is high -10.8db. I dropped it down to -18DB on the right channel same thing, it tells me the signal is too high. I set it to -23DB and it comes back registering Level OK -16.2DB in green. I click start measure and it pegs the meter 0.0 headroom in red saying it clipped. 

The only thing I can think of is maybe my radioshack meter is messed up and giving me an inaccurate reading to 75DB which is causing this problem. I don't get why REW is telling me the signal is ok to measure at -16.2DB and then runs out of headroom on the MIC when it tries to measure. Should the MIC calibration file be loaded during all of this configuring of the levels or no? I have the soundcard cal and the mic cal both running when I am setting the levels. 

Start FREQ HZ is 0 and End Freq is 20,000.


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## brucek (Apr 11, 2006)

The answer to your problem is rather long winded.

The short answer is that you're trying to measure your entire response from the sub all the way to the top 20KHz, and your sub has a large peak that throws off the levels.

The long answer is that you have to keep in mind that the _Check Levels_ routine in the Measurement screen and the _Check Levels_ routine in the Settings screen use different values to generate the pink noise level setup test signal.

The Check Levels routine in the the Settings screen uses the fact that you have the _*Check/Set levels with Mains*_ pull-down selected to generate its test pink noise for setting the levels - (this a band limited signal with a low and high cut of 500Hz - 2000Hz) - but then you are measuring full range from 0 up to 20000Hz in the Measurements screen, and the sub has a large peak that creates a large signal clip. 

REW double checks and verifies that the _*end frequency*_ of the sweep that you have selected in the Measurement panel roughly matches the test signal setting (for sanity). Its Check Level routine uses the end frequency as the test pink noise hi-limit cutoff. So if you had 20KHz as the end frequency to measure for your sweep, you can see how the pink noise would be different (between the two Check Levels) and as such the different level signal you're experiencing.

The way around it is to test the mains using a measurement sweep from 200Hz-20000Hz, and test the sub with a measurement sweep of 0Hz-200Hz.

If your response was a bit smoother without the subs peak, then this problem wouldn't occur.

brucek


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## jmalto (Jan 26, 2008)

Thank you again Bruce, your knowledge is amazing in this area.

I actually did give it some thought and could tell that the subs may be the issue because every time it spiked I could hear my projector screen rattle from the subwoofers and the headroom would peak. 

I will measure them individually going forward until I can get the EQ a little more under control. I will give it a shot and let you know how it goes.

Thanks.


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## jmalto (Jan 26, 2008)

Bruce - I have the calibration and level input issues all resolved finally, thanks for the help there. I was wondering if you could give me some advice on how to measure my room? I have 3 MFW-15 subwoofers (2 in the back corners and one by my right main) and 4 individual seating positions. What would be the best way to tackle this room? I was thinking of individually measuring the mains in each seating position and comparing the graphs and then individually measure each sub for each seating position, but that is a lot of measurements and comparing. It seems what may be good for one chair might not be good for the other and vice versa since I know for a fact there is a bad null in the front-right chair. 

Any tips or advice is appreciated on how to accurately start. I am thinking of just putting the mic in between each set of seats and measure the mains and each subwoofer, but that is still a lot of measuring.


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## brucek (Apr 11, 2006)

> Any tips or advice is appreciated on how to accurately start.


Three subs is a tough job. I can't really comment until you tell us what you propose to do to obtain your best response. Positioning or EQ or both?

brucek


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## jmalto (Jan 26, 2008)

Bruce - Both if possible, but EQ is my main concern right now. I am having an issue with my center channel where you have to strain a bit to hear vocals (everything is level matched perfectly) and I did not have this problem until I bought the home theater chairs and moved my speakers around. I also have a bad bass null in my front right seat while my back seats are being pounded with bass (mainly due to the subs being right beside the chairs in the back corners.)

I would assume positioning would be the first hurdle to tackle and then EQ?


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## brucek (Apr 11, 2006)

OK, you may not like my idea, but I would at least try this for fun.

Start by measuring just one of the three subs. Find the best spot in the room that is the best compromise for your four seating positions. I would use the RTA for this, as it might quicken the process. (I can tell you how to use the RTA if necessary). I suppose the easiest way to test would be to sit the sub in chair #1 and walk around with the mic while the RTA is running to find the best spot. Then move the sub to chair #2, and so on until you feel you have the best overall location. Then I would stack those three subs in that one spot and equalize using the Average feature of REW.

Then I would proceed to the mains by themselves and find the best positioning for them for all the seating positions. You won't have as much leeway with the mains, because they basically have to be at the front of the room in a typical position, but you can still make a fair bit of change in the response by moving them a bit and towing them.

Then measure the sub and mains together and adjust the phase of the sub(s) in unison to get the best crossover region.

brucek


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## jmalto (Jan 26, 2008)

I like your idea and it makes sense, but I if I stack all 3 subs on top of one another that will limit where I can place them because of my projector. I originally did the "crawl" test for the front row seats and found the back-right and back-left corners of the room were the strongest overall DB bass so I put one in each corner, but the back seats next to them are often overwhelmed with bass and it is very easy to localize (Thus what started my REW adventure.)

I have TrueRTA and have a flat calibration file (I plugged the 1/4 connector into the Line-In and Line-Out and followed the volume instructions.) I didn't ever calibrate it with my SPL meter though so I will need to do that. When you calibrate with a RS meter should I be doing 80DB or 75DB? I was using 75DB before since I was trying to get a frequency response of my Mains with my Subs and 75DB is how I level calibrated them using pink noise.

I have some room treatments on the sidewalls and the backwall behind the primary listening positions, but using the "mirror trick" after re-arranging my room I found a couple more spots that are reflecting that I will need to treat. 

What would classify the "best spot" for the main speakers, an overall flatter frequency response?

Thanks again Bruce!


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## brucek (Apr 11, 2006)

> When you calibrate with a RS meter should I be doing 80DB or 75DB?


Set the listening position to 75dB. Be sure to use the subwoofer test signal for the sub and also for the sub + mains when measuring from 0-200Hz.



> but I if I stack all 3 subs on top of one another that will limit where I can place them because of my projector.


Side by side, or two side by side and one on top. The goal is to create a single sub with enormous headroom in the best position in the room. Then you equalize them as one sub.



> What would classify the "best spot" for the main speakers, an overall flatter frequency response?


Yep. You should try and have then out from the front wall and away from the side walls.

brucek


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## jmalto (Jan 26, 2008)

Ok, I just about have what I need to start Bruce. What test tone or song do you recommend to test the subs with in TrueRTA while I am trying to find the best spot?

Wouldn't having the subs in multiple locations help eliminate nulls? That was my original vision when purchasing 3, I was trying to remove the null in the front row which adding a third did help tremendously. I am hoping if I stack them it does not re-create the null.


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## brucek (Apr 11, 2006)

> What test tone or song do you recommend to test the subs with in TrueRTA while I am trying to find the best spot?


TrueRTA? What's that.

Use REW in RTA mode with it's own pink periodic test signal. I can give you all the settings if you need it.



> Wouldn't having the subs in multiple locations help eliminate nulls?


If you're able to pull it off for the entire listening area, then that would be great. Let us know how it goes. 

brucek


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## jmalto (Jan 26, 2008)

brucek said:


> TrueRTA? What's that.
> 
> Use REW in RTA mode with it's own pink periodic test signal. I can give you all the settings if you need it.
> 
> ...


Doh, I thought you were talking about the program TrueRTA, sorry 

Yeah I will need some steps for REW and what I need to look out for if you have a few minutes that would be great. :hail:


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## brucek (Apr 11, 2006)

To use the RTA feature of REW, you must first setup the levels as if you were about to make a standard response measurement. 

This involves running the Check Levels routine for the mic at the listening position to 75dB and the Calibrate routine. You can even run a measure to have a reference.

Then, select the Spectrum tab in REW and setup the controls as shown in the picture below.

Then, select the REW Generator and select Pink PN and start the signal using the play button.

Press the Red record button on the Spectrum page.

You can monitor your Input Level on the Spectrum page in the bottom left hand corner. 

If it reads clip at any time (because you're moving the mic around when using the RTA), then simply turn down the input level a bit.

Below shows a response measure I took (that gets transferred to the RTA graph), and the black line is the RTA itself. Now I'm ready to move the mic around and watch the black line change.

*RTA feature of REW*









brucek


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## jmalto (Jan 26, 2008)

Excellent write-up Bruce, I will give this a shot this afternoon if I get a chance and report back the results I find.


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