Calibration – Cant get a Flat response ? CM-140 & DSP1124 & REW

[tapeshc]

Please help me figure what am I doing wrong. See this is what I am doing.

1. I take my first measurement, as expected comes way off.
2. Go though find peaks, assign filter and save filters to DSP1124.
3. Do another measurement, much better.
4. Do the step 2 again to iron out left over peaks. Save new filters to DSP1124.
4. Do another measurement, now its worse then 2nd measurement.

This cycle never ends.

My guess is, after applying filters 1st time, very few corrections are needed but when those corrections are written to DSP(based on 2nd reading), they over write the corrections(from 1st reading) on which these values were based so, this cycle never ends.

Any suggestions ? I am sure I am missing a step or two here.

I am using Galaxy 140 that I got recently from Sonnie.

I did go through all the steps of calibrating laptop sound card, checked levels, checked if I get flat response from laptop loopback . Loaded Galaxy’s calibration file and unchecked C-Weighted box. Also made sure those filters are being written to DSP1124(I have the new EPROM).

Please help, I am going crazy.

Thanks

Tapesh

[Otto]

I think I see what you're saying in your guess...

If you are doing a second measurement, and then allowing REW to find and apply filters, there can be trouble if you are already have filters engaged in the BFD. REW won't know what filters are on the BFD, so its newly determined filters (from your second sweep) will be off.

You can continue to sweep and tweak filters. I usually do it by hand, though. I tweak the filter values that were determined in the first sweep, reload them to the BFD, tweak again, etc.

[Sonnie]

I'm with Otto on how to tweak.... if you tweak it the first time around, you probably won't have to tweak again. Generally my predicted response is dead on.

Let REW determine the filters... then let REW optimize those filters... then start tinkering with it yourself, before loading it to your BFD. Make sure you are measuring only your sub without mains and surrounds.

[tapeshc]

Thanks for quick responses. I was just thinking it will be just measure, save and have fun but nothing is that easy.

I have to first understand and get a feel of how changes affect the curve.

Do we have some guide lines on how to tweak a curve manualy ?

Thanks again.

Tapesh

[Otto]

I don't recall any instructions on how to tweak manually. There may be some in the REW and BFD guides...

Really, there are only three parameters per filter to adjust: frequency, bandwidth and gain. When possible, I like to re-use existing filters that are nearby any peaks (or dips) that I want to adjust. If those don't work, I will dispatch a new filter to hit my target area. Usually I take a measurement, make a change and see what happened (and I maintain a way of going back). I generally only change one thing at a time, so that I know how it affected the overall response; too many changes and you'll lose track of the big picture.

After making a change, I remeasure and reassess. Did it cut too much? Reduce gain. Did it cut too wide? Reduce bandwidth. It's pretty easy to hit the right frequency, so there's usually not much problem there. Still, you might play with sliding a narrow-bandwitch filter around in problem spots.

Overall, it can take a while once I get going, if only because I enjoy tweaking with it. Of course, you can use REW's filter suggestions right out of the box and be pretty close.

Have fun!

[tapeshc]

I just read in another thread that CM-140 should be set to fast not slow, is this correct or it was in some other context.

I had it in C-weighted, slow mode and range was 50-100 db.

In Rew I had C-Weighted unchecked.

Please advice if any of these settings are wrong.

[Sonnie]

Yes... set it to Fast... all else is correct.

[Wayne A. Pflughaupt]

Otto wrote:

I don't recall any instructions on how to tweak manually. There may be some in the REW and BFD guides...

All you need to do is get the filter panel up (forget how to do that).





You can change the parameters as you like for any given filter by clicking the arrow buttons, or entering a value in the field. To activate a filter, change the “Filter Type” from “None” to “PA.”

Wayne

[tapeshc]

Thanks everyone ! I was able do it. but I think a very flat response doesnt sound good. I had the whole curve between 72-74 db, but didnt sound good.

I am reading about house curve, lets see if that can do the trick.

Thanks

Tapesh