Can ETC display ms and distance on same axis? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 09-28-11, 02:36 PM Thread Starter
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Mike R

Join Date: Jan 2008
Location: SE, Michigan
Posts: 127
Can ETC display ms and distance on same axis?

John;
I have a thread @ avs forum titled Using energy time curve for acoustic analysis: developing a Home Theatre primer .

One thing that has come up upon reading the ETC charts is I need to whip out my calculator and convert ms to distance to trouble shoot "bad" early reflections, http://www.avsforum.com/avs-vb/showt...1#post21012231

It occured to me, why not have both ms and distance on same axis?
So, can REW display ms and distance on the horizontal axis, as that is in the end what we need to correlate to, one above and one below the axis line.

Then, there would be no need to whip out the calculator every time to say "sound from speakers travels 1,125ft in one second, and I have this spike at 7ms I need to hunt down, so that's 1,125/1000*7= 7.875 feet dist traveled", when it can be plotted for you to easily use....
(of course for our cuzins in metric regions display in meters not feet)

If REW can already do that, please advise how to turn that feature on, I could not find it.

example of 1 of my many ETC charts in that ongoing thread
mtbdudex is offline

Old 09-28-11, 04:08 PM
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John

Join Date: Apr 2006
Location: UK
Posts: 6,303
Re: Can ETC display ms and distance on same axis?

You can't show a distance axis instead of (or as well as) a time axis (on my list of things to do though), but if you use the delta/zoom box it will show both time and the equivalent distance. To bring that up press the Ctrl key, hold down the right mouse button and drag.
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Old 09-28-11, 05:32 PM Thread Starter
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Mike R

Join Date: Jan 2008
Location: SE, Michigan
Posts: 127
Re: Can ETC display ms and distance on same axis?

Quote:
JohnM wrote: View Post
You can't show a distance axis instead of (or as well as) a time axis (on my list of things to do though), but if you use the delta/zoom box it will show both time and the equivalent distance. To bring that up press the Ctrl key, hold down the right mouse button and drag.
That works for now, thx for quick response, as I'm in the thick of doing this.
mtbdudex is offline
Old 09-28-11, 06:41 PM
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SAC

Join Date: Dec 2009
Location: CST
Posts: 516
Re: Can ETC display ms and distance on same axis?

Just to help those who may be interested in the same issues...

Just be aware that if you use the display as shown (aside from needing to rather dramatically window the response with regards to both time and gain...), with the arrival of the direct signal set to T=0, the time will not be accurate, meaning you are not obtaining the time of flight (TOF) of the signal.

Not only MUST you employ the hardware propagation delay loopback correction that remains in place for all tests, you must also tell REW about its use and NOT translate the time axis so that T=0 corresponds to the direct signal arrival (as is done in the posted display). Thus, the direct signal will arrive at some time after T=0 that will correspond to the Time of Flight (TOF) of the test stimulus and also the distance {T(ms) x 1.13 ft/ms}. Each indirect energy wavelet TOF thereafter will be directly read with respect to time and the corresponding distance of flight calculated in the same manner determined in the same manner {T(ms) x 1.13 ft/ms}.

Without making those adjustments (which I wish were the default as many do not understand this) we are left with a scenario that exists thusly:

Two men run a race. Given that they both run at a velocity equal to 10 meters a second, the first runner arrives at the precise moment that you start the stopwatch. The other arrives precisely 3.2 milliseconds later.
How far did they run?

Unfortunately, we can't tell!

Simply knowing that one arrives 3.2 ms x1.13 ft/ms = 3.62feet ahead of the other runner does not help you - unless you happen to be establishing delay offsets to bring the two distributed elements into phase alignment at the point of measurement. Handy as it may be for that particular application, it is useless for determining the total time of flight and the corresponding TOF distance. (And without the hardware propagation delay loopback compensation, we also be adding some arbitrary unknown value inherent to the particular platform - and if that is not enough, that value will vary due to Windows reset, but that is another complicating topic in and of itself...)

If you are going to use the time domain measurements, they should be configured and calibrated accurately. So this is the situation you have unless one has taken all the steps to correct for hardware propagation delay issues, and told REW to start the time scale at the time of test signal origination, not arrival by setting: Analysis->Preferences -> Use Loopback as Timing Reference.

Last edited by SAC; 09-29-11 at 02:30 AM.
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