Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 353 - AVS Forum
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post #10561 of 11895 Old 06-24-2014, 05:30 PM
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Originally Posted by JohnPM View Post
You seem to be getting time and frequency signals a bit confused here, perhaps as a result of plots like the waterfall that combine the two. Apologies if what follows is teaching grandma to suck eggs, but just to be clear on terminology...

A waterfall shows the evolution of the frequency content of a signal over time. It is not a time signal, but a plot of frequency content with a second time axis.

The impulse response is a time signal, it has different values at different times, just as a sine wave is a time signal. The impulse response itself is a plot of the time history of what would be recorded at the measurement position if the speaker were capable of emitting an impulse. It is no more meaningful to talk about the t=0 value of an impulse response telling you something about its frequency content than it would be to say the same about the t=0 value of a sine wave (which would always be zero!). They are both simply the value of a time signal at a single moment in time. If the sine wave happened to be at 1 kHz and you passed it through an octave band filter centred at 1 kHz it would emerge pretty much unaltered. If you passed it through an octave band filter centred at 500 Hz its amplitude would be reduced by about 20 dB, which is the approximate attenuation of the 500 Hz octave band filter at 1 kHz. It would still pass through zero every 0.5 ms (at the zero crossing of each half cycle) but the peak positive and negative amplitudes it reached at 0.25 ms, 0.75 ms, 1.25 ms, 1.75 ms etc would be one-tenth as large as those of the original signal. The filtered IR plots are, just as the name says, the result of passing the impulse response through a filter. It remains a time signal, and looking at the effect the filter had on the sample value at a particular time just isn't meaningful.
OK, here is what I have done and where it got me:

Sweep #1 333hz - 666hz (1 octave with 500hz center)
Sweep #2 666hz - 1333hz (1 octave with 1K center)
Sweep #3 1333hz - 2666hz (1 octave with 2K center)
Sweep #4 2666hz - 5333hz (1 octave with 4K center)

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This is the overlayed SPL plots just to show how each sweep is broken down into the frequencies covered.


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Here is each sweep filtered 1/1 as per its center frequency (blue - 4k, green = 2k, orange = 1k, gray = 500hz)

Notice that each one now aligns its peak at t=0? That is, the time deviations have disappeared. Notice how the deviation in an un-normalized environment now are very close in magnitude at t=0? And correspond to there relative magnitude represented in the frequency response graph?

John? Marcus? Can I do it this way? Have I accomplished anything?


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post #10562 of 11895 Old 06-25-2014, 08:25 AM
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Is there somewhere in this thread where it shows how to use loopback to set speaker distances? This was recommended in the Datasat thread but I cannot find instructions. I used the search this thread feature but haven't come across an explanatory post. Thanks!


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post #10563 of 11895 Old 06-25-2014, 08:31 AM
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Is there somewhere in this thread where it shows how to use loopback to set speaker distances? This was recommended in the Datasat thread but I cannot find instructions. I used the search this thread feature but haven't come across an explanatory post. Thanks!
I recommend you pose this question in the HTS REW forum. Personally, I am unfamiliar with the procedure you reference.

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post #10564 of 11895 Old 06-25-2014, 11:26 AM
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Originally Posted by jim19611961 View Post
Have I accomplished anything?
I'm not sure you have. Measurement sweeps start an octave below the range of interest and end an octave above, so the impulse responses reflect a broader frequency span than the range displayed. That aside, I'm not sure how you plan to interpret the resulting plots and what they tell you? Why not make a normal measurement and generate a spectrogram or waterfall with a narrow window and short rise time?

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post #10565 of 11895 Old 06-25-2014, 11:33 AM
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I'm not sure you have. Measurement sweeps start an octave below the range of interest and end an octave above, so the impulse responses reflect a broader frequency span than the range displayed. That aside, I'm not sure how you plan to interpret the resulting plots and what they tell you? Why not make a normal measurement and generate a spectrogram or waterfall with a narrow window and short rise time?
That is 2 octaves, not 1. I thought a filtered 1/1 meant 1 octave of range. But changing my sweeps to a 2 octave range is pretty simple if you think that will yield better data.

I have played and played with these and cant get Spectrograms or Waterfalls to show me what I want in the detail I want.

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I assume you mean something like this. I find this impossible to extract the data I want. It would help things some if a rainbow pattern could be assigned to the slices so each stands out some. But even then, how can I determine, lets say, how many db down 500hz is at 10ms? And then be able to compare it to 2K at 10ms. And so on. This nor a Spectrogram will tell me that precisely. Not even vaguely. And that is the kind of data I am after.

The filtered IR/Envelope displays the data exactly the way i want to see it. The hurdle seems to be if the data is accurate and what the measurement and control panel properties need to be to make it accurate.


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post #10566 of 11895 Old 06-25-2014, 01:04 PM
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Can you post one of your measurement mdat files?

It is the sweep which spans extra octaves, not the filter - e.g. if you ask REW to measure from 500 Hz to 1500 Hz it will generate a sweep that spans from 250 Hz to 3000 Hz to avoid artefacts at the edges of the desired measurement range.

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post #10567 of 11895 Old 06-25-2014, 01:17 PM
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Can you post one of your measurement mdat files?

It is the sweep which spans extra octaves, not the filter - e.g. if you ask REW to measure from 500 Hz to 1500 Hz it will generate a sweep that spans from 250 Hz to 3000 Hz to avoid artefacts at the edges of the desired measurement range.
This site wont let me post mdats. But go here and you can get them:

http://www.gearslutz.com/board/10184220-post386.html


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post #10568 of 11895 Old 06-25-2014, 01:54 PM
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how can I determine, lets say, how many db down 500hz is at 10ms? And then be able to compare it to 2K at 10ms. And so on.
Part of the problem is that this is not possible. It is meaningless to ask "what is the frequency of the sample 10ms after the peak", for example, because a single sample of a time signal can't have a frequency. To try and decide what frequencies are present you have to examine a series of samples over a time interval, and the frequency resolution you then achieve (using FFT approaches) is the reciprocal of the time span you examine - so if you looked at the sequence of samples from 5 ms to 15 ms, a span of 10 ms, you can only determine the frequency content with a resolution of 1/0.01 s = 100 Hz and what you get is effectively the average frequency content over that span.

In any event, here are some views of your data, along with the settings that produced them. The decay plot is probably easiest for what you are trying to achieve.

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You would also need to change the Spectral Decay Right window to Gaussian in Analysis preferences for these plots.

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post #10569 of 11895 Old 06-25-2014, 02:35 PM
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Quote:
Originally Posted by JohnPM View Post
Part of the problem is that this is not possible. It is meaningless to ask "what is the frequency of the sample 10ms after the peak", for example, because a single sample of a time signal can't have a frequency. To try and decide what frequencies are present you have to examine a series of samples over a time interval, and the frequency resolution you then achieve (using FFT approaches) is the reciprocal of the time span you examine - so if you looked at the sequence of samples from 5 ms to 15 ms, a span of 10 ms, you can only determine the frequency content with a resolution of 1/0.01 s = 100 Hz and what you get is effectively the average frequency content over that span.

In any event, here are some views of your data, along with the settings that produced them. The decay plot is probably easiest for what you are trying to achieve.

Attachment 135353

Attachment 135361

Attachment 135369

Attachment 135377

Attachment 135385

Attachment 135393

You would also need to change the Spectral Decay Right window to Gaussian in Analysis preferences for these plots.
I haven't thanked you for your time yet, so thanks for sharing how all this stuff works

Its evident how many assumptions I (we) make that are utterly wrong (made apparent by some of your comments). For instance, when I looked at a filtered ETC/IR @ 500hz and 10ms and the graph says its at -22db, I assume because thats what the data says, that it is right. I dont mean to sound critical, but if what the graph says is wrong, how is it valuable? Or is it the case where one is supposed to be sufficiently versed in what one is looking at to know when your looking at bad data? I think most of us here assume that the data we see is always good data and its up to us to figure out what it means. I think we also assume that acoustic software wouldn't offer features that present bad data in the first place.

All (most) of my acoustic comrades here have been silent so far in our conversation John. But I assume some of them are looking and watching Maybe anyways I hope they are gaining something from our conversation.

John, you have about convinced me to abandon using filtered ETC/IR as a tool for reflection spectral analysis. But before I do, please address specifically what that data means if it doesn't mean what it says as it pertains to the graphical data presented both in magnitude and in time. You have said in different ways repeatedly that how I was interpreting the data isnt accurate or possible to do. So what do those graphs mean then?

As to your illustrations of my mdat data, I do admit you got it to look in ways I hadn't. Helpful ways Appreciated!


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post #10570 of 11895 Old 06-25-2014, 03:06 PM
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John, you have about convinced me to abandon using filtered ETC/IR as a tool for reflection spectral analysis.
We have been observing, Jim, but unfortunately much of the conversation has been beyond my comprehension level. I can understand your statement above, and I can only say, Wow.

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post #10571 of 11895 Old 06-25-2014, 03:22 PM
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when I looked at a filtered ETC/IR @ 500hz and 10ms and the graph says its at -22db, I assume because thats what the data says, that it is right. I dont mean to sound critical, but if what the graph says is wrong, how is it valuable?
The data is right, it is your interpretation of it that is wrong

Applying a filter to a time signal doesn't turn it into a frequency domain signal, it is still a time signal, you are trying to assign frequency domain characteristics to it that it doesn't possess.

The filtered IR plot is primarily about reverberation times, aka RT60 data. An RT60 figure is how long it takes the sound energy to decay by 60 dB, derived from the impulse response. (Partly) because impulse response measurements often don't have 60 dB of dynamic range (they almost never did back when most of the work on RT60 was done) this isn't as simple as just moving along the plot until it drops 60 dB below its peak. The RT60 figures are instead derived by measuring the rate at which the level is decaying over a defined portion of the response (for example, from the point where it is 5 dB below the peak to the point where it is 25 dB below) and then extrapolating that to come up with a figure for the time to drop by 60 dB. The time signal itself jumps back and forth between positive and negative values, so it is hard to see the decay from the time signal directly. Instead, the calculation is done by looking at either the envelope of the time signal or by a process of backward integration of the signal named after Schroeder, who invented the process around 1965.

Typically the RT60 figures are broken down into frequency bands so that treatments can be targeted at those portions of the band where decay time is excessive. Those band figures are obtained by the same process as the overall decay figure, except that the impulse response is passed through an octave or one-third octave filter before the envelope and/or Schroeder integral are calculated. The figures then indicate the average energy decay over the range of frequencies that remain within the filtered impulse response (707 Hz to 1,414 Hz for the octave filter band that is centred at 1 kHz, for example). It's not a hard cutoff of course, filters have slopes outside their pass band rather than cliff edges, but the roll off is fairly fast (3 dB down at 707 Hz and 1,414 kHz, but typically 20 dB down at 500 Hz and 2 kHz for the 1 kHz filter, for example). I repeat that it is still a time signal though, just one with a more limited spectrum.

All that is somewhat irrelevant for what you are trying to do however, which is to examine the variation in frequency content of a signal over time and in particular to see what content it has at the time your engineered reflection arrives. Such time-frequency plots are subject to the uncertainty trade-off I mentioned previously - if you want to be precise about timing you will get very vague data for frequency, being precise about frequency means being vague about time. The spectral decay, waterfall and spectrogram plots are all ways of examining that variation of frequency content over time, with adjustable window widths to provide control over the time-frequency trade-off.

John

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post #10572 of 11895 Old 06-25-2014, 03:33 PM
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@JohnPM is there any value in limiting the bandwidth of the sweep here?

For example say I sweep 500-1000Hz (equates to 250-2000 in practice?) and then 4000-8000 (2000-16000 in practice?) with loopback on. I now have 2 etc's but I no longer have the window trade off to make because one doesn't need to window out anything to see the relevant spectral content. Meanwhile loopback can be trusted to give temporal alignment of the two measurements.

This seems intuitively right (but intuition is often wrong!)
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post #10573 of 11895 Old 06-25-2014, 03:35 PM
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The data is right, it is your interpretation of it that is wrong

Applying a filter to a time signal doesn't turn it into a frequency domain signal, it is still a time signal, you are trying to assign frequency domain characteristics to it that it doesn't possess.

The filtered IR plot is primarily about reverberation times, aka RT60 data. An RT60 figure is how long it takes the sound energy to decay by 60 dB, derived from the impulse response. (Partly) because impulse response measurements often don't have 60 dB of dynamic range (they almost never did back when most of the work on RT60 was done) this isn't as simple as just moving along the plot until it drops 60 dB below its peak. The RT60 figures are instead derived by measuring the rate at which the level is decaying over a defined portion of the response (for example, from the point where it is 5 dB below the peak to the point where it is 25 dB below) and then extrapolating that to come up with a figure for the time to drop by 60 dB. The time signal itself jumps back and forth between positive and negative values, so it is hard to see the decay from the time signal directly. Instead, the calculation is done by looking at either the envelope of the time signal or by a process of backward integration of the signal named after Schroeder, who invented the process around 1965.

Typically the RT60 figures are broken down into frequency bands so that treatments can be targeted at those portions of the band where decay time is excessive. Those band figures are obtained by the same process as the overall decay figure, except that the impulse response is passed through an octave or one-third octave filter before the envelope and/or Schroeder integral are calculated. The figures then indicate the average energy decay over the range of frequencies that remain within the filtered impulse response (707 Hz to 1,414 Hz for the octave filter band that is centred at 1 kHz, for example). It's not a hard cutoff of course, filters have slopes outside their pass band rather than cliff edges, but the roll off is fairly fast (3 dB down at 707 Hz and 1,414 kHz, but typically 20 dB down at 500 Hz and 2 kHz for the 1 kHz filter, for example). I repeat that it is still a time signal though, just one with a more limited spectrum.

All that is somewhat irrelevant for what you are trying to do however, which is to examine the variation in frequency content of a signal over time and in particular to see what content it has at the time your engineered reflection arrives. Such time-frequency plots are subject to the uncertainty trade-off I mentioned previously - if you want to be precise about timing you will get very vague data for frequency, being precise about frequency means being vague about time. The spectral decay, waterfall and spectrogram plots are all ways of examining that variation of frequency content over time, with adjustable window widths to provide control over the time-frequency trade-off.
So my interpretation falicy is trying to pin it down in both time and frequency? Can you define which tools are best at time vs which are best in frequency?

The tradeoff of time vs frequency sounds like quantum mechanics trade off of a particle position and velocity Still, since you didnt mention it, is magnitude/amplitude a variable as well? A trade off as well?


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post #10574 of 11895 Old 06-25-2014, 03:39 PM
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We have been observing, Jim, but unfortunately much of the conversation has been beyond my comprehension level. I can understand your statement above, and I can only say, Wow.
I am not digesting all the detail either. Rather, I am trying to view it conceptually. I am open to changing my view of things when evidence dictates I must. This maybe one of those occasions.


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Applying a filter to a time signal doesn't turn it into a frequency domain signal, it is still a time signal, you are trying to assign frequency domain characteristics to it that it doesn't possess.
But band-limited impulse data does have certain frequency domain characteristics because of being filtered. I'm not saying that other plots won't be more meaningful but filtered ETCs aren't entirely wrong either.

Anyhow, what we're looking for is a method to determine timing, level and spectral distortion of (first) reflections.

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but band-limited impulse data does have certain frequency domain characteristics because of being filtered. I'm not saying that other plots won't be more meaningful but filtered etcs aren't entirely wrong either.

anyhow, what we're looking for is a method to determine timing, level and spectral distortion of (first) reflections.


+1000
(In my case, my first significant reflection is at 25ms. But the spirit of the comment still holds true)


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Originally Posted by jim19611961 View Post
So my interpretation falicy is trying to pin it down in both time and frequency? Can you define which tools are best at time vs which are best in frequency?

The tradeoff of time vs frequency sounds like quantum mechanics trade off of a particle position and velocity Still, since you didnt mention it, is magnitude/amplitude a variable as well? A trade off as well?
There is no "best". With FFT there's a tradeoff between time and frequency resolution. If you "make the window longer" then frequency resolution will be higher but you lose time resolution. If you "make the window short" then you have good time resolution but frequency resolution will be coarse.

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There is no "best". With FFT there's a tradeoff between time and frequency resolution. If you "make the window longer" then frequency resolution will be higher but you lose time resolution. If you "make the window short" then you have good time resolution but frequency resolution will be coarse.
So, if one is interested in fairly high frequencies, 500hz and above, shorter windows make more sense. If your looking at the bass region, longer windows are required, but at the sacrifice of precise timing?


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post #10579 of 11895 Old 06-25-2014, 03:54 PM
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+1000
(In my case, my first significant reflection is at 25ms. But the spirit of the comment still holds true)
in your case, scrolling the window over to that 25ms reflection while maintaining the width of the window should work perfectly then shouldn't it? ie you have already isolated the event you want to analyse so shift the left side of the window out and leave the right side of the window at some long enough period to gain frequency resolution. The problem is if you want to zoom in on arbitrary bits of time while maintaining frequency resolution.
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post #10580 of 11895 Old 06-25-2014, 03:57 PM
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in your case, scrolling the window over to that 25ms reflection while maintaining the width of the window should work perfectly then shouldn't it? ie you have already isolated the event you want to analyse so shift the left side of the window out and leave the right side of the window at some long enough period to gain frequency resolution. The problem is if you want to zoom in on arbitrary bits of time while maintaining frequency resolution.
While my 25ms reflection is a primary interest, it is not my only one. So I need to grasp the big picture to know what I have to do when looking at other areas.

For instance, I go this using John's guidance to show the spectral content of my kicker. I tried to limit to show only those reflection which were -12db or greater (i.e., those great enough to constitute or trigger the Haas effect)

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post #10581 of 11895 Old 06-25-2014, 04:00 PM
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While my 25ms reflection is a primary interest, it is not my only one. So I need to grasp the big picture to know what I have to do when looking at other areas.
what is the "big picture" exactly? rate of decay by frequency or something else?
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post #10582 of 11895 Old 06-25-2014, 04:05 PM
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what is the "big picture" exactly? rate of decay by frequency or something else?
By "big picture" I meant being able to look at any frequency, and know HOW I need to shape all the graph parameters in order to look at it properly, both in the time and spectral domains.


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post #10583 of 11895 Old 06-25-2014, 04:20 PM
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My question is what does "look at it properly" mean? What are you looking for?

Ultimately rew doesn't have many controls beyond the window params so once you grasp those then it is all intepretation and knowing when to apply one view or another.
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post #10584 of 11895 Old 06-25-2014, 04:30 PM
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My question is what does "look at it properly" mean? What are you looking for?

Ultimately rew doesn't have many controls beyond the window params so once you grasp those then it is all intepretation and knowing when to apply one view or another.
If others are like me, I learn best through illustration.

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4ms window

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10ms window, while I gain a lower frequency limit, the time element is starting to smear.

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20ms window, a still lower frequency limit, even more time smearing.


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post #10585 of 11895 Old 06-25-2014, 05:24 PM
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Following intently Jim. Haven't felt that I can contribute significantly yet (and probably only confuse things further), hence remaining quiet. Thanks for getting this line of conversation started.

I like the spectrogram view from John for your kicker, and the decay plot for that matter. I think I can manipulate the waterfall to track these early reflections, but don't have REW available atm to provide examples.
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post #10586 of 11895 Old 06-25-2014, 05:33 PM
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Following intently Jim. Haven't felt that I can contribute significantly yet (and probably only confuse things further), hence remaining quiet. Thanks for getting this line of conversation started.

I like the spectrogram view from John for your kicker, and the decay plot for that matter. I think I can manipulate the waterfall to track these early reflections, but don't have REW available atm to provide examples.
It should be evident that I have been groping in the dark a bit here.

John's illustrations were illuminating, definitely. Just goes to show that all those variables are there for a reason.

I dont know what Jerry is going to do in terms of the guide as we discover that so many different looks/views/control panel settings are needed to see certain data correctly. Definitely complicates matters.


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post #10587 of 11895 Old 06-25-2014, 06:21 PM
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I think I have mentioned previously, let's leave the defaults as they are (the guide) for first time posters in this thread, but when tracking down problems, we need to adjust the graphs to suit us, not stick to some previous guideline, under the impression that it helps. And we obviously have a little way to go, to find some new defaults for tracking early reflections at specific frequencies.

If I'm not mistaken, the full range ETC is exactly like it was previously concerning our interpretation of it. It's only when trying to track down specific frequencies, that we need to look outside the box.

Groping in the dark has been good, knowledge is good

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post #10588 of 11895 Old 06-25-2014, 06:30 PM
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It should be evident that I have been groping in the dark a bit here.

John's illustrations were illuminating, definitely. Just goes to show that all those variables are there for a reason.

I dont know what Jerry is going to do in terms of the guide as we discover that so many different looks/views/control panel settings are needed to see certain data correctly. Definitely complicates matters.
As I said previously, the discussion has been over my head. So it is unrealistic to expect a Guide update from me at this time. If, however, we can take theses new learnings and come up with something that is appropriate for inclusion in a guide update, I would be glad to help. My feeling is that this is pretty advanced theory, and may be out of place in what is intended to be a beginner's guide. HST, if something in the Guide is wrong or misleading, it should be corrected.

I am open for suggestions.

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post #10589 of 11895 Old 06-25-2014, 06:39 PM
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As I said previously, the discussion has been over my head. So it is unrealistic to expect a Guide update from me at this time. If, however, we can take theses new learnings and come up with something that is appropriate for inclusion in a guide update, I would be glad to help. My feeling is that this is pretty advanced theory, and may be out of place in what is intended to be a beginner's guide. HST, if something in the Guide is wrong or misleading, it should be corrected.

I am open for suggestions.
I didn't mean the guide needs to be undated tomorrow. I agree we may need some time to understand Filtered IR, Frequency Limited Sweeps, and the advanced windowing for Decay, Waterfalls and Spectrograms more before we jump into advising anyone. But all these areas have been blown wide open here recently. I guess I inadvertently found Pandora.

In truth, I for one at least, am not sure I understand these things well enough to advise anyone, much less guide them. JohnPM isnt going to hold our hand much longer on this I suspect. I have been playing with John's ideas on my own graphs to see if I can reverse engineer what he provided to us. I think Marcus has a better grasp of these recent issues than I do.


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post #10590 of 11895 Old 06-25-2014, 06:48 PM
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I agree, Jim. I am looking forward to continued discussion--let's see where it takes us.

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