Spectrum Labs - different slope than room/subwoofer frequency sweeps - Page 2 - AVS Forum
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post #31 of 41 Old 08-30-2013, 11:28 AM
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Quote:
Originally Posted by desertdome View Post

Except that frequency response graphs produced by REW, Omnimic, ARTA, CLIO, and HOLMImpulse use logarithmic sine sweeps. What is irrelevant then is using Spectrum Lab graphs in a direct comparison to their frequency response graphs.
Great job! Are those taken with a loopback like Mark suggested? Loopback vs direct from file don't really matter, but I'm curious.
Exactly what I'm saying!

When maximum output is measured in program that uses a linear power spectrum (such as Spectrum Lab) is directly compared to a frequency response graph, this is what is represented using the max ouput chart from Skyfall:



When maximum output is measured in a program that uses a logarithmic power spectrum (such as TrueRTA or REW), this is what is represented:



The second chart provides an apples to apples comparison with the measurements at data-bass.com. When you put the subwoofer in a room and experience room gain, then this is possible:



I mentioned before that there are several implications when one converts a linear power spectrum graph so that it can be directly compared to a logarithmic power spectrum graph.
  1. In-room bass output peaks somewhere in the 40-50 Hz region and flattens or rolls off as you go down in frequency
  2. Subwoofers with limiters can still accurately produce most of the recorded bass without having a "variable frequency response" since the logarithmic maximum output graphs match very well with a lot of the subwoofer measurements at data-bass.com - even at higher output levels.
  3. Ported subwoofers are producing more of the original bass than thought which corresponds with most listener impressions at subwoofer GTG's.
  4. Studios are viewing a different representation of the audio than what Spectrum Labs presents

We've reached the point where betting is required. tongue.gif

A single PB 13 will accurately track any movie to 14 Hz at the LP at 0dBRL with no distortion and no compression.

How much is your wager?
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post #32 of 41 Old 08-30-2013, 01:32 PM
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Quote:
Originally Posted by Ricci View Post

Correct...

+1 to Mark's post.

The point DD is trying to make is that since the peak and average traces in SL are linear based while most measurements of the response of subs are typically log sine based, that making direct data comparisons of spectral weight/balance of content in movies captured in SL to response charts of subwoofers will be skewed somewhat if the differences in the way the data is presented are not accounted for. Both are accurate but the presentation is different.

This is where we disagree, simply because that's a flawed assertion. There is no difference in the way data is displayed other than the obvious, that one is a trace and the other is a waterfall. The 2 apps arrive at the same result through different methods, but the results from each can most certainly be directly compared

You don't use SL to measure FR and you don't use REW to make a waterfall of a movie scene. Yeah, I got that without the help of this thread. But, why can't the in-room FR be compared to the SL result? We have folks tilting graphs, etc. and it's gone on to the point where a single PB-13 is gliding right through your movie collection calibrated to reference level at the LP without the need for limiters and no distortion.

DD claims that content peaks at 40 Hz and rolls off as frequency decreases (even though he tilted the graph to errantly claim that the peaks from Hulk are flat from 8-80 Hz).

If that is the case, then I need to put a 3dB/octave HPF in front of my mic and misrepresent the content so that PB-13 owners will feel good?
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Originally Posted by Ricci View Post

In the case of Bosso or anyone else for that matter using SL to capture movie signals direct and then setting up a mic and capturing the reproduction by the system, both are taking place in SL so that is perfectly valid since the comparison is of how close the system matches the electrical signal.

Correct, except the problem claimed with adding my FR into the comparison escapes me.

REW crunches the math fed it by mic'ing subs at the LP when the subs are fed a logarithmic sine sweep. The result is a trace on a graph that can be log or linear in scale. That trace accurately displays my FR at the LP.

SL crunches the math fed it by mic'ing subs at the LP when the subs are fed the summed bass content from all channels of a soundtrack. The result is a waterfall on a graph that can be log or linear in scale. That waterfall accurately displays my subs reproduction of that content at the LP.

Tell me again why the two results cannot be directly compared?

In the meanwhile, send me a PB-13 and I'll add that into the comparison (but, only after DD agrees to a sizable enough wager), cuz it'll be loads of fun.

Once again, here's my REW FR at the LP displayed on a linear scaled graph matched to the SL scene cap and another SL cap with sine tone inputs as noted.



The soundtrack effect is encoded so that each frequency in any given effect has a relative level. SpecLab shows that relationship just as it receives that data from the disc or the mic. That resultant waterfall corresponds exactly to the measured in-room frequency response, whether that FR trace is displayed on a log scale or a linear scale graph. The fact that the REW result is derived from a completely different input from the input used by SL is not relevant to the comparison.
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post #33 of 41 Old 08-30-2013, 03:05 PM - Thread Starter
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Quote:
Originally Posted by bossobass View Post

A single PB 13 will accurately track any movie to 14 Hz at the LP at 0dBRL with no distortion and no compression.
  1. I only showed Skyfall in my previous post. Other movies vary just like Spectrum Labs indicates.
  2. 0dBRL stands for decibel return loss. Probably a typo since I'm pretty sure you mean 0dBFS.
  3. The 2nd chart takes the real maximum output of the PB13 Ultra in 15 Hz tune per data.bass.com and shows how much of the signal in Skyfall one can play when playing to the max capability of the PB13 Ultra. It makes no claims about reference volume or 0dBFS.
  4. The maximum output signal as graphed is not all played at once.
  5. I referenced 17 Hz, not 14 Hz.
  6. 0dBFS doesn't correlate to reference level. Also, the acoustic output of 0dBFS is system specific.
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Tell me again why the two results cannot be directly compared?
You make several references to log or linear scale. You obviously misunderstand the issue here. Log or linear scale make no difference for a comparison.

Representing the data with as linear power spectrum or as a logarithmic power spectrum is the issue. Spectrum Lab is incapable of representing the data as a logarithmic power spectrum.
Quote:
Once again, here's my REW FR at the LP displayed on a linear scaled graph matched to the SL scene cap and another SL cap with sine tone inputs as noted.
Once again, the sine tones you use are linear so obviously they will be the same level on a linear scale system such as Spectrum Lab.

Your example of showing linear sines flat in Spectrum Labs and logarithmic sines flat in REW illustrates my point exactly. wink.gif
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post #34 of 41 Old 08-30-2013, 03:21 PM
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Quote:
Originally Posted by Ricci View Post

In the case of Bosso or anyone else for that matter using SL to capture movie signals direct and then setting up a mic and capturing the reproduction by the system, both are taking place in SL so that is perfectly valid since the comparison is of how close the system matches the electrical signal.

Yes. And since a PB13U (et al.) will not match the full bandwidth electrical (or digital) source signal captured in SL as well as the sealed systems, I wonder what the practicality of any points trying to be made in this thread is exactly.
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post #35 of 41 Old 08-30-2013, 07:42 PM
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Quote:
Originally Posted by desertdome View Post

  1. I only showed Skyfall in my previous post. Other movies vary just like Spectrum Labs indicates.
  2. 0dBRL stands for decibel return loss. Probably a typo since I'm pretty sure you mean 0dBFS.
  3. The 2nd chart takes the real maximum output of the PB13 Ultra in 15 Hz tune per data.bass.com and shows how much of the signal in Skyfall one can play when playing to the max capability of the PB13 Ultra. It makes no claims about reference volume or 0dBFS.
  4. The maximum output signal as graphed is not all played at once.
  5. I referenced 17 Hz, not 14 Hz.
  6. 0dBFS doesn't correlate to reference level. Also, the acoustic output of 0dBFS is system specific.
You make several references to log or linear scale. You obviously misunderstand the issue here. Log or linear scale make no difference for a comparison.

Representing the data with as linear power spectrum or as a logarithmic power spectrum is the issue. Spectrum Lab is incapable of representing the data as a logarithmic power spectrum.
Once again, the sine tones you use are linear so obviously they will be the same level on a linear scale system such as Spectrum Lab.

Your example of showing linear sines flat in Spectrum Labs and logarithmic sines flat in REW illustrates my point exactly. wink.gif

Ported subs can't do what you suggest. I've been there years ago and you have no proof to negate my findings. Sorry, but I find this part of the discussion to be actually a bit insulting.

I mean decibel from reference level. If you aren't talking about reference level, then define what you are talking about. I'm not familiar with the method to John's madness in generating the PH/A graphs, but all of my waterfalls are generated through the subs at reference level and all of my discussions of SL are from that perspective.

The charts you posted are irrelevant.

You wouldn't want SpecLab to represent the input from the SW jack as a logarithmic power spectrum. Well, apparently, you would, but no one else would find that data useful.

I'm not misunderstanding anything. I discovered this years ago when I ran a measurement sine sweep through SL. I went through the same tunnel you're in, but it didn't take me as long to get to the light at the end of it.

Once again, the linear sines are a good example of what a soundtrack is feeding SL. A measurement sine sweep is not. That's why you use a measurement sine sweep with REW to measure the FR and you use movie soundtrack content through the SW jack with SL to create a waterfall graph of scenes from Movies With Bass.

I really don't know what your point is, actually. Can you boil it down to a sentence? It appears that you are claiming that an accurate measurement from SL cannot be directly compared to an accurate measurement from REW because SL waterfalls exaggerate the low end. How exaggeration of any kind can be called accurate escapes me. You can always compare accurate to accurate regardless of the format.
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post #36 of 41 Old 09-03-2013, 10:30 AM
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post #37 of 41 Old 09-03-2013, 12:02 PM - Thread Starter
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Quote:
Originally Posted by bossobass View Post

Can you boil it down to a sentence?

Odd question. I have purposely done so in almost every post:

Post #1: Because Spectrum Labs has a different slope than other measurements you can't directly compare the charts to room measurements or subwoofer measurements such as those on data-bass.com.

Post #6: Both are accurate, but when you compare the charts of one system vs another you need to tilt the graphs by 3 dB for an apples to apples comparison.

Post #7: In order to make an FFT spectrum match a 1/3rd octave spectrum, the graph must be tilted by 3 dB/octave or you are not making a direct comparison!

Post #18: One must change the slope of the Spectrum Lab graph to match the SPL charts.

Post #29: What is irrelevant then is using Spectrum Lab graphs in a direct comparison to their frequency response graphs.

Post #33: Representing the data as a linear power spectrum or as a logarithmic power spectrum is the issue.

Others have also understand what I'm saying and rephrased in a single sentence:

Post #10 (J_Palmer_Cass): Desertdome is talking about the difference observed between an FFT measurement and an RTA measurement when a broadband signal is being used as a signal.

Post #28 (Ricci): The point DD is trying to make is that since the peak and average traces in SL are linear based while most measurements of the response of subs are typically log sine based, that making direct data comparisons of spectral weight/balance of content in movies captured in SL to response charts of subwoofers will be skewed somewhat if the differences in the way the data is presented are not accounted for.
Quote:
Originally Posted by bossobass 
Once again, the linear sines are a good example of what a soundtrack is feeding SL. A measurement sine sweep is not. That's why you use a measurement sine sweep with REW to measure the FR and you use movie soundtrack content through the SW jack with SL to create a waterfall graph of scenes from Movies With Bass.
Yes, they are different. And their visual display of the data cannot be directly compared. This has been explained to you in the past when you tried to overlay the SL graph with an ELC graph. You are acting like both are using the exact same method of graphing the data when they aren't.

Let me rephrase the issue again for the 7th time smile.gif: The digital display of a DVD/Blu-ray in Spectrum Lab does not match the acoustic output when measured with an RTA.
Quote:
Originally Posted by "bossobass 
You wouldn't want SpecLab to represent the input from the SW jack as a logarithmic power spectrum. Well, apparently, you would, but no one else would find that data useful.
It is useful when you want to compare the Spectrum Lab graph to a subwoofer measurement and make a meaningful correlation.

Many do want it represented as a logarithmic power spectrum. Here is a thread by audio engineers discussing how they like the data represented: Spectrum Analyzer Slopes.

RME realizes the issue and their Digicheck software does the following:
Quote:
The 'bands' of a FFT have a constant frequency distance, which results in the higher area in numerous bands, in the lower frequencies in very few bands. Some manufacturers try to work around this problem, but even then a sine will show an un-symmetrical display, and an insufficient separation between bands.

RME's Spectral Analyser performes a true bandpass filter calculation, as usual in all professional devices (hardware). The frequency distance between the filters is not linear, but scaled according to human hearing.
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post #38 of 41 Old 09-04-2013, 11:40 AM
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This is madness.

I'd call out JPC but he still doesn't know how to calibrate and set up SL for movies with bass. Like this thread, he just latches onto some minuscule datum and twists it to ever lovin' death.

I'm calling out Ricci to be more specific as his comments may or may not relate to using SL to analyze an audio file accurately.

This is what SL is used for, and to millihertz accuracy when properly calibrated and set up... analyzing audio files, as in movies on disc. It's absurd to even suggest that the analysis is so inaccurate as to require tilting the graph or whatever the nonsense written in this thread suggests.

It's clear you'll remain stuck where you are, which is fine and dandy, so you go ahead and take all of the waterfalls and peak hold vs average graphs Max, others and I have posted to date and recolor them, tilt them, whatever floats your boat. rolleyes.gif

I'm only posting here so that other members may not actually believe Husker wasted his time, money and effort in upgrading his system and that he is experiencing no difference in reproduction of movie content than he would from a PB-13 Ultra, suggesting that his comments to the contrary are psychoacoustic or psychotic in nature.

First, I want to clarify one point I made because I dislike being incorrectly explained by anyone who is as confused as you seem to be:

I overlaid the ELC curve to show that people who mix sound automatically bump the low end because they have ELC built into their hearing, which is where ELC comes from. The fact that the low end in every recording and every live show (House Curve) since Fred Flintstone has the low end bumped by mix engineers, re-recording engineers and FOH sound men is because that's how it sounds best to humans, not because SL tilts the analysis. My use of that fact has nothing whatsoever to do with pink noise, white noise or "method of graphing".

Like an RTA(nalyzer), SpectrumLab is a tool for analyzing audio and audio files. The difference between the 2 is that the best of RTAs available only work to 10 Hz and are nowhere near the resolution available from SL.

In days of yore, we used TrueRTAs peak hold function to capture scenes from movies. Those caps will be close to Maxmercy's SL peak hold caps, except that Max's graphs reflect far higher resolution than 1/24 octave and extend far lower in frequency.

Here's a very short clip of a scene from WOTW using the REW RTA app set to 1/48 octave. The graph is set to match SLs linear graph @ 5dB per division, both are set to use Hann windowing and the 2 graphs are normalized to exact scale. Since REW has no peak hold function in its RTA app, the scene was shortened and the button hit and this process was repeated and averaged. Either way, we never saw this tilted phenomenon you insist is there.



You said this:
Quote:
That is a Spectrum Lab graph which has a 3 dB/octave rising slope as you go down in frequency vs REW, Omnimic or any other measurement software. This is why some of the current movie graphs at data-bass using actual disc data show a rising tail down below 10 Hz. In the 3-4 octaves below 10 Hz there is an increase of 9-12 dB.

REW says this:
Quote:
REW V5 Help
The Measurement Sweep signal is used by REW when measuring system response. It consists of a logarithmic sweep from the start frequency to the end frequency. The sweep duration is set using the Length control. If the start frequency is below 20Hz the signal begins with a linear sweep from DC to 10Hz, followed by a logarithmic sweep from there to the end frequency. This signal is selected automatically to make sweep measurements.

Again... SpecLab is a tool for analyzing audio and audio files to millihertz resolution. I focus on DC to 120 Hz. Look at any of my SCs from SL and you'll see my systems roll off precisely reflected, not 20dB bumped below 4 Hz or whatever.

We are not analyzing measurement sine sweeps, pink noise, white noise or tone burst noise. We are analyzing audio files in various formats, showing a waterfall, peak hold trace, average trace and amplitude waveform. The results precisely correlate with a REW in-room FR measurement as I've shown many times sin the past.

The energy is encoded on the disc and is reflected in my systems reaction to that data when it is converted to analog and fed to my subs. It is certainly not -12dB lower than SL analysis shows. Bring your ported sub over and I'll prove it.
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post #39 of 41 Old 09-04-2013, 02:57 PM - Thread Starter
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What are the two spectrum analysis techniques?
"There are primarily two common spectrum analysis techniques: Constant Bandwidth (CB) and Constant Percentage Bandwidth Analysis (CPB). CPB Analysis can be implemented with analog or digital signal processing, while CB analysis is usually implemented using the digital FFT (Fast Fourier Transform) technique." (from Brüel & Kjær's technical documentation for hand held analyzers)

What type of analyzer is Spectrum Lab?
Spectrum Lab is a fast Fourier transform (FFT) and therefore is a Constant Bandwidth analyzer (also know as linear power spectrum)
  • "Constant Bandwidth – linear rendering of bandwidth (FFT calculates filters with constant BW)" (from Sound Reinforcement System Design by D. G. Meyer 2011)
  • "The FFT generates frequency domain data that is linearly-spaced as a function of frequency, unlike our hearing, which primarily perceives frequency in a logarithmic manner" (from FFT Fundamentals by Rational Acoustics)

What type of analysis is presented by REW, Omnimic, data-bass.com?
These present the data as Constant Percentage Bandwidth (also known as logarithmic power spectrum or octave analysis)
  • Octave analysis shares some similarities with power-spectral analysis, but in spectral analysis all frequency bands (bins) occupy equal bandwidth. For example, in a 1000-point FFT that covers DC to 10 kHz, each bin occupies 10 Hz. The frequency resolution is linear over the entire range. Because of its log scale, octave analysis can show results over multiple octaves, allowing you to see and compare signal levels over a broad frequency range. (Analyze Signals Octave by Octave by Sam Shearman)

Why do REW, Omnimic and data-bass.com use Constant Percentage Bandwidth?
"Traditionally the frequency analysis of sound, and occasionally vibration, is made using constant percentage bandwidth (CPB) filters. The filters may be 1/3-octave (approximately 21%) or 1/1-octave (approximately 70%) band pass filters. Sometimes CPB analyzers are called ‘Octave’ analyzers for this reason. The constant percentage bandwidth matches human perception of sound, and the filters are well defined by international standards, ensuring consistent results. Constant Percentage Bandwidth frequency analysis is appropriate where the frequency scaling is logarithmic and where octave analysis is specified, such as in acoustic or human vibration measurements" (from B&K technical documentation)

"Octave band analysis has been defined as a standard for acoustic analysis. This type of spectrum is called constant percentage band (CPB) because each frequency band is a constant percentage of its center frequency. In other words, the analysis bands become wider in proportion to their center frequencies. 1/1 and 1/3 octave spectra are the most frequently used formats in acoustical measurements - three 1/3 octave bands encompass one octave" (from SpectraPLUS-SC FFT Analysis software)

How does one get the highest resolution data?
By using an FFT to measure the signal.

If I use an FFT to measure the signal how can I compare the data to something that uses Constant Percentage Bandwidth?
You can convert from Constant Bandwidth to Constant Percentage Bandwidth with many acoustic measuring solutions.

Does Spectrum Lab allow one to change the display of data from Constant Bandwidth to Constant Percentage Bandwidth?
No

Do any FFT programs allow one to change the display of data from Constant Bandwidth to Constant Percentage Bandwidth?
Yes
  • SpectraPLUS-SC - "SpectraPLUS-SC is a digital analyzer and utilizes the FFT algorithm to compute the narrowband (linear) frequency spectrum. This linear frequency spectrum is then apportioned among the octave bands to produce the corresponding octave spectrum."
  • SIA-SmaartLive® measurement platform - "We have shown that the frequency domain spectral data from an FFT is distributed on a constant, linearly-spaced interval in frequency. If we evenly distribute the FFT data onto a graph, we will need to employ a frequency axis that is equally spaced in frequency, that is, there is an equal interval per Hz. In contrast, our hearing perceives musical pitch on (approximately) equal frequency ratios, yielding a logarithmic frequency axis for equal-interval analysis (an equal interval per octave) . . . SmaartLive contains several methods for improving
    the logarithmic-frequency view of FFT data, which vary according to the analysis task at hand."
  • Voxengo GlissEQ, SPAN, and SPAN Plus - allows one to change from linear power to logarithmic power display
  • Blue Cat's FreqAnalystPro -allows one to change from linear power to logarithmic power display
  • RME's Spectral Analyzerr - "RME's Spectral Analyser performes a true bandpass filter calculation, as usual in all professional devices (hardware). The frequency distance between the filters is not linear, but scaled according to human hearing."
  • Meyers Sound's SIM 3 FFT Analyzer - allows one to view the data as linear power or logarithmic power density

Aren't these programs distorting the data?
No

Is there any info on how to convert Constant Bandwidth to Constant Percentage Bandwidth?
There is a document called Versatile FFT Supports Accurate 1/3 Octave Analysis that explains how to convert to 1/3 octave constant percentage bandwidth. However, the previously mentioned programs perform the conversion with a much higher accuracy.
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post #40 of 41 Old 09-04-2013, 05:46 PM
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This quote grabbed me: :What type of analysis is presented by REW, Omnimic, data-bass.com?
These present the data as Constant Percentage Bandwidth (also known as logarithmic power spectrum or octave analysis)
Octave analysis shares some similarities with power-spectral analysis, but in spectral analysis all frequency bands (bins) occupy equal bandwidth. For example, in a 1000-point FFT that covers DC to 10 kHz, each bin occupies 10 Hz. The frequency resolution is linear over the entire range. Because of its log scale, octave analysis can show results over multiple octaves, allowing you to see and compare signal levels over a broad frequency range. (Analyze Signals Octave by Octave by Sam Shearman).

I'd add that log presentation of the data, at least once you get used to it, is more in line with what we hear. I assume the 10 Hz per band concept is just a simplification for purposes of the discussion. Because the jump from 20 Hz to 30 Hz is eight notes on the piano (so one step in the frequency analyzer covers a lot of distinct notes. On the other hand, at 1000 hz(ish) the next note up on the piano is 160 Hz higher (so one note covers 16analyzer steps), and the sound 10 Hz higher is probably well within tuning error for typical pianos or among instruments in any group (if I understand correctly, 1010 Hz would be one cent sharp of 1000 Hz. Used to be people got along fine with magic electronic guitar tuners with a stated 3 cent tolerance . . . . http://www.phy.mtu.edu/~suits/notefreqs.html

I also think that looking at the long term average frequency content of any particular program material doesn't tell you anything about the method being used to analyze and present the long term average. It just is what it is, and like any average may precisely represent exactly zero milliseconds of the real content. Part of the reason pink noise is the test signal of choice is because it more closely reflects the real power contents of real world content. So a downward slope in average levels of frequencies is to be expected, not a result of a testing methodology.
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post #41 of 41 Old 09-05-2013, 07:33 AM
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Quote:
Originally Posted by bossobass View Post


Like an RTA(nalyzer), SpectrumLab is a tool for analyzing audio and audio files. The difference between the 2 is that the best of RTAs available only work to 10 Hz and are nowhere near the resolution available from SL.


That is not the primary difference between and RTA display and a straight FFT display.



Quote:
Originally Posted by bossobass View Post



In days of yore, we used TrueRTAs peak hold function to capture scenes from movies. Those caps will be close to Maxmercy's SL peak hold caps, except that Max's graphs reflect far higher resolution than 1/24 octave and extend far lower in frequency.


Refresh our collective memories. Max recommends the use of what approximate bandwidth for each FFT bin when he used Spectrum Lab? At low frequencies (say octave from 20 to 40 Hz), how much difference is there between the FFT bin bandwidth in SL and a 1/24th octave RTA bin bandwidth in a program like TrueRTA?



Quote:
Originally Posted by bossobass View Post



Here's a very short clip of a scene from WOTW using the REW RTA app set to 1/48 octave. The graph is set to match SLs linear graph @ 5dB per division, both are set to use Hann windowing and the 2 graphs are normalized to exact scale. Since REW has no peak hold function in its RTA app, the scene was shortened and the button hit and this process was repeated and averaged. Either way, we never saw this tilted phenomenon you insist is there.





Just what RTA settings did you use in REW for that comparison?
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