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Author Topic: 5.1 level calibration  (Read 9483 times)

cheflaco

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5.1 level calibration
« on: February 27, 2006, 11:38:40 AM »

Hello,
I need to do 5.1 music mixing for DVD-V's and DVD-A's.
I was planning on calibrating the system to -20dBfs = 85dB SPL. The SMPTE recommends -18dBfs = 83dB SPL for cinema applications (which ends up being the same, right?)
Even though there is no standard reference level for music applications I would like to know what is the community doing for 5.1 calibration on a music mixing studio.

Another question. If I calibrate each monitor to 85dB SPL, wouldn't the sum of all main monitors add up to 97dB SPL (85dB+3dB*4) ? Wouldn't this be a little too "hot"?  

Thank you very much
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bobkatz

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Re: 5.1 level calibration
« Reply #1 on: February 27, 2006, 03:07:15 PM »

cheflaco wrote on Mon, 27 February 2006 11:38

Hello,
I need to do 5.1 music mixing for DVD-V's and DVD-A's.
I was planning on calibrating the system to -20dBfs = 85dB SPL. The SMPTE recommends -18dBfs = 83dB SPL for cinema applications (which ends up being the same, right?)
Even though there is no standard reference level for music applications I would like to know what is the community doing for 5.1 calibration on a music mixing studio.

Another question. If I calibrate each monitor to 85dB SPL, wouldn't the sum of all main monitors add up to 97dB SPL (85dB+3dB*4) ? Wouldn't this be a little too "hot"?  

Thank you very much




The sum depends on whether you use uncorrelated or correlated pink noise. The idea is that the SUM should be hotter than the individual. You then mix to the calibrated monitor gain and the more loudspeakers, since it will sound louder to your ears, your "RMS" levels will be lower on the individual channels. This gives you more headroom and cleaner sound!

As for the calibration, I could write an essay...  

The "new" smpte RP200 calibration is 85 dB SPL with -20 dBFS measured on an "average responding voltmeter" (like a "VU" meter). It sucks, "average responding" is not a real standard, but that's what we have to live with. But at least they specified a measurement bandwidth of 20-20 kHz, but without defining the slope of the skirts.

The previous iteration of that document, also called RP 200 specified 85 dB SPL with -18 dBFS RMS 20-20 kHz measured on an RMS-calibrated meter. Yes, this is the same as 83 dB SPL with -20 dBFS RMS.

Best bang for the buck is to get Tom Holman's test tapes/discs because he also provides a narrow band (500 Hz to 2 kHz) pink noise signal that takes the worry out of whether the bass energy in your room or microphone errors are causing errors in your measurement. If you're mixing for a film to be reproduced in a real theatre and mixing in a large soundstage, then follow the real RP200 as closely as you can.

But if you are mixing 5.1 music for the home in a small room, I recommend you use the evolving K-20 "standard" that requires an RMS meter, for it will be more accurate as there are so many variants on "average". And RMS is more accurate measurement of energy level of a noise source. You could be as much as 2 dB or more off in apparent loudness anyway due to a lot of other contributing factors including size of room, bandwidth of measurement, and distance from loudspeakers. So go for the best "average" you dan.

BK
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fader8

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Re: 5.1 level calibration
« Reply #2 on: February 27, 2006, 10:12:25 PM »

The "Recommendations for Surround Sound Production" generated by Phil Ramone & co. is here:

http://www.grammy.com/Recording_Academy/Producers_and_Engine ers/Guidelines/

it's an interesting, albeit not very helpful read. It lacks enough technical merit in the calibration scheme to void its credibility, IMO.

cheflaco:
    ""If I calibrate each monitor to 85dB SPL, wouldn't the sum of all main monitors add up to 97dB SPL (85dB+3dB*4) ? Wouldn't this be a little too "hot"?""

It would be very close to a 7dB increase in SPL. You can figure it out for yourself too:
10Log(dB1/dB2) gives you the delta, in this case the increase in SPL when instantiating the additional sound source. In the real world if you manage to get above 6dB with all 5 running, you're doing well.

I presently use Mr. Katz method of calibration, thank you Bob for allowing us a credible alternative for music than the RP 200, but I'll switch over to RP 200 if I'm producing a stem for film.

These days I'm a sound designer, but I spent 12 years of my career as an acoustician and being anal is a job requirement/prerquisite in that field. As such, I'm somewhat disappointed that there isn't better deinition with regard to how we translate analog concepts into the digital world, and that the methods for monitor calibration, as offered, lack sufficient hard data to allow for true compatibility. No offense Mr. Katz, you've done a great job. In my opinion however, there are still too many variables in the process.

I mean, let's face it, the telephony world has it all over us in this respect.  One thing that would help is a little clearer definition of the tools. For example:

-Agree on a standard test signal. There are several methods available for generating Pink noise and none really come close to the ideal, but this wouldn't matter if we just specified the method of generation, gaussian distribution, or even just say, "use white noise using "X" method, filtered 10dB/decade. Don't care, just agree on something. I'm inclined to think the signal should be clamped to provide a constant crest factor, say 12 dB or something. That would help alleviate variables in the electrical and acoustic measurement side of things, eg SLM integration times and impulse effects, i.e you wouldn't be stuck having to buy a B&K meter.

-Metering... yea there's a can-o'-worms. Another good argument for a fixed crest factor test signal. Can we even do "True" RMS measurements in the digital domain? The best Agilent True RMS digital meter does an analog conversion first to figure this out. OK if you're just using sinewaves, but any complex waveshape.....

-Are we using the 0dBspl=20 uPascal standard?

-Why measure C-weighted? The measurement should be linear if you want to correctly calibrate the LFE. C is 3dB down at 30Hz, 6dB at 20. A lot of room for error, especially if test signals are inconsistent.

-Microphone incidence correction?

-SLM integration time? (to the standards, not the Radio Shack variety).

-RTA or TDS? Both? Hanning, Hamming?

Maybe I am being too anal. Poor cheflaco just wanted some help and I give him a rant. Ah, there's some single-malt in the cabinet. I'd better have one now. Anyway, follow Mr. Katz procedure and your stuff will more than likely sound great on other peoples systems. So, maybe all the tech stuff really doesn't matter.
Randall



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bobkatz

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Re: 5.1 level calibration
« Reply #3 on: February 28, 2006, 11:40:01 AM »

Fader8 wrote on Mon, 27 February 2006 22:12

The "Recommendations for Surround Sound Production" generated by Phil Ramone & co. is here:

 http://www.grammy.com/Recording_Academy/Producers_and_Engine ers/Guidelines/

it's an interesting, albeit not very helpful read. It lacks enough technical merit in the calibration scheme to void its credibility, IMO.




As you see, once you start specifying things they REALLY have to be specified.

Quote:



I presently use Mr. Katz method of calibration, thank you Bob for allowing us a credible alternative for music than the RP 200, but I'll switch over to RP 200 if I'm producing a stem for film.




You have to, as long as the room is large enough... Otherwise it doesn't translate anyway, so you might as well go with the "K" standard, which is more repeatable.

Quote:



offense Mr. Katz, you've done a great job. In my opinion however, there are still too many variables in the process.




I think that an AES committee on "small room monitor standard calibration" would be very desirable. But by the time it gets out of committee there will already be a de facto standard in place. That's what happened with Dolby and the theatres!

Quote:



-Agree on a standard test signal. There are several methods available for generating Pink noise and none really come close to the ideal, but this wouldn't matter if we just specified the method of generation, gaussian distribution, or even just say, "use white noise using "X" method, filtered 10dB/decade. Don't care, just agree on something. I'm inclined to think the signal should be clamped to provide a constant crest factor, say 12 dB or something. That would help alleviate variables in the electrical and acoustic measurement side of things, eg SLM integration times and impulse effects, i.e you wouldn't be stuck having to buy a B&K meter.




It's not quite as squawky as that. If you agree on a standard measurement method (RMS) and a standard measurement bandwidth (20-20K skirts of????), then the crest factor of the test signal can vary to some degree and not matter. It seems to me...

Quote:



-Metering... yea there's a can-o'-worms. Another good argument for a fixed crest factor test signal. Can we even do "True" RMS measurements in the digital domain? The best Agilent True RMS digital meter does an analog conversion first to figure this out. OK if you're just using sinewaves, but any complex waveshape.....




Absolutely positively you can do true RMS measurements in the digital domain. Ask Ralph Kessler. Ask Jim Johnston. Ask BJ Buchalter.

Quote:



-Are we using the 0dBspl=20 uPascal standard?




I believe so. Are there any SPL meters that differ from that?

Quote:



-Why measure C-weighted? The measurement should be linear if you want to correctly calibrate the LFE. C is 3dB down at 30Hz, 6dB at 20. A lot of room for error, especially if test signals are inconsistent.




I'm more than willing to change the standard test signal to be 500 Hz to 2 kHz pink noise RMS level -20 dBFS. Then you can measure C weighted to your heart's content with no errors.

Quote:



-Microphone incidence correction?





If we amend it to narrow band pink noise then the incidence does not matter anymore as long as the diaphragm is reasonably small.

Quote:



-SLM integration time? (to the standards, not the Radio Shack variety).




Is "C" "Slow" a standard?  Is the SPL meter an RMS meter? Does it matter? I believe so. If the SPL meter uses averaging instead of RMS it will vary in measurement.

Quote:



-RTA or TDS? Both? Hanning, Hamming?




No need if we use the narrow band test signal. Or rather, sure, we could need it, but let's reduce the complexity of the equipment required to do the measurement and increase its availability by the average user. Since most things are digital nowadays, standardized meters (K system!!!!) can be used and there are already 4-6 K-system approved plugins and systems available.

Hope this helps a little bit. The devil is in the details (The definition of a "camel" is that it is a horse designed by a committee).

BK
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fader8

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Re: 5.1 level calibration
« Reply #4 on: February 28, 2006, 07:06:00 PM »

Thanks for sharing Bob, good stuff.

BK: ""I think that an AES committee on "small room monitor standard calibration" would be very desirable. But by the time it gets out of committee there will already be a de facto standard in place. That's what happened with Dolby and the theatres!""

Yes, reminds me of the adoption of 44.1k for CDs. What a monkey wrench that was, and still is.

BK: "" If you agree on a standard measurement method (RMS) and a standard measurement bandwidth (20-20K skirts of????), then the crest factor of the test signal can vary to some degree and not matter. It seems to me...""

That's correct if it's specifically stated as "True RMS" not just "RMS". That can cause confusion with many meters that state RMS and actually only derive it based on calculation assuming a sine wave. Most Beckman handhelds, for example. I think you're fundamentally right, but I've noticed peak to RMS ratios varying considerably between pink noise sources. I'm still "thinking" about this one.

BK: ""Absolutely positively you can do true RMS measurements in the digital domain. Ask Ralph Kessler. Ask Jim Johnston. Ask BJ Buchalter.""

OK Bob, I believe you! I'm just not familiar with the methods used.

-Are we using the 0dBspl=20 uPascal standard?

BK: ""I believe so. Are there any SPL meters that differ from that?""

Well, there used to be. I think when the old GenRad calibrators were used alot. The pistonphones were always 1 Bar calibrators, as opposed to the more affordable 1 Pascal devices, so they referenced some uBar value that I don't recall, but was physically different from 20uPa. So, the point is, it's not necessarily the SLM that references it but what calibrator is used and what it's referencing. I know, confusing. With SPL measurements it's always assumed that the SLM is calibrated on the spot. Period. Cal dates for a SLM are never ref'd but the cal date for the calibrator always is as well as what 0dB reference it uses. (reminds me that I have to send my 4230 out for cal).

BK: ""I'm more than willing to change the standard test signal to be 500 Hz to 2 kHz pink noise RMS level -20 dBFS. Then you can measure C weighted to your heart's content with no errors.""

That might be on the right track, although extending it to 1 decade wouldn't hurt, 500-5000. But then what signal do we use for the LFE? Regarding subs, shouldn't they be aligned to the total spl of the other 5 speakers operating together? Does this make sense?

BK: ""If we amend it to narrow band pink noise then the incidence does not matter anymore as long as the diaphragm is reasonably small.""

True. This is in line with my theory of limiting crest factor to compensate for inaccurate metering. Pink noise can have some frequent wild excursions which can "confuse" some meters, as well as producing varying results from different loudspeakers.

-SLM integration time? (to the standards, not the Radio Shack variety).

BK: ""Is "C" "Slow" a standard?""

Yes, if the SLM manufacturer specifically states that the SLM meets ANSI S1.4 Type 1 or its equivelant IEC standard.

BK: "" Is the SPL meter an RMS meter?""

No.

BK: ""Does it matter? I believe so. If the SPL meter uses averaging instead of RMS it will vary in measurement.""

It doesn't matter if it meets the standard above. It will be the same type/method of averaging. This is where that ugly crest factor thing can sneak in again.

BK: ""No need if we use the narrow band test signal. Or rather, sure, we could need it, but let's reduce the complexity of the equipment required to do the measurement and increase its availability by the average user. Since most things are digital nowadays, standardized meters (K system!!!!) can be used and there are already 4-6 K-system approved plugins and systems available.""

So, can I assume from all of this that the K-System meter in my UAD-1 Precision Limiter is capable of True RMS?

BK: (The definition of a "camel" is that it is a horse designed by a committee).

Yet, it is a most practical beast.

Randall
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bobkatz

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Re: 5.1 level calibration
« Reply #5 on: March 02, 2006, 06:37:12 AM »

Fader8 wrote on Tue, 28 February 2006 19:06

Thanks for sharing Bob, good stuff.





It's easy to get me to shoot from the hip, but get me to find time and patience to sit on an AES committee... the red tape is enormous!

Quote:

  SNIPPING REPEATED STUFF...

BK: "" If you agree on a standard measurement method (RMS) and a standard measurement bandwidth (20-20K skirts of????), then the crest factor of the test signal can vary to some degree and not matter. It seems to me...""

That's correct if it's specifically stated as "True RMS" not just "RMS".




Right!

Quote:



BK: ""I'm more than willing to change the standard test signal to be 500 Hz to 2 kHz pink noise RMS level -20 dBFS. Then you can measure C weighted to your heart's content with no errors.""

That might be on the right track, although extending it to 1 decade wouldn't hurt, 500-5000. But then what signal do we use for the LFE? Regarding subs, shouldn't they be aligned to the total spl of the other 5 speakers operating together? Does this make sense?




I vote for 500-2 K, stays out of the X curve issue. See, we're arguing already, is this beast going to get out of committee  Smile  ? Tom Holman fought 15 years for the first version of RP200, it came this close to getting adopted, then someone from Dolby used their 150 pound gorilla influence and "averaging metering" is what we get.

LFE and bass management should be integrated with an RTA at least or Melissa-style "semi-anechoic" if possible. I think all other methods are big approximations.

Quote:



True. This is in line with my theory of limiting crest factor to compensate for inaccurate metering. Pink noise can have some frequent wild excursions which can "confuse" some meters, as well as producing varying results from different loudspeakers.




Well, you're right, we have to specify crest factor. But what crest factor, and how do you justify the choice? I would think that specifying TRUE RMS detectors for the SPL meter would be easier to decide on than what crest factor the P.N. should have.

Quote:




So, can I assume from all of this that the K-System meter in my UAD-1 Precision Limiter is capable of True RMS?




Not yet! I'm working with them and have not yet seen the latest beta.

BK
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minister

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Re: 5.1 level calibration
« Reply #6 on: March 03, 2006, 08:37:05 PM »

cheflaco wrote on Mon, 27 February 2006 10:38

Another question. If I calibrate each monitor to 85dB SPL, wouldn't the sum of all main monitors add up to 97dB SPL (85dB+3dB*4) ? Wouldn't this be a little too "hot"?
to add to the posts here...

that is the beauty of a calibrated room.  leave the monitor level where it is and : is this sound too loud?  bring it down.  is it too soft?  bring it up.  you use your ears and not the meters!

a film mix is designed to use a wide dynamic range.  85SPL is comfortable, but can be on the edge of comfortable. 105 SPL (FULL SCALE) is LOUD!!  but it is nice to have that headroom for loud scenes, explosions, gunshots, etc.

look at film dialogue levels... the average around -7 to -2 VU or -28 to -28 LEQa...doesn't seem like much. but in a -20=0VU=85 world, it is PLENTY!!

no american dub stage ever calibrated to -18=83.  in europe and a few other places, they use -18.  but in america, it has always been -20=85.
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fader8

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Re: 5.1 level calibration
« Reply #7 on: March 03, 2006, 09:18:33 PM »

BK: ""It's easy to get me to shoot from the hip, but get me to find time and patience to sit on an AES committee... the red tape is enormous!""

Maybe that's why Phil Ramone published his through the Academy!

BK: ""I vote for 500-2 K, stays out of the X curve issue.""

OK, I agree. So there's 2 votes. See? I'm easy.

BK: ""LFE and bass management should be integrated with an RTA at least or Melissa-style "semi-anechoic" if possible. I think all other methods are big approximations.""

That still leaves the question of what test signal is used for the LFE. I'm inclined to think broadband pink noise, all speakers running, 1/3 octave RTA-no weighting. I think that's simple, practical, reasonably accurate and within the realm of what most users could do.

BK: ""Well, you're right, we have to specify crest factor. But what crest factor, and how do you justify the choice?""

There's some precedent. NFPA, while developing the standard for loudspeakers in fire protective signaling systems, (UL1480) established that a band limited pink noise with a 6dB CF approximated speech for the purposes of power testing loudspeakers.  From the work I used to do in designing industrial sound systems for speech intelligibility, I can verify that they were pretty close to the mark. Zero to peak voltage is roughly double the RMS for moderately compressed speech.

So, perhaps measuring a comprehensive sampling of various musical styles-genres might lead to some acceptable average.

BK: ""I would think that specifying TRUE RMS detectors for the SPL meter would be easier to decide on than what crest factor the P.N. should have.""

Hmmm... a new SLM standard. If you think the AES is bad, just try getting anything through the ASA!

RT: So, can I assume from all of this that the K-System meter in my UAD-1 Precision Limiter is capable of True RMS?

BK: ""Not yet! I'm working with them and have not yet seen the latest beta.""

Looking forward to it!

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bobkatz

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Re: 5.1 level calibration
« Reply #8 on: March 03, 2006, 11:05:41 PM »

Fader8 wrote on Fri, 03 March 2006 21:18



BK: ""LFE and bass management should be integrated with an RTA at least or Melissa-style "semi-anechoic" if possible. I think all other methods are big approximations.""

That still leaves the question of what test signal is used for the LFE. I'm inclined to think broadband pink noise, all speakers running, 1/3 octave RTA-no weighting. I think that's simple, practical, reasonably accurate and within the realm of what most users could do.




Or, for those with simple sound level meters, a standardized, filtered test signal. Give it away for free.  Smile

Quote:



BK: ""Well, you're right, we have to specify crest factor. But what crest factor, and how do you justify the choice?""

There's some precedent. NFPA, while developing the standard for loudspeakers in fire protective signaling systems, (UL1480) established that a band limited pink

snip




Hardly matters what you define, it will seem "arbitrary" to some interest or other.
The key is to find a STANDARD that is easily generated and widely accepted. Give away a test signal, I guess, make it easy to download.

Quote:



So, perhaps measuring a comprehensive sampling of various musical styles-genres might lead to some acceptable average.




Sounds very "academic" and ultimately time-wasting to me. Any reasonable crest factor would do if you can make it STICK!

Quote:



Hmmm... a new SLM standard. If you think the AES is bad, just try getting anything through the ASA!




Right.... Well, it seems to me if we can't get the SLMs to use RMS averaging, then what is the point in my insisting on RMS for the generated signal?  Sure, it will be consistent with an RMS meter no matter what the crest factor of the source signal, but if the SLM is a simple average (median), then we have to define crest factor for consistency in SPL measurement.

Well, I guess the main thing I have learned from this discussion is we have to have a standardized pink noise crest factor to get more consistent SPL readings, right?

BK
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seriousfun

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Re: 5.1 level calibration
« Reply #9 on: March 06, 2006, 03:29:44 PM »

The band-limited calibration tones from Blue Sky are reliable.

(Spam Alert) If anyone wants to buy the TMH surround DTRS test tape - our CheckMAX - please contact me. We may be discontinuing this soon.
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bobkatz

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Re: 5.1 level calibration
« Reply #10 on: March 07, 2006, 07:28:34 PM »

seriousfun wrote on Mon, 06 March 2006 15:29

The band-limited calibration tones from Blue Sky are reliable.




Reliable?  Well, they did not specify the method of averaging that was used to determine the level of the pink noise. By measuring with a true RMS meter the pink noise from blue sky is 0.7 to 0.8 dB too low. Perhaps they used a VU meter to measure it? And the crest factor of the pink noise has not been specified. This is a hot topic.

Based on that, I'd suggest Tom Holman's test files which I can certify are accurate or the file you can download from the digido website.

BK
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Re: 5.1 level calibration
« Reply #11 on: March 09, 2006, 02:11:49 AM »

bobkatz wrote on Tue, 07 March 2006 18:28

seriousfun wrote on Mon, 06 March 2006 15:29

The band-limited calibration tones from Blue Sky are reliable.




Reliable?  Well, they did not specify the method of averaging that was used to determine the level of the pink noise. By measuring with a true RMS meter the pink noise from blue sky is 0.7 to 0.8 dB too low. Perhaps they used a VU meter to measure it? And the crest factor of the pink noise has not been specified. This is a hot topic.

Based on that, I'd suggest Tom Holman's test files which I can certify are accurate or the file you can download from the digido website.

BK
also, the BLUESKY noise has a buzz in it!

the TASCAM monitor controller genrated PN is off too.

and the Pro Tools Signal Genny...well...

i use the DOLBY generated pink noise.

PM me if you want it.
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seriousfun

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Re: 5.1 level calibration
« Reply #12 on: March 09, 2006, 01:27:12 PM »

bobkatz wrote on Tue, 07 March 2006 16:28

seriousfun wrote on Mon, 06 March 2006 15:29

The band-limited calibration tones from Blue Sky are reliable.




Reliable?  Well, they did not specify the method of averaging that was used to determine the level of the pink noise. By measuring with a true RMS meter the pink noise from blue sky is 0.7 to 0.8 dB too low. Perhaps they used a VU meter to measure it? And the crest factor of the pink noise has not been specified. This is a hot topic.

Based on that, I'd suggest Tom Holman's test files which I can certify are accurate or the file you can download from the digido website.

BK


Of course, I agree that Tom's tones are the most accurate (and I forgot about your downloadable tones - this? http://www.digido.com/User/Assets/Active/General%20Links/004 97-Pink_min_20_dBFS_RMS_uncor_st_441.WAV). Maybe Blue Sky fixed the files, but in our tests crest factor seemed much like everything else (that's why a pulse-based test tone like MLSSA uses - no crest factor, no room buildup - seems like the beset idea), and I didn't hear a buzz.

Most commercially available devices and tone-sources disagree by that 0.7 dB or so, probably for the RMS/VU issue - conformity and awareness are rare, I agree. That's why we distribute the TMH tones.


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bobkatz

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Re: 5.1 level calibration
« Reply #13 on: March 11, 2006, 06:33:11 PM »

seriousfun wrote on Thu, 09 March 2006 13:27

[


Of course, I agree that Tom's tones are the most accurate (and I forgot about your downloadable tones - this?  http://www.digido.com/User/Assets/Active/General%20Links/004 97-Pink_min_20_dBFS_RMS_uncor_st_441.WAV).





I'm certain my tone agrees with Tom's because we both used very accurate RMS measures. However, my tone is full bandwidth. That's another hot topic  Smile

As for crest factor, if you're going to use a VU, then someone is going to have to specify crest factors, because the RMS has a predictable and portable measure of the actual energy, but the VU does not.

I think if everyone is using Westons in Hollywood and 20 kHz bandlimited noise we won't be more than a dB apart anyway  Smile
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Re: 5.1 level calibration
« Reply #14 on: March 12, 2006, 12:18:30 AM »

Alex's original post mentions multiple formats.  Some formats will apply a +10 dB in-band gain to the sub.  Depending on you mixing process, you must keep this in mind, shouldn't you?
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