Calibrating a HT system to play at Doby reference levels (75dBC)

Discussion in 'Archived Threads 2001-2004' started by Jones_Rush, Jul 11, 2001.

  1. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Dolby reference levels, that was.
    I've talked to a guy who mix and transfer 5.1 DD/Dts DVD soundtracks for hollywood movies, he told me that for HT, Dolby reference levels are 75dBC for mains,center and surround, and 82-85dBC for the LFE (since the LFE channel is recorded at the studio at 10dBC lower than the rest of the channels, so it could use more headroom when you'll turn it 10db higher to compensate).
    The most important thing when calibrating your 5.1 system to play at reference levels (or at least to know how far from reference you are) is to use test tones which their dynamic range is known. VE for example, contains test tones with 75db of dynamic range, which makes it perfect for reference calibration. On the other hand, most of the test tones floating around the web are not published with their dynamic range, so one can not tell how far above/below reference he is. A month ago I downloaded some test tones from a site, calibrated my HT to 75dBC with it, and played a movie, the sound was way too loud (as it should, at reference levels). When I calibrated my HT with the test tones of my DD decoder, the sound in the movies was much less louder, the difference between these two test tones is quite dramatic. and now for the question:
    Does anyone know of a place on the net which contain downloadable test tones with a published dynamic range ?
    [Edited last by Jones_Rush on July 11, 2001 at 07:40 PM]
     
  2. Jack Gilvey

    Jack Gilvey Producer

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  3. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Jack,
    You are right about the fact that our DD 5.1 decoder is set to play the LFE channel at 10db higher than the rest,
    the reason for this is that at the studio the LFE channel is recorded 10db lower than the rest. The audio engineer I've talked to yesterday say that according to his handbook, Dolby recommends setting the SUB channel 7-10dBC higher than the rest of the speakers "Since our hearing is not as sensitive to low frequencies as it is to highs, leaving it at the same volume as the mains would make it sound out of balance".
    According to this engineer, the reference levels for HT are 75dBC for mains+center+surround and 82-85dBC (according to your liking) for the sub. You have to understand that this guy mix and transfer 5.1 DD/Dts soundtracks for living, the information he give can not become more credible. If you want the links to the actual thread:http://forums.consumerreview.com/crf...i96^4@.eeaa026
    The guy's name is Terrence (maybe you know him).
     
  4. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Jack, the article you mentioned pretty much contradict what I've been told by the audio engineer regarding to the sub calibration. The article clearly say not to add 10db to the sub channel over the rest channels. I find it odd that this article doesn't mention the fact that humans are less sensitive to the bottom octaves hence the sub need to be calibrated to a higher level. I find it even more odd that the handbook that Dolby publishes regarding level calibration is not consistent.
    On the other hand, the article say:
    "there are many important differences between SPL (used in HT) and RTA (used in theaters) calibration (which are too complex to discuss here). The close quarters of home theater, not to mention the limitation of the hardware, can make this (85dBC) a very high level at which to watch a two hour movie."
    This pretty much explains why the engineer told me that for HT, Dolby reference is set to 75dBC and not 85dBC.
    [Edited last by Jones_Rush on July 12, 2001 at 05:37 AM]
     
  5. Jack Gilvey

    Jack Gilvey Producer

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    quote: You are right about the fact that our DD 5.1 decoder is set to play the LFE channel at 10db higher than the rest,
    the reason for this is that at the studio the LFE channel is recorded 10db lower than the rest.[/quote]
    Correct, that's why.
    Some people do set the sub level up a few db for more impact. We do have decreased sensitivity down low as the F/M curves show, but people usually do it because it's more fun. [​IMG]
    Nope, I don't know who this "terrence" is or what his credentials are. It's interesting that he refers to "calibrating the LFE", which you can't do on the consumer equipment I've seen. It's always a good idea to try and get confirmation of things you read from posters in AR threads, of course, or any thread, for that matter.
    quote: The article clearly say not to add 10db to the sub channel over the rest channels[/quote]
    Yes, for the reason given in the first quote up top. I'm not aware of Dolby's official position on "goosing" the sub level a bit, but it usually sounds better.
    As far as I know, "reference level" calibration results in peaks of 105db from the main channels and about 120db from the sub channel (when speakers are set to small). I calibrate 10db under that.
    If you have problems with the article, go over to avsforum.com. The author,Brian, regularly posts there, and I'm sure he'd be glad to explain it to you if you address a question to him.
     
  6. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Well, I know this guy (through the net, of course) for 3 years, he is highly regarded by all the regulars at AR, but that doesn't mean that everything he say must always be right.
    At the end, I'll do what sounds best to me, but for now, I just want to find out what Dolby's instructions are.
    I asked him to read this article, it's going to be interesting to see what he will say about it.
    btw, I found out that there is a nice calibration program called "THX Optimode" which come as an extra feature in several DVD movies and contain test signals at -30dBFS, meaning 75dBC. If someone want to use this program efficiently, he should first read this article:http://www.smr-home-theatre.org/optimode/index.html
     
  7. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Here is Terrence response after reading the article:
    "I'll have to admit, I setup all of my systems based on Dolby's guidlines for the production environment. It may not always square with what is written on different web sites. I also incorporate professional and consumer products in my setups, so they may not always produce the same results as the average consumer system does.
    In saying that, I am staring right now at dolby's production handbook for calibrating speakers. It plainly says that the sub channel should be calibrated so that a RTA display reads 90-91db over the 20-120hz range. This is in relationship to the 85db for the mains. This is a measurement from a RTA, with a SPL meter, it is closer to 5-7db over the mains. That is my reasoning for the 7-10db's over the mains. That is a "adjustment range"
    My suggestion to you is experiment with what I advice, and what the article suggests. See which works well for you. I personally follow dolby's guidlines and get excellent results."
    Well, Jack, I am sure going to experiment, but when talking strictly at Dolby's will, unless Terrence is a phony who for the last 3 years just pretended to be a 5.1 audio engineer, how can one argue with the above ?
     
  8. Pete Mazz

    Pete Mazz Supporting Actor

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    If you were to set your sub level 10db higher than your mains, music would sound bass heavy if you have your speakers set to small. It seems something isn't translating well between the guidelines for engineers/theaters vs. home gear.
    Pete
    ------------------
     
  9. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Pete, (unless you are referring to DD 5.1 music)
    This whole thread is only about Dolby Digital calibration, not regular 2 channels stereo (at regular stereo there is no discrete LFE, and the dynamic range is usually much narrower, so the calibration is different).
     
  10. Don O'Brien

    Don O'Brien Stunt Coordinator

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    This is an apples and oranges comparison. He is quoting you the technique to calibrate based on "theater" presentation, while we are calibrating home theater products. Consumer products ideally would be set with all channels producing the same dbSPL. The accuracy of this calibration and then resultant sound is impacted by room modes common in small venues (residential sized rooms), and the accuracy of the radio shack meter that many of use for testing and calibration. Go the dolby web site and look at the instruction for calibration in a professional environment. It will confirm what I have written. The consumer decoders already allow for 10dbSPL more potential output by default. In much the same way that compensations were made for a pair of surround speaker during DPL days that would have potentially produced a 3db gain for the combined surround channels. This is also factored into the equation to allow us to set all the channels to the same level.
    ------------------
     
  11. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Don, the engineer I've quoted knew I was referring to a home setup and not to a theater setup.
    But, you are right and I can't ignore the fact that the "dolby's production handbook for calibrating speakers" he was using showed SPL (?) readings which were taken with a RTA unit, which is used usually in theaters and not home setups. That being said, this guy has worked for years in the audio industry, and it will surprise me if he still can't make the right distinction between HT and theaters.
     
  12. Vince Maskeeper

    Vince Maskeeper Producer

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    This sort of issue comes up, seemingly, every 2 weeks around here. I guess I should just break down and write a FAQ or a series of articles (I've started them multiple times, but I simply don't have time).
    There is a confusion here about what 75db means, about what the difference between Studio and PLayback calibration differences are, and some other general concepts.
    I will try my best to explain where the problem is occurring- and hopefully it will help clear the issue up a bit...
    And, once again, this confusion is because he is calibrating for studio environment. In the home environment- the boost is not something needed to execute manually... The DD decoder in your receiver/preamp will place this needed boost into the LFE signal automatically (provided the LFE pad is in the proper position).
    These dolby guidelines he is sighting are to setup the mixing environment- which does not have the auto boost previded by a decoder (since no DD decoder is involved)- and so the studio environment must be properly calibrated to reflect this.
    Again- this is a really simply concept. The LFE channel needs to be 10db higher than the main channels in output at the same encode level. At the mixing stage- this is done manually (setting up the mixing environment with the LFE 10db over manually)-- and then it must be replicated in the plaback environment (in that case the DD spec receiver/decoder does this boost for you).
    No need to manually boost the level in the home- the decoder does it. Calibrating with VE with all channels to 75 sets all speakers to their proper calibrated playback level. If you choose to goose the lfe a DB or two beyond that- this would be your choice- however it is not really "ref" and is not "necessary".
    So, I will say this one more time for those interested:
    "The information presented by Terrance reflects specifically the DOLBY MIXING ENVIRONMENT and calibration levels needed for setting up the DOLBY MIXING ENVIRONMENT. FOr the home environment, this needed LFE boost is provided AUTOMATICALLY by any and all DOLBY DECODER products- it is part of the spec system.
    I would assume most here in ADVANCED already understand these concepts, so thanks for reading
    Vince
    PS: I don't know if this is really an "advanced topic"; if this thread doesn't end here I might move it to hardware.
     
  13. Julian Data

    Julian Data Second Unit

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    Jones,
    I have contacted Vince to take a peek into this thread but he doesn't want to because it is getting monotonous for him explaining over and over again. I can see his POV.
    OOPS! He replied! [​IMG]
    Anyhow, here's one thread you might want to read.
    Bass Management
    WARNING: this thread is awfully long. It does break down on the recording and such.
    You might want to a visit to the 'archives' section at HTF.
    ------------------
    [Edited last by Julian Data on July 12, 2001 at 09:33 PM]
     
  14. Jack Gilvey

    Jack Gilvey Producer

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  15. Mario_C

    Mario_C Stunt Coordinator

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  16. Jones_Rush

    Jones_Rush Stunt Coordinator

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    Vince, Thank you very much for your extensive explanation.
     
  17. Mario_C

    Mario_C Stunt Coordinator

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  18. Brian Florian

    Brian Florian Stunt Coordinator

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    Ya...ya...ya. OK. I'm the guy who with the gracious assistance of a sound engineer at Dolby Labs put together the aforementioned article on the LFE channel. Writing a piece on that subject which is both accurate and accessible to all levels of readers was a challenge. The fact is, I could have gone on writing forever but it got to a point where we had to say "that's enough" and get the article out there. I've been adding to it for the past 14 months (has it been that long?) since writing it but that's not to say I'm unhappy with it. Not a week goes by I don't get email from someone thanking us for clearing up a very fuzzy topic.
    All that to say that this thread has prompted me to buckle down and create a "directors cut" of the essay. [​IMG] Stay tuned.
    But in the spirit of this "advanced" forum, we can safely (I hope) dig a little deeper right here and now.
    Vince has done an excellent job of explaining how the actual volume you use to calibrate is irrelevant and how what is important is that the relative levels of the 6 channels are correct. He reiterates in bold letters (with my firm support) the truths of home Dolby Digital decoders.
    What I would like to stress (and maybe this will help some people's understanding) is that "LFE level" and "Subwoofer level" are 100% unrelated. Kind of. Sort of.
    We also need to talk about why RTA and SPL calibration are different. I think that might clear up what 'Terrence' is saying. One thing at a time...
    Subwoofer level, in the strictest sense, is the level of bass from a subwoofer, but specifically bass which has been taken from a main channel.
    LFE level is the level of the LFE channel. It could come from a subwoofer, or some of the main speakers.
    There is absolutely no question about what the levels of these two should be in principal. The subwoofer level should be the same as the main channel from which the bass is sourced. The subwoofer therefore is an extension of that channel and together they should yield a uniform, flat response to the listener. Have a look at the response graphs in my essay.
    The LFE channel, regardless of what is actually producing it, should play 10dB above all the other channels. If the subwoofer has been balanced with the main channels, then by definition the LFE data must play 10dB higher than the basic subwoofer level.
    Although stressed by Vince (ad nausea almost), I will say it again: Consumer Dolby Digital decoders of any flavor, during the decode process, raise the LFE data by 10dB!! If the gears are turning in your head, it should already be clear that by balancing a subwoofer to a main channel, LFE data flowing through it will have already been raised by 10dB. This is why "LFE channel" is never part of the set-up routine. Many consumer decoders let you manipulate it a little but 99% of the time you can only reduce it (there are exceptions). This functionality is NOT provided so that you can "calibrate" the LFE channel. It is there so you can protect a less than capable subwoofer from potentially rambunctious bass content in the LFE channel.
    So the ONLY question on anyone's mind should be "how to correctly set the subwoofer level relative to a main channel". The more inquisitive may also want to ask "how do they do it in a mixing facility or movie theater". One at a time.
    Let me spin you around and start with mixing facilities and cinemas. In either case, there is no convenient built in LFE offset. More important, they don't use convenient little tools like Video Essentials and AVIA to setup their gear with a $30 Radio Shack SPL meter. They have expensive tone generators and even more expensive Real Time Analyzers (RTA). At least....they should. Not all do (keep this in mind for later).
    An RTA can be thought of as a whole bunch of SPL meters in one box with each SLP meter ONLY paying attention to a narrow frequency. An SPL meter, such as the RS model, can only tell you the SPL of all frequencies combined. So if you are looking at an RTA of a speaker which is "flat" at 85dB for example, the SPL meter might read 91dB. The RTA is measuring each band at 85dB, the SPL meter is measuring the sum of all those bands at 91dB.
    STOP RIGHT THERE. Do not go home thinking that everything you measure with an SLP meter should be higher than RTA. I will shortly explain why the opposite is true.
    So the guy at the consol feeds wide band noise at -20dB into each channel and balances them with the RTA so that each band reads 85dB. As Vince pointed out more than once, this yields the reference level playback of 105dB peak (from each channel). When it comes to any channels which have been crossed over to the subwoofer, you feed the same wide band pink noise through the channel in question and adjust the subwoofer's level to yield a nice flat response.
    You then feed the same level of pink noise into the LFE channel (but band limited to the range of the LFE channel 20-120Hz). Using the RTA you adjust the level of the LFE channel (NOT the subwoofer level) so that each band on the RTA read 10dB higher than the same band did with a main channel.
    I quote from Dolby's guide to setting up a mixing room: "The LFE channel is calibrated such that each 1/3 octave band between 20 and 120Hz is 10dB higher than the equivalent 1/3 octave bands for any of the full-range speakers."
    But what if you don't have an RTA? It is possible for mixing facilities to still set things up using an SPL meter but the numbers have to be fudged. Remember we said that the RTA gives an SPL reading for each band and an SPL meter reads the entire spectrum? That's a problem. If we measure 85dB on an SPL meter for the main channel, that reading is including energy from 20 to 20,000 Hz. For the LFE, we only have energy which goes from 20 to 120Hz. We therefore do NOT shoot for 95dB LFE level as we do with an RTA. We shoot for 90-91dB. This I believe is what Terrence was talking about but which has been misunderstood.
    Again, I quote from Dolby's guide for setting up a mixing facility: "If an RTA is not available, setting the LFE channel higher (e.g., ~ 90 to 91dBc for the [LFE] channel when the Center channel measures 85dBc), can give an approximate level with an SPL meter. This level varies with the quality of the meter being used."
    Now, the fundamental differences when we move to the home are:
    1) We don't use wide-band pink noise. We use tools like AVIA and Video Essentials which are tailor made for us home users.
    2) We don't use RTA's but that's ok because of #1
    3) We do not calibrate the LFE channel.
    While doing the DVD benchmark is Seattle a month ago, Colin and I took advantage of the fact that we had the very expensive Audio Precision at our disposal and looked at the exact contents of the test noise on both AVIA and Video essentials. What we found was not wide band pink noise. In both cases the noise for reading the main channel SPLs was band limited, rolling off well before 120Hz. In reciprocal, the subwoofer or LFE test noise was rolled off well before 120Hz. This mean two things:
    1) These test are specifically tailored for a simple SPL measurement without any fudge factor at all.
    2) They "work" regardless of your crossover point (within reason).
    AVIA does not use the LFE channel for setting up the subwoofer. All its sub set up noise is on main channels. This is good because as noted, once you set the subwoofer relative to a main, the LFE is correct. Period.
    Video Essentials does put it's sub test noise on the LFE track BUT it is recorded 10dB lower than the noise on all the main channels. When boosted by the consumer Dolby Digital decoder, all 6 channels' noise comes out at the same level.
    AVIA recorded there noise at -20dB. VE at -30dB. Calibrating the former to 85dB and the latter to 75dB is the exact same thing.
    In both cases your goal is to set all channels and the subwoofer noise to the same level.
    Further though, we noticed that VE's sub noise was flat to 20Hz. AVIAs actually rolled off a little. Because the Radio Shack SPL meter rolls off as you get towards 20Hz, it is now our opinion that the subwoofer noise should be set a couple dB, as many as 3dB, lower than the main channel to be absolutely 100% correct.
    Cheers and above all, happy listening!
    ------------------
    Brian Florian
    Editor, Canada
    Secrets of Home Theater and High Fidelity http://www.hometheaterhifi.com
    [Edited last by Brian Florian on July 16, 2001 at 11:27 AM]
     

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