The mysterious, mythical, 'magical' -18dB FS = 0VU calibration ITB

Discussion in 'Mixing and Mastering' started by No Avenger, Feb 12, 2020.

  1. No Avenger

    No Avenger Audiosexual

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    Preface: After some minds may have cooled down, and new questions have arisen, I decided to start this thread.


    Ever since I first heard about this calibration for plugins and figured out what it was all about, I thought it was beginner's stuff (like I were back then), but my online (not only here) and offline experience has surprisingly taught me a different lesson.

    This calibration was originally made to calibrate digital hardware to analogue hardware - for broadcasting purposes.
    It wasn't intended for ITB productions at all.

    I made and edited a nice little screenshot to show you how it works ITB
    [​IMG]
    Although I think the picture is pretty self-explanatory, I will talk you through.

    Since originally a sine wave with a peak of -18dB FS was used for this calibration, hence the name, I have of course also done this.
    And before you start shouting 'no, this is wrong, this is wrong!', please read first and then complain.

    On the right side you see Reaper's Master channel. At the top it shows a peak value of -18dB FS, at the bottom (you may have to zoom in to see it clearly) -21.
    And please, let's not talk about the 0.1dB difference, nothing in life is perfect (you included [​IMG]).

    - VU meter top row: mode Peak, reference level (top left corner) 0 and as you can see, it correctly shows a peak of -18dB, like the Master meter does.

    - VU meter second row: mode RMS, reference level again set to 0, results in a measured RMS of -21dB, same like in Master.

    - VU meter third row: mode VU, reference level the triple-m -18. And surprise, surprise it shows 0VU.

    Now you might ask 'but where's the proof whether it shows 0VU for the peak or for the rms?' Well, a VU meter can't measure peaks, it's just to slow (integration time of 300ms). So the only logical conclusion is, it shows -21dB RMS = 0VU (for this calibration).
    But to be on the safe side I used this setup additionally for a triangle, square and sawtooth wave, a pink and white noise, drums, bass and vocals. And whenever I adjusted the RMS to -21dB the VU meter showed 0VU.
    On top of that I even made this test with two more (free) VU meters, all with the same result (please keep in mind that the more movement the level has, the less precise this alignment becomes).

    Once again, for a calibration of -18, -21dB RMS = 0VU applies.

    Simple as that and end of story.

    I hope this is clear now once and for all (but I'm afraid it's not :winker:).
     
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  3. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    :mates:thx for posting
     
    Last edited: Feb 12, 2020
  4. jhagen

    jhagen Producer

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    ITB calibration is useless, in digital domain 0 is always 0 and 1 is always 1
     
  5. No Avenger

    No Avenger Audiosexual

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    I've never said otherwise. :winker:
     
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  6. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    i thought whole time VU is a analog true peak meter..:deep_facepalm:
    youre peak,rms,vu pictures showed me im such a fool

    dbvu:deep_facepalm::facepalm:it meassure volume units ...what a douche

    edit oh gott..unglaublich
     
    Last edited: Feb 12, 2020
  7. No Avenger

    No Avenger Audiosexual

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    Mh, should I rate this 'like' or 'agree' or... :unsure: [​IMG]
     
  8. KungPaoFist

    KungPaoFist Audiosexual

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    I feel like some knowledge just happened but can anyone explain what this means to the mathematically challenged? Thanks in advance
     
  9. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    i dunno..something like 0dbtp = +3dbvu ?
    ..
    pretty much for me it means i got no use for vu meters itb....you can calibrate for the low end mix trick with it (which better for non electro material imho , so no use either for me) , you can see round bout rms in vu meter but i use rms meter then
     
    Last edited: Feb 12, 2020
  10. No Avenger

    No Avenger Audiosexual

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    Just that a VU meter is an RMS meter and that for the famous -18 calibration 0VU equals -21dB FS RMS, that's all.
     
  11. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    its an volume units meter with it you can also determine rms...isnt that more right ?
     
  12. No Avenger

    No Avenger Audiosexual

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    No, no, VU means Verkehrsunfall, 0 Verkehrsunfall means all is good. :rofl:

    Yes, but I don't think it's helpful in this context (doesn't make things easier).
     
  13. Sinus Well

    Sinus Well Rock Star

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    Okay, I'm gonna do a little recon here.

    What is dB?
    Decibel is a relative logarithmic level unit. "Relative" means it's unrelated. So if someone says something is 6dB loud, then this information is useless because the context is missing - it's just a scale!. 6dB of what? Sound pressure? Volts? Digital Peak?
    Only when dB is put into a context it gets a meaning.
    This is achieved by giving the dB a suffix. If we mean with 6dB the Sound Pressure Level, then we give the decibel the suffix SPL.
    This makes our dB specification absolute! 6dB SPL is therefore an "absolute level" and 6dB is just a "relative level".

    What is Peak?
    A peak is the maximum displacement of a wave. In the Box a distinction is made between "peak" and "true peak". But an analog peak is always a true peak. If the meter ballistics is fast enough - and the meter level is correspondingly high-resolution - then the TP is displayed. Ballistics is no problem in the age of the LED.

    What is RMS?
    RMS is the abbreviation for Root Mean Square. It is a measured average value.
    Since the level of analog equipment is always voltage based - and voltage is always measured in RMS - all analog voltage levels (such as dBu, dBv or VU) are automatically RMS levels. In general, all analog levels are measured in RMS (if not explicitly marked with PEAK), whether they're based on voltage or pressure or whatever.

    What is VU?
    Volume Unit is a relative level. The statement a level has 0VU can only be understood by another person if this person knows what this 0VU refers to. VU meters show an average level with a slow response of 300ms attack & fall time. Why so slow? VU meters were originally developed in the 1930s to standardize the transmission level over telephone lines. as the attack and fall times are similar to human hearing, the balistic characteristics of a VU meter are perfectly suited to assess speech levels

    What is dB VU?
    There's no such thing as dB VU. The unit dB as well as the unit VU are different relative level units, which only become "absolute" by a meaningful suffix.
    While it is sufficient to append the suffix directly to the dB (example: dBv, dBu, dB SPL, dB RMS, dB FS, etc), the entire reference must be appended to the VU.

    Analog Example:
    -7VU (+4dBu).
    This means that the VU meter is currently displaying -7,
    but the VU meter at "0VU" takes the nominal voltage level of +4dBu (1,228 V - 600 ohms) as reference.

    Digital Example:
    +6VU (-18dB FS).
    Current VU deflection: +6VU
    Reference: -18dB FS = 0VU

    There are several reference standards for VU, but a VU meter can be calibrated to just about any reference level you want.
    It is only important to specify the "absolute" reference value in the suffix!
    Nice to know: In German broadcast, +4dBu is not used as a reference, but +4dBm. Since dBu and dBm at 600 ohms are congruent, the result is the same.
    For example:
    With my current hardware my VU meter is set to 0VU(-12dBFS), because when I measure the line outs with a dBm meter at 600 ohms resistance, I get exactly +4dBm at -12dBFS.
    So my digital pre-fader level is max -12dBFS PEAK, not -18dBFS.

    [​IMG]




    VU also has benefits for ITB. Of course you can also use RMS meters, but these are two completely different things. While it is true that VU displays an RMS value, dBFS-RMS is not the same as VU. Both meters have a completely different ballistic and therefore show levels with a different timing.

    Even if you calibrate a VU meter with a static 1kHz sine wave to 0dBFS RMS,
    the meters would still differ in level when you measure a dynamic signal, as you can see in the post below!
     
    Last edited: Feb 14, 2020
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  14. Sinus Well

    Sinus Well Rock Star

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    The RMS+3 meter above is flat (Z-weighted).
    The VU meter below is calibrated to 0dB FS.
    A static sine of 1kHz would therefore give the same value for VU, RMS+3 and Peak meter.
    However, NOT for a dynamic signal:

    [​IMG]

    And, YES, that time window of the rms meter (600ms) is not equal to that of the VU meter (300ms).
    And, YES, equal time windows would bring the displayed levels closer together but still doesn't change the fact that the ballistics of these two meters are different.
     
    Last edited: Feb 13, 2020
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  15. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    thx for youre time and explanation
     
  16. Sinus Well

    Sinus Well Rock Star

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    you're welcome. maybe it will help some of you to understand the material more deeply.
    Most of the people here probably work purely ITB, but a basic understanding of the analog world should be there to understand what is happening ITB...
     
  17. Gyro Gearloose

    Gyro Gearloose Audiosexual

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    it will help and is helping many ppl:cheers:
    :bow:
     
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