What *IS* the TRUE dBFS level when the red LED lights

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This topic contains 20 replies, has 6 voices, and was last updated by Profile photo of volounteer volounteer 3 months, 3 weeks ago.

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  • #85347
    Profile photo of volounteer
    volounteer
    Participant

    The documentation is confusing.
    Is the red Led equal to 0dBFS or is the red Led actually -18dBFS ? or is is something else?

    I take it to be -18 or else there would not be 18dB headroom the document specs claim.
    If it is really 0 then going into the yellows, which the manual says is fine, could be a problem.
    Certainly riskier for clipping than I would be comfortable with.

    There is no marking on the board for the RED Led that I can see, and the yellow lights (+3, +6, +12) are not equally spaced so unsure how much farther the Red Led is from the last yellow light. Is the Red 3 , 6 , 9 , 12, 18 , other dB higher than the the +12 yellow?

    #85348
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Which red light? What documentation? Why dbfs?

    The Qu metering/lights are dbVu, but whichever way you prefer to measure, the user guide simply states that the red LED associated with the metering system lights 3dB before the onset of clipping for either dBfs or dBvu. See attached screen shot:

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    #85352
    Profile photo of volounteer
    volounteer
    Participant

    @DickRees

    I mean the red light on the top of the pseudo-peak/peak-like meter towards the right hand corner.
    This is not a VU meter with known ballistics, and I really do not care about dBu which is related to volts,
    but do care about dBFS and clipping.

    The manual relates the 8 green (-40, -20, -16, -12, -9, -6, -3, 0 ) 3 yellow (+3, +6, +12) and no value (“Pk”) red ,
    to dBFS.
    And also says there is 18dBFS headroom. I infer from your comment that that the channel green means -18dBFS if the manual meant dBVU instead of saying dBu

    Clipping starts at 0dBFS.
    dBu relates to voltage levels. The image you show does not have any dBvu just dBu.

    The question is what the red Pk light on the LR meter in the top right means in terms of dBFS

    The channel meter lights are nice but rather crude although a good supplement to the LR out meter.
    But do not seem to match those LR lights.
    They turn on at -40 while the channel meter says -26dBu is signal detected. How do you detect voltage in a digital computer?
    Their last green one is labelled 0 no units, while the channel green labelled 0dBu (voltage ) would be is -15dBFS (relative to digital max) I as know headroom in the digital realm if they had said dBVU not dBu.

    dBu is of no value to us in the digital world, we need to know dBFS. dBu will happen after the D/A conversion.
    The studio recording level (pro audio) of +4 dBu means a voltage of 1.228 volts. And was useful with an analog board.
    Not so much with the digital mixer.

    Really looks like some FH engineers confused analog and digital when they designed this device and rewrote the manual.

    #85354
    Profile photo of Dick Rees
    Dick Rees
    Participant

    V…

    ***content removed – personal comment that does not contribute to the thread*** Seemingly simple statements such as “clipping starts at 0dBFS” is so incorrect and misleading. For one thing, the “0dBFS” point is not a “starting point”. A digital signal by definition cannot exceed 0dBFS, thus in your terminology it would be an ending point. It s the proverbial “brick wall”. Such seemingly small discrepancies are anathema to those seeking a fuller understanding.

    Clipping can occur with a live sound system at many points and may or may not be subject to detection by metering. Most such metering is RMS if for no other reason than the longer measurement sample is more “real world” than transient peak measurement…which is very difficult to implement.

    The meters can’t tell you how it sounds. You still have to use your ears.

    Next time you cite some document or white paper on any subject, be so kind as to provide complete citation and a link. As is, all we have is your interpretation of the un-named source, comparing it to “internet opinion”…***content removed – personal comment that does not contribute to the thread***

    #85355
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Addenda:

    The “AudioWiki” http://www.sengpielaudio.com/calculator-db-volt.htm

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    #85357
    Profile photo of volounteer
    volounteer
    Participant

    @DickRees

    0dBFS is the maximum in the digital world that does not cause problems.
    dBFS is always measured down from 0dBFS.

    0dBFS is where clipping starts — unless you lower the final before conversion with D/A.
    So yes, you could go to +ndBFS but that will not convert to the correct analog version.

    There is a big difference between analog and digital. And yes clipping can occur at many places.
    Which is why you should not be running digital anywhere near 0dBFS in any place.
    And you can not set channels anywhere near the top and then expect to add them together without problems.

    My rule of thumb is to never go over -12dBFS anywhere.
    You could do -18dBFS or even some more as there is plenty of DR and very good S/N for a huge range.

    Many digital devices now use peak metering to ensure you do not go over 0 and clip.

    We do use our ears, but meters can help stop us from ever hearing that it was distorted or worse due to clipping.
    Our goal is to prevent it not fix it.

    ***content removed – personal comment that does not contribute to the thread*** It came from a major mike manufacturer. Not sure where they found it.

    Your png image agrees with what I said. There is no direct volts to dBFS conversion. But there is a maximum dBFS before clipping and the question is the relation to 0dBFS and the red light on the Qu32 mixer in the upper right for LR out.

    #85359
    Profile photo of xyz
    xyz
    Participant

    Good articles!

    #85360
    Profile photo of Mfk0815
    Mfk0815
    Participant

    Hmm, i read the LEDs and especially the Pk LED simply in a practically way. If the red light goes on on the desk (and so the manufacturer too) says me „ui, too much lower down the volumn“, if the yellow( or orange) LEDs starts to light, the desk sys me „attention, its ok but you are close to danger“. Green means that every thing is fine, no LED means no or too less signal level.
    Based on what happens after the DA conversion the manufacturer defined -18dBFS as an equivalent to 0 dBu. So Pk is -3 dBFS for me but i don not care, because…. the above.
    So, you can think about theoretical stuff the whole day but PK means you are too loud are your PA is to small for that location.

    #85362
    Profile photo of Ryan
    Ryan
    Participant

    In one of the recent posts about recording levels to a USB drive, someone from A&H said that the “0” green LED on the meter corresponded to -18dbFS in the recording.

    #85363
    Profile photo of xyz
    xyz
    Participant

    @mfk0815
    I agree
    Same! ditto!
    Unless someone is requesting specific instructions that the end result should be XYZ level.
    I use my ears and NEVER hit the red
    Check all amps when powering during in an event to check as well!

    And I’ve had to run my QU at 124 db [monitor mix] on a very large “on stage SPL” outside stage.
    Analogue meter.

    #85367
    Profile photo of volounteer
    volounteer
    Participant

    @ryan

    That is how I read the spec sheet and documentation.

    I was hoping someone from Ah would give a definitive answer.

    *IF* the green peak is -18dBFS then rarely touching it would not be a problem. And that would normally only be some extreme event that caused a peak like a gunshot in a movie.

    But if it were only -3 Then there is a lot of risk that TPs would cause clipping.

    Right now we are barely into the green on the bottom but would do better if we occasionally touched the yellow, as the manual also said was normal operation. I really wish this thing also had a real VU meter.

    The MD now wants us to use a handheld SPL meter to judge the loudness that the audience is hearing , which is much more than we hear up in the balcony. If we had a number in the balcony that we knew related to the audience levels then the audio team folks could have consistent loudness for the audience week to week without being too loud or too soft. It might be too hard for us to hear without using the earphones but it would be right for the audience and the church service.

    #85368
    Profile photo of volounteer
    volounteer
    Participant

    @ryan

    If 0dB green light is -18dBFS then I would never want to ever see a yellow light even if one or two of them was not yet a problem.
    But that is me.

    I know digital since I had it in grad school some 50 years ago. I know that there is plenty of DR and S/N and there is no reason to push 0 for any reason unless you are a rock band trying to make your mix louder than every other band because you think louder is better.

    #85371
    Profile photo of Ryan
    Ryan
    Participant

    @williamadams why are you lecturing me about never wanting to see a yellow light? Did you mean to direct that comment at NZdave?

    That *IF* comes across as kinda rude, I know what the post I said saw, ***content removed – personal comment that does not contribute to the thread***. Here’s the post by someone from A&H, ***content removed – personal comment that does not contribute to the thread***: https://community.allen-heath.com/forums/topic/usb-b-output-volume-control#post-45342

    #85374
    Profile photo of volounteer
    volounteer
    Participant

    @ryan

    Not lecturing.
    Telling you my comfort zone for risking clipping.

    Your post said << someone from A&H said that the “0” green LED on the meter corresponded to -18dbFS in the recording. >>
    so if that is true then I would not want to go into the yellow. So my answer << If 0dB green light is -18dBFS then I would never want to ever see a yellow light even if one or two of them was not yet a problem. But that is me.>> stands.

    I have seen the USB volume control post. No idea if that really applies to the main LR out lights. But the DAW level is a separate issue from what is happening in the mixer which was my question.

    And again, *IF* the 0 green led on the LR out is really -18dBFS then *I* do not want to go into the yellow ever. But that is just my comfort level no matter how many rock bands want to push right up to 0dBFS with their gear. And I see some go over 0dBFS and ignore the distortion from clipping. I guess when its that loud you cant really hear what is happening anyway.

    #85377
    Profile photo of DavidCo
    DavidCo
    Participant

    Red light comes on at +15dB (-3dBFS). Clipping occurs at +18dB (0dBFS)

    DC

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