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dB, dBFS, dBu, VU

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Category: General
Forum Name: Newbie Discussion
Forum Description: Newbie Discussion/Questions. Look less stupid here...
Printed Date: 06 June 2023 at 8:46pm
Software Version: Web Wiz Forums 12.06 -

Topic: dB, dBFS, dBu, VU
Posted By: Norseman
Subject: dB, dBFS, dBu, VU
Date Posted: 30 January 2023 at 10:17am
hey everyone.
sorry if this has been done before, ive tried searching and not coming up with much helpful.
I come from a studio background but never needed to consider much of this, working a lot in the box and only using outboard gear for colour etc.

I am trying to figure out the gain structure for my rig, and am a bit OCD once something gets in my head, often its unnecessary crap but something i need to sort out!

essentially i am running my gear as follows

sources: turntables, CDJ --> allen and heath s2 mixer --> ras efx reggae pre --> i-nstruments VU meter (monitoring) --> powersoft x8.

now, whats getting me stuck is the differences in db measuring.  I shall go through some points one by one...

allen and health s2:
output from xlr balanced is the standard "professional" +4dBu, all very well.
On the back there are some recessed pots for attenuation on channel gains, but also it has a "master output limiter" whos settings have thresholds ranging from +6 to +18dBu....
The master level indicators go from -20 to +10! (no mention of what units) with the sensible suggestion to keep average level at 0, with peaks up to, but no higher than +6 (+10 is the clip red light)

so number one question is wtf are these units...
secondly, if the main output is +4dBu, what does it mean to have a limiter threshold set at +18dBu...(seemingly higher than the output level...does this make sense to anyone???)

now, from here i run into the RASEFX pre, 12 o'clock is basically "pass through" ie no gain added, however EQing and so on will add some gain.
Here (at home) i usually attenuate the music level by keeping that knob at 11 o'clock to allow for flexing the EQs and so on.  all well and good.

This then sends an out to the i-nstruments VU meter, which is a copy of the Dorrough rack mount thing.  it has 0db at the top of the yellow meters, after which its red...
doesnt say dBu, just dB.  its a studio centric device, so i assume this is just studio 0dB (probably +4dBu professional as when the s2 meters are at zero w small peaks, its also at zero w small peaks)

now, the powersoft x8 default setting on inputs in armonia have two metering options, dBFS and dBu.  ive not really messed with these and just used the age old green light orange light red light as a metering to know whats going on, but, yeah, ocd....soooo

it seems that by default on the input meters, they have 0dBfs set at 24dbU (and it says its max input acceptance is 27dBu...?)

from my reading, it seems in order to maintain a decent amount of headroom for peaks, 18to 20db of room is about standard, and therefore that that the RMS average should be -20dbFS, or +4dBu (0dBfs - 20dB = -20dBFS ~OR~ 24dBu - 20dB = +4dBU...)

then if the max tolerance without clipping the input is +27dbu...are they just setting the 0 dbfs (which cannot be exceeded?) to 24 for safety?

1. is this thinking even right?
2. does this then mean that driving this amp to max will occur when my levels as shown on my i-nstruments meter are at 0 (signifying the S2/RAS poutput is +4dBU??)
3. Is there some untapped power or room for more level here...
4. im sure im just getting confused with it love someone to be able to explain this like they are talking to a ten year old!  



Posted By: Conanski
Date Posted: 31 January 2023 at 2:53pm
Hopefully this helps and doesn't just add to the confusion. 

If you come to the conclusion that the audio industry can't make up thier minds you wouldn't be wrong, but this is a technology industry and technology changes and sometimes in hugh ways such as when digital audio appeared. 

Db is a ratio between values and in that form it is unitless, when a 3rd character is added that provides a reference. dBu is referenced to 0.775v and dBv is referenced to 1v. dBFS is the digital scale and it is referenced to digital clipping of the highest level that a piece of equipment can generate. That is a fine idea in theory but different pieces of equipment can and do have different maximum output levels so you have to know what that is to make a relative comparision, so in practice this can lead to confusion. 
Modern digital components have a large amount of signal headroom compared to analog components and people get worked up about "using all the bits" and other nonsense, but the reality is this gear has a very wide usable range where there is very little audible penalty so it is perfectly fine to use a digital signal level that is well below digital clipping. 
So bottom line if you use +4dBu as the goal and keep the peaks below clipping of your analog components(no red) you are good, and that would correlate to around -18dBFS average in the digital realm with peaks hitting -10db perhaps. 

Posted By: Norseman
Date Posted: 02 February 2023 at 6:01am
thanks, both. those images are very helpful.

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