is dbfs the same as rms?

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mixaholic

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i was wondering if dbfs means the same thing as rms? also i record with a presonus firepod and i see people say that when recording vocals it's good to get about -18dbfs but i don't have a meter on the firepod so how will i be able to tell when i achieve this level. thanks alot guys
 
FS means full scale.


RMS means root mean squared. For all intents and purposes, that means average.

You want the recording levels to be -18dbFS RMS. Or, in other words, you want the average recording level to be around -18 on your digital scale in your DAW.
 
No. DBFS is dB Full Scale which referes to digital full scale readings. Zero is the top of the scale and it can not be exceeded. RMS is Root Mean Squared and referes to the average level, not the peaks which can be much more than the average level. The -18 dB is the dBFS reading of the VU equivelant.
 
thanks guys. i'm a little confused tho by reading both responses lol.
 
so when people say they record vocals at -18dbfs they mean that on the VU meter, it is going to average about -18?
 
A VU meter will only be on analog equipment. Your recording levels should be around 0dbVU. For the purposes of this discussion, -18dbFS is the digital equivelent of 0dbVU. (the exact calibration of your unit will vary, if your manufaturer even publishes the spec) They are two completely different scales measuring the signal two different ways.
 
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mixaholic said:
so when people say they record vocals at -18dbfs they mean that on the VU meter, it is going to average about -18?
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No, I think it means that it will average at about 0.

I believe 0dbVU (analog reading) is = to -18dbFS (digital reading).

Which is why you want to average at about that level so you're not overdriving your pres.

Someone correct me if I'm wrong.
 
RMS is primarily used to calculate the average 'real' power in a signal, like amplifier wattage. You measure the peak to peak voltage of a sine wave for example, and the RMS voltage is about .707 of the peak voltage. if your amplifier puts out 10 volts of P-P voltage, the RMS voltage is about 7 volts.

dBFS came about with the popularity of digital equipment. It simply means the very top of the scale, before distortion occurs. Digital distortion is harsher than most analog stuff, so it's been kinda accepted to keep the average signal levels down around -18 dBFS to match the analog levels and prevent distortion.

Farview, Danny, and Trackrat all have it right.
 
mixaholic said:
so when people say they record vocals at -18dBFS they mean that on the VU meter, it is going to average about -18?
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No, that'll work out to 0 or +4 dBU on an analog VU meter.
 
thanks guys i'm think i get it now. i record vocals with a presonus firepod but the firepod does not have any meters on it so is there a way to meter the signal so i'll know it's averagin 0dbvu
 
Well, you can always crank it up till you hear distortion, then back it off a couple of numbers. Doesn't the firepod feed software or another device that does have meters?
 
i'm not sure. to reach 0dbvu, i find myself having to turn the gain up almost all the way on the firepod and it picks up alot of background noise even the slightest movement. is this normal?
 
mixaholic said:
thanks guys i'm think i get it now. i record vocals with a presonus firepod but the firepod does not have any meters on it so is there a way to meter the signal so i'll know it's averagin 0dbvu
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If Presonus bothered to publish full specifications on the Firepod, you could find out how their converters are calibrated and then use the digital meters in your computer software to read it properly. Unfortunately, they don't specify (at least not online) the maximum converter level or the conversion calibration level.

You can make a fairly educated guess though, give or take a couple of dB. Set the input level sliders in your software to "unity gain" (zero boost/zero cut) so that you're reading the unaltered level coming into your computer. Then adjust the Firepod gain so your averaging (not the peaks, but the main cruising level) somewhere between -20 and -12dBFS. Of course if you're peaks are clipping, turn it down a bit on the Firepod. Also of course, if you sound like you're distorting a bit, turn it down on the Firepod until it does not distort. Stay away from the clipping and the distortion, and keep the average meter reading somewhere in the -teens dBFS, and you should be OK.

G.
 
thanks alot glen! when u say not the peak meter but the main one which meter will that be in soundforge?
 
mixaholic said:
thanks alot glen! when u say not the peak meter but the main one which meter will that be in soundforge?
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No, it's the same meter (the ONLY meter :) ). What I meant was when looking at it, don't use the peaks of the signal (the short, fast, highest point) to judge RMS, instead just take your best guess at the average level that the meter is riding at between the big peaks. Only look at the peaks to determine if they are hitting zero or not.

G.
 
Farview said:
-18dbFS is the digital equivelent of 0dbVU.
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Not to be pedantic, but there's no absolute correlation between digital and analog levels. Some manufacturers ship their products calibrated to -18dBFS = 0dBVU, though not all. You should figure out what cal level works best in your room and always check the cal of your new ADCs and DACs - even if you choose to operate at the factory levels, you never know if the channal balance is correct until you check it...
 
I edited the post
Farview said:
For the purposes of this discussion, -18dbFS is the digital equivelent of 0dbVU. (the exact calibration of your unit will vary, if your manufaturer even publishes the spec)
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On things like the firepod, there is no user calibration and the manufacturer does not provide any clue as to how it is calibrated. -18dbfs equaling 0dbVU is just a 'conventional wisdom'-type, safe bet sort of thing.

If someone is having a hard time getting their head around the various db scales and meters, they obviously don't have the background to "figure out what cal level works best in your room and always check the cal of your new ADCs and DACs". And pointing out that the correlation between scales is to some extent random, will probably prove counter productive in his effort to put all the pieces together.
 
Farview said:
If someone is having a hard time getting their head around the various db scales and meters, they obviously don't have the background to "figure out what cal level works best in your room and always check the cal of your new ADCs and DACs". And pointing out that the correlation between scales is, to some extent, random will probably prove counter productive in his effort to put all the pieces together.
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I understand, but remember that there are hundreds (if not thousands) of folks who will read this thread besides those who post. I'd rather people walk away with questions in mind searching for answers than walk away with an overly simplistic (and potentially incorrect) view of the subject.

Your post wasn't wrong, per se, but I wanted to clarify for the masses...
 
One could test the calibration by running a +4dBu, 1kHz sine wave into the analog front and then see what comes out the digital back.

But then again, if one is savvy enough to actually run that test, they're probably savvy enough to have figured that out themselves already ;) :).

G.
 
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