Help with decibels

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mshilarious

mshilarious

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Let me preface this by saying I have read both Prorec articles on this topic several times. In fact, that is the source of the problem--only enough knowledge to be curious about the fact that I don't understand this.

Let's say I have a preamp that operates at +4dBu, and has a listed headroom of +28dBu, which means that its meter is labeled as +24dB max (+28dB is shown as clipping).

OK, now my AI3 converter manual says that, set at +4dBu operating level, -15dBFS is .138V, which translates to -15dBu. Thus +0dBFS is +0dBu, not +4dBu, which means that my effective headroom on the preamp meter is -4dB, not +24dB.

It seems that at least 0dBFS should be +4dBu, not +0dBu.

Yet the manual claims that the maximum input level is +10dBV, which is +12.2dBu. I suppose that means the analog circuit won't clip until that level, but what is the point if the digital signal clipped 12.2dB ago?

Consequently it seems the output from the converter is never going to exceed +0dBu. The manual claims that the maximum output level is +17.5dBu, but I have no idea how that is supposed to happen.

Here's what confuses me the most: if I set the selector switch to -10dBV operating level, then it actually outputs a stronger signal (0dBFS = 0dBV = +2.2dBu)!

Is it just me, or is this manual smoking crack?
 
wow, this is getting a bit too technical for me,
but as far as i know:

i got a +4 and -10 DB switch on allmost all my outboard gear,

and indeed, if you set EVERY piece in your signal path to +4,
then the recorded signal will be louder

i read that everything has got to be OR + OR -,
logic, i guess,

i even got some jumpers on my soundcard (dman 2040) , without the jumpers i got the +4 setting, then i can record clippin and stuff,
with the -10 db jumpers on i CAN NOT RECORD CLIPS !!!!
at first i thought there was some sort of hard limiter built in that card,
but no, i just recorded at a low volume, no clipping, plenty of headroom, and this card sounds so clear that you can easily boost the volume afterwards with software plugins.....


i hope this can make things a bit more clear for you, but guess not,
just my Two pennies....

earworm
 
The operating level can be looked at as the sensitivity of the unit. (when speaking interms of input)

So basically there is a 14db senstivity difference between -10 and +4

Of course the real reason for this is so that if you have gear that outputs at -10 you match it up with a device that inputs at -10 and likewise with +4

else what you will run into is an output device outputing at +4 and you running your input device to read at -10 and introducing a GREAT possibility of clipping and the like.

So as a simple rule of thumb you match devices inputs and outputs. (when going from one to the other) -10 to -10 and +4 to+4. These are design conventions as most consumer gear operates at -10db Output and pro gear at +4.

Hope that helps.

Bryan Giles
 
You went wrong on one key point, mshilarious.......

You're equating VU, dBu, and dBFS, and basically, they're completely different scales.....

More details here in one of my articles --> Meters, Signal Level, and Headroom

But the bottom line is - given a uniform gain structure (ie, all gear running operating at either +4dBu, or all at -10dBV), a signal that reads 0VU on a mixer or preamp, will read somewhere around -15 to -12dBFS.
 
Golly... I'm gonna take a chance at screwing this up too... first off, the difference between -10 and +4 is like 11.5 db as it's a logrithmic scale... neither here nor there.

What you need to kinda understand is that 0dbfs is the end point of what you can record. So, in setting levels in digital you're talking about headroom beneath 0dbfs... so to match the headroom of the gear you're using to the available headroom of the digital input you would set +4dbu [which will often equate to 0VU] to -24dbfs thus giving you 24 db of headroom on the digital scale.

Is that any better?
 
Fletcher....

If you have the ability to calibrate input levels, then yes - setting it to -24dBFS (or any value) is possible -- but in the case of gear with fixed input levels (for ex, a Masterlink), the manufacturer often calibrates 0VU to be anywhere from -15 to -12dBFS at a given gain structure............
 
Blue Bear Sound said:
You went wrong on one key point, mshilarious.......

You're equating VU, dBu, and dBFS, and basically, they're completely different scales.....
Click to expand...

I don't believe I was, it was given in the manual: -15dBFS = .138V when set at +4dBu mode, and -15dBFS = .178V when set at -10dBV mode. Those voltages both convert to -15dB in their respective scales.

OK since my computer's not up yet, I set up an effects loop in my preamps to test this. Despite those figures above, the converter does output +12dB hotter in +4 mode, and the clipping mark on the preamp (+28dBVU, which I assume, possibly incorrectly, is +28dBu based on the preamp manual) does correspond with the clipping mark in the converter.

Thus I know that the converter manual must have been somewhat confused, but I still don't know exactly what level clips the converter since I don't have a VU meter calibrated to anything I can verify.

Having said that, the preamp & converter meters match up, so I guess there's no worries.
 
mshilarious said:
Thus +0dBFS is +0dBu, not +4dBu, which means that my effective headroom on the preamp meter is -4dB, not +24dB.

It seems that at least 0dBFS should be +4dBu, not +0dBu.
Click to expand...
This is the part that is wrong as far as I can see (whether it's you or the manual is unclear to me).... anyways - at either gain structure, 0VU is *NEVER* 0dBFS unless you're adding an additional gain stage in between.
 
Blue Bear Sound said:
This is the part that is wrong as far as I can see (whether it's you or the manual is unclear to me).... anyways - at either gain structure, 0VU is *NEVER* 0dBFS unless you're adding an additional gain stage in between.
Click to expand...

Yeah I think that was my conclusion, the Alesis manual was either very misleading or dead wrong.
 
Nope, no idea what you guys are on about. Is it something to do with sport?
 
Note - that's only true if the manufacturer calibrates to that convention.... some do, others don't.... Alesis calibrates their Masterlink where 0VU = -15dBFS.......
 
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