0dB For Recording Vs. 90bB range for CD standards: Confused

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Mike Freze

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Hi! In my home recording books, I always read that you should not let your input levels for any instruments exceed 0bB because of clipping/distortion problems. Getting it close to 0dB for the best, full signal at the beginning gain stage...I understand.

Yet I also understand that the audible range for hearing sound is between 0bB (the "noise floor") and 120dB (pain threshhold level, although some people hear more than a 120bB range). Again, understand. I also realize that CD burning limits a range to no more than 90dB (and you use compression to sqeeze the range to boost the overall volume even more). OK. Above a 90dB range for Cds, distortion: too low, can't hear the lows sufficiently.

Why, then, do we not record near a 90dB limit to avoid distortion rather than a 0dB limit? I thought 0dB is where human hearing of sound begins to kick in. At least that's what my books say about the audible range for human hearing concerning sound.

Mike Freze
 
The scale is in negative numbers, with 0dB being the highest and -90dB being the softest sound a CD can capture.
 
Just 'cuz you see "dB" at the end of a number...it's NOT always refering to the same thing.
There are many dB scales...and their numbers do NOT match up.

One example:
Clipping after "0dB" refers to the digital scale (which is actually dBFS)....but on an analog system you CAN go well beyond the "0" mark and NOT clip.

So you have identify which dB scale is being used before considering the numbers.

Here's some "light" reading off of Google...;)...it can get confusing:

Understanding dB

dB: What is a decibel?

Decibel - Wikipedia, the free encyclopedia
 
Thanks, guys! Both explanations are helpful, especially the bit about negative numbers (-90dB) and the highest recording level (0dB). Quick question, though.

Again, if 0dB is where human sound begins to kick in, how can you record at, say, -60dB and still hear that low of a sound?? Why, in fact, do they use negative numbers for digital when expressing the dynamic range for audible sound? Seems like if you have a 90dB range, it should be -40dBs on that end up to 50dB on the positive end. Then 0dB would be about in the middle of the hearing range. How can you record (or "hear") anything in the negative range??

Mike
 
Again...they are different dB scales...so 0dB SPL is not the same as 0dBFS.

Think about temperature scales...two scales, same temps...yet much different numbers.

"dB" is just a logarithmic unit, used in various audio scales.
 
One thing to understand about decibels is that using them is just a way to keep from thinking in big numbers. e.g -3db means 1/2 of the power, or whateever is being measured, and +3db means 2x it. Likewise 10db is 10x, 20db is 100x, and so on. The other thing to understand is that there is always a reference level that corresponds to the 0dB. In the case of electrical power (dBm) the reference is one milliwatt, so 0dBm = 1 milliwatt. In the case of acoustic pressure, 0dB reference (according to Wiki) a sound pressure level of .0002 microbar, which is defined as the threshold of human hearing, in a normal person I suppose. I don't know what meter function display on the D3200 is based on -- maybe dBu (originally called dbv) with 0dB reference being .7746 volts into a 600 ohm load, and it really isn't approrpate anyway IMO because the track meters show somethng iin the digital domain which of course can not be measured in the analog fashion.

One thing I have noticed is that there seems to be a safety factor in the Korg meter indications. When setting the meter display to CH IN (as I do when recording tracks) the meter will show peaks to 0dB well before the red warning LED hear the TRIM control lights, and I have seem some tracks that appear just a little too loud (peaks over 0dB on the meter indication) but have no distortion that I can hear.
 
Mike Freze said:
Again, if 0dB is where human sound begins to kick in, how can you record at, say, -60dB and still hear that low of a sound??
Click to expand...

That's not the threshold of human hearing. Zero dBFS represents the largest binary number that a digital audio system can accommodate.

Mike Freze said:
Why, in fact, do they use negative numbers for digital when expressing the dynamic range for audible sound
Click to expand...

Negative numbers are used in scales that refer to electrical signal levels, not audible sound waves.
 
Yeah this stuff can be as confusing as hell

For digital you need to think about 0 dBFS as the ceiling. This is the largest number that the digital system can represent. If you go above that you get digital clipping because the system can not represent the number anymore. It's like typing 99,999,999 +1 on your old LCD calculator and getting a response of 1.0E, That is not the correct answer it's just you have exceeded the calculators ability to represent the size of the number and so it throws up an error (clipping)

This has noting to do with sound moving in air which is usually is expressed in dBSPL (Sound Pressure Level) at a specific distance, for example 100 dBSPL at 3 Feet. This number will vary as you get closer or further away from the source so this sound that is 100 dBSPL at 3 feet could be almost in audible at 500 yards for example

Analog equipment is also expressing signals as a dB measure but in this case it is about voltage reference levels
most adio gear is references in dBu with a reference voltage of 0.773 volts and is described of having a nominal line level of +4 dBU. this means that line level RMS is approx 1.23 volts RMS.

Things get even more cconfusing when you then start looking at analog VU meters which usually have numbers that go down to -20 and up to +3.
These are calibrated to yet another scale. In this case they are calibrated so that 0VU = +4dBU so that when the meter receives a 1.23 volt signal it will read 0 on the VU meter.

All of this is why audio guys were called "Engineers". to do it right you need to have a pretty good understanding all of this electrical voltage stuff
 
Thanks, Bristol! You mentioned SPL (sound pressure level). The other thing you said was 0dBFS.

What does FS stand for? Must have something to do with just digital.

Mike Freze
 
Mike Freze said:
Thanks, Bristol! You mentioned SPL (sound pressure level). The other thing you said was 0dBFS.

What does FS stand for? Must have something to do with just digital.

Mike Freze
Click to expand...

"FS" means full scale. It's the largest number a particular digital system can produce, corresponding to the loudest sound it can represent, called 0dBFS. Any other value is a negative number. In digital 0dBFS is the absolute highest level possible.

"SPL" means sound pressure level. It is a measure of acoustic energy, the volume actual sound traveling through the air.

"dB" is a relative scale. "0dB" is specified by referring to some absolute value, and values above or below that are relative. In digital systems 0dB is the highest possible level, but in analog systems it refers to some voltage value or the like. That voltage is the "normal" level above which there is some headroom. Since digital has no headroom above 0dBFS you have to pick a lower level as your "normal" tracking level.
 
And while we're at it --
Getting it close to 0dB for the best, full signal at the beginning gain stage...I understand.
Click to expand...
Although I can't really "blame" people for not understanding this part (as there are books, magazines, even owners manuals that tout that very point), it's really not the case. -0dBFS assumes an awfully reasonable amount of headroom for the analog input chain -- Driving a signal close to -0dBFS can take the front end well out of it's "comfort zone" where it was designed to run.

More (rather basic, but some) here if you're bored... Proper Audio Recording Levels | Rants, Articles | MASSIVE Mastering
 
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