Understanding Decibels

[filmismymedium]

I would like to use this thread to better understand the physics and math behind the term decibel. Let me start with a story:


I was laying off some tapes at work from Final Cut to our Beta deck. I had adjusted our audio levels to the -12dB range on the FCP meter and began to output to the deck. On the Beta deck the levels showed exactly the same, about -12dB. *and when I say this I mean it would peak at slightly above the -12 mark but almost perfectly fell at around -12.*

Now my boss came in and told me that the level for the beta deck must be much higher than -12 because it is an analog signal. He suggested 0 for my baseline.

Now when we output to our digital deck (dsr-80) we have always done this at -12dB and it was fine.

Basically I have two questions,
will a -12dB signal coming from FCP and recorded to a beta deck showing -12dB be too low for analog broadcast?

and secondly, I was under the impression that a "decibel" was not a unit of exact measurement but a way to reference from someone else's decibel level. For instance, you couldn't go up to a person and say, "Hey can you sing me a song at a level of 10dB" but you could go up to a person and say, "Hey, here is a tone @ 10dB and this other tone is at 15dB (on my specific software) and is thus twice as loud.

(I believe this is because dB are on a logarithmic scale and an addition of 5dB doubles the "volume".)


please clarify before I explode.

 

[bob15489]

From what I understand, digital decibel meters work in dbFS (decibel full-scale). When the meter hits 0 all the bits that can be used are turned on and anything above that clips and becomes distorted. Analog decibel meters work in VU (volume units). The analog 0 has to do with tape saturation and the "headroom" an analog machine has is how high above 0 it can record without clipping. You are right that decibels are a relative, logarithmic scale. Check out some of these links and hopefully they will help.

http://www.teamcombooks.com/mp3handbook/11.htm
http://en.wikipedia.org/wiki/Decibel
http://www.sencore.com/newsletter/Oct06/MeasDigAudLvls.htm

 

[Oleg 2]

what beta you use, digi beta , 70 or 2800?

 

[someday]

FCP has peak meters. You don't get the idea of the LOUDNESS which can only be shown on RMS (average) meters. Analog betas have RMS meters.
There is a huge difference between peak and RMS levels.
Uncompressed speech has up to 20 dB difference!!!
So you see that your program is at -12dB but it's average level is maybe at -24dB
Aactual pop and TV advertising audio has a peak to RMS ratio of 6dB.
6dB peak to RMS level is the maximum you can have without distortion.
To maximise the loudness, in other words to diminuish the peak to average ratio you need compressors and brickwall limiters.

For a normal TV program usually you don't need all that compression. You can compress in order to have a peak to RMS ratio of 9 or 12 dB. Ask the client what they want.


.

 

[Michael T]

I use to layoff a lot of video and audio out of FCP to Beta SP 2800 and a 2000 and we used -6 for FCP and our clients liked the signal strength. We did some tests and sent them to the clients and they liked -6 the best. Some clients were cable and some broadcast. I remember reading somewhere that -6 digital equals 0 anolog but I have read the -12 signal should be used. Dynamic rage can differ a lot coming out digital to analog so you should do some testing for your clients to see what they want. Some wanted a hotter signal and some clients recompress everything you give them so the wanted the -6 signal.
Good luck.

 

[Oleg 2]

usially for broadcasting, programs mixer with picks about -10dbfs after compretion during the output limmiters station use on its outputs .
for analogit abit more complicated as vu meter not allow you actualy se the picks so better keep the everege les the ovu if compression wasnt envolved
for europian standart -18dbfs =0vu
for otghers -20 dbfs =0vu
if you try to import from fcp the reference of -12dbs should be something around -6 db vu

 

[Petrus]

Bel as a unit is logrithmic to base 10, which means the when the sound or dignal level get bigger by one bel, the level is 10 time higher.

For practical reasons we use ten times smaller fractional unit desiBel (just like desimeter, desiliter), with that 10 means ten times more, 20 hundred, 60 million. For doubling the ratio is 3 (10*log2), not 5.

 

[filmismymedium]

what beta you use, digi beta , 70 or 2800?

2800

 

[filmismymedium]

I use to layoff a lot of video and audio out of FCP to Beta SP 2800 and a 2000 and we used -6 for FCP and our clients liked the signal strength. We did some tests and sent them to the clients and they liked -6 the best. Some clients were cable and some broadcast. I remember reading somewhere that -6 digital equals 0 anolog but I have read the -12 signal should be used. Dynamic rage can differ a lot coming out digital to analog so you should do some testing for your clients to see what they want. Some wanted a hotter signal and some clients recompress everything you give them so the wanted the -6 signal.
Good luck.

ok, one more question:

I usually lay off to our decks using Print to video function in FCP, but the last time i did it I just placed the bars and tone, slate, etc on the timeline and played from the timeline. The bars and tone are auto set @ -12dB and all of my audio content of the show is -12 as well. On the beta deck when I adjust the input values to either match dB to VU or to make the -12dB about -6VU on the deck it works fine but the actual content is far lower on the VU scale. In other words, Bars/Tone and the show on the timeline are both -12dB digital but on the VU scale the bars and tone are far higher. Why is this?

 

[wabbit]

Re-read Someday's post. Digital FS is only a peak measure, VU meters measures RMS. Additionally, FS meter tops at 0 while 0 is the average optimum range for VU. It varies from equipment to equipment but analog dbu can reach as high as +20 before distortion becomes a problem.

Cheers

 

[filmismymedium]

Re-read Someday's post. Digital FS is only a peak measure, VU meters measures RMS. Additionally, FS meter tops at 0 while 0 is the average optimum range for VU. It varies from equipment to equipment but analog dbu can reach as high as +20 before distortion becomes a problem.

Cheers

I understand the difference between the peak measure in FCP and the VU on the deck but what I was wondering was why two clips on the timeline whom both share the same level on the peak measure show vastly different levels on the VU.