backini
Newbie
Joined: 10 Sep 2003
Posts: 2
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| meters constantly in the red on analog desk |
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Hi, I inherited a situation and I'm not sure if its right. I have an Amek Einstein desk and a tascam mx24 digital multi-track. On playback, the mx24's meters read between -6db and 0, bet the bar meter on the desk is constantly in the red past +4db. Now I'm aware that the actual volume of a digital signal at 0db is much higher as analog, but whats the point of having a meter bridge if it just constanly reads red ? Ultimately I've been trusting my ears although I'm starting to doubt the over all quality of the sound and I am getting some occasional unexplained clipping, although not enough to suggest where the problem is. I think someone once said that there was a switch inside the desk somewhere that attenuated the incoming signal but I've no idea where it is and I may have just dreamt it.
Can anyone help ? |
Wed Sep 10, 2003 2:18 am |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Calibrating Your Studio |
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Often equipment can be calibrated at two different settings.
semi pro is -10 dBu
Pro is +4dBu
These figures are referenced to 0dBu which is 0.775 volts.
So if a machine (or the studio) is calibrated to run at -10dBu this really means that when "0" is read on the meters it means that it is actually -10dB below 0.775 volts.
Likewise when a +4 machine is running at "0" it actually means +4dB above 0.775volts.
Get it?
Now this may or not be your problem but it's a good start.
Often machines can be "switched" between the two, some cannot. The difference in calibration levels is purely down to cost, it costs more for higher grade components etc. Also the signal is significantly stronger in +4dB.
Sounds like you have a mismatch like this.
If you can't change your calibration level, you could try to re-calibrate the meters (usually pots inside), or you lower (by calibration) the digital levels. Or if you can do niether of these you could use a pre-amp/attenuaton circuit on the in/out of the desk to compensate.
More info here:
http://www.audiocourses.com/article50.html
Does that help?
_________________
It's all in the ears. - Learn the concepts not the software.
Audio Courses is a way into the music business for you
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Wed Sep 10, 2003 3:55 am |
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Will.W
Joined: 03 Oct 2002
Posts: 101
Location: NOTinfrontofcRuN |
| popular misconception? |
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Hi,
The semi- pro setting is actually -10dBv not -10dBu! This is a topic I debated with colleagues and lecturers when I did my city and guilds.Just check out any compressor manual or similar,go to spec. page and u will see -10dBv(or +4dBu) .Just wanted to clear that up.
Cheers
Will
Last edited by Will.W on Wed Sep 10, 2003 7:29 pm; edited 1 time in total |
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Wed Sep 10, 2003 4:26 am |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Reference Levels |
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Will thanks for that, you are “bang on the money” and it’s a common error made that comes with late nights. Here is a complete run down of all reference levels commonly encountered.
Audio Reference Levels
dBV – is decibels related to 1 volt
dBu – is decibles relative to 0.775 volts
dBm – is decibels relative to 1 milliwatt
dBSPL – is decibels relative to 20 micro Newton metres squared
What Does a Decibel Mean?
These are really cool things and very useful for defining audio reference levels. A decibel itself is not something that exists, the term only has meaning when we use a reference level with it, such as volts or watts.
An audio reference level is a known quantity that is “set in stone”. Alexander Graham Bell (the dude who invented the telephone) is where the term is named from. The “Bel” is the unit and the decibel equates to one tenth of a Bell.
Think of the decibel as a unit to describe relative loudness. Relative loudness is a vital concept in sound engineering.
In audio circles we use the term decibel in conjunction with audio reference levels, this can be sound pressure levels (SPL), voltage (V), current (I) and power (P). As all audio signals are in fact scientific quantities it stands to reason we would use a scientific language to describe them right?
Any signal can be described in decibels.
* A decibel is not itself a unit.
* The internationally agreed SPL (sound pressure level) is :
0dB = 20 micro Newton metres squared
0dB = 20µ N/m2
* For most people this is the quietest audible sound.
* Therefore, 120 dB SPL is not in itself a measurement, it means that it is 120 dB louder than 0 dB SPL.
* Real dB can only be measured against one another, not on their own.
Back to Reference Levels
* For an electrical signal 0.775 volts = 0dBu (dBu is the most often used).
* Also for electric signals this alternative is sometimes used: 1 volt = 0dBV.
* Almost every time this one is used you'll find it being used incorrectly, often mistaken for 0dBu (this is Will's Point, and a good one!)
Table 1
dB ( Decibel ) Multiply by
0 - 1
3 - 1.414
6 - 2
12 - 4
18 - 8
20 - 10
i.e. An electrical signal of 0.3v increased by 20 dB = 3v
Table 2
dB ( Decibels ) Divide by
0 - 1
-3 - 1.414
-6 - 2
-12 - 4
-18 - 8
-20 - 10
Formula
A handy formula to work out the increase or decrease in decibels between two voltages on your calculator:
dB = 20 log (V1/V2)
1) divide one voltage by the other.
2) press the log button
3) multiply by 20
The answer is the number of dB V1 is greater than V2
Decibels that must be remembered
0 dB stays the same
+ 6 doubling of voltage
+ 20 multiply by 10
- 20 divide by 10
- 6 halve the voltage
What’s the difference between dBV and dBu?
Well the practical difference is a loudness one, or a voltage difference, if you like. Remember once the audio is in the system it is in fact voltage, so any changes we make with faders etc is actually a change of voltage.
Let’s look at a worked example.
0dBu = 0.775 volts
0dBV = 1 volt
Here we see a difference of 0.225 volts, which is actually significant. Now lets use the decibel formula to establish the difference between –10dBu and –10dBV.
What is –10dBu?
V1 = 0.775 Volts
V2 = ?
dB =20 log V1/V2
dB = 20 log 0.775/V2
rearranging we get:
-10dBu = 20 log V1/V2
-10/20 = log 0.775/V2
-0.5 = log 0.775/V2
0.32 = 0.775/V2
V2 = 0.32 * 0.775
-10dBu = 0.248 Volts
What is –10dBV?
V1 = 1 Volt
V2 = ?
-10dBV = 20 log V1/V2
-10/20 = log 1/V2
-0.5 = log 1/V2
0.32 = 1/V2
V2 = 0.32 * 1
-10dBV = 0.32 Volts
So notice how there is a difference between 0dBv and 0dBu, and that the difference is not 0.225, but in fact is a relative scale which changes depending on the difference between the levels.
And Practically?
Well here you probably will not need to worry about the math, you simply get yourself calibrated using a screwdriver.
As an example, if you have an old mixing desk and a new digital recorder you would normally calibrate the inputs down on the digital recorder. One thing to be careful of is that in the digital world and analogue world there is a difference in the meters. Typical analogue meters are VU meters, which are not to be confused with 0dBu or 0dBV, they are in fact altogether a different system. VU meters have a slow attack time and are an indication of perceived loudness. Digital meters (which are usually calibrated differently from both VU and other digital meters) are an indication of the peak loudness, not perceived loudness.
To get your system calibrated set a tone up on your desk, usually 1Khz is used and set this so your meters are on 0. Take the lid off your digital box and find the input calibration pots, (check the manual). Take a screwdriver and turn them all down so they also read 0, or some other useable figure you are happy with, bear in mind your desk meters will go well above 0 at times.
Does that clear things up?
Thanks again Will, it is attention to detail like this we all need!
Cheers
_________________
It's all in the ears. - Learn the concepts not the software.
Audio Courses is a way into the music business for you
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Wed Sep 10, 2003 3:10 pm |
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backini
Newbie
Joined: 10 Sep 2003
Posts: 2
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| cheers. |
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Thanks alot chaps, I'll check it out. |
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Thu Sep 11, 2003 3:11 am |
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