decibals and project

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Pepsiiuk

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hey,

I have been give a specification for an audio signal but I'm a little confused about it. The specification was that it was 0dBm into 600 ohms.

From what i have read on-line (we are not being taught about the dB scale in the module) this means that there is 1mW through the resistor? Now using p=(V^2)/r i calculate it to be 1.2*10^-3 volts?

Any info on this or the dB scale would be useful (I have searched google and on here but didn't really find anything that helped).

Thanks
 
770mV. They are talking about standard line level. Decibels suck.

0db into ANYTHING ought to be NOTHING, of course. The term seems to be thrown around more to confuse people.

DECIBELS SUCK.
 
0 dBm means the ratio between the power and 1 mW is 0, i.e. 0 dBm = 10*log(P / 1 mW)

where your P would satisfy that equation. To answer your question, solve for P that would give you this case, then use the power equation that you typed in there to find whatever information you need?
 
The little m tells you what the reference is. dB is just a ratio. dBm, dBi, dBc, etc. are all decibel measurements used for certain applications and imply a reference.
 
Manufacturers use them to hide things from you. For instance, I had a transformer and on the spec sheet it said "-3db insertion loss". Doesn't sound like much, right? What that means is, IT LOSES HALF THE POWER. Instead of coming outright with it - "50% efficient" - they try to hide it with db. This seems to be the main use of the db, to hide or otherwise obfuscate straightforward data.

DECIBELS SUCK.
 
The little m tells you the audio level of 0dBm is a level of power, not voltage.

Translated, corresponds to 0.77 Volts RMS across that 600 Ohm resistance. Search for articles "Understanding decibels" on the net.
Miguel

--Good, duffy --
 
There's a good tutorial on decibels at Decibel - Wikipedia, the free encyclopedia. It states: "The decibel is useful for a wide variety of measurements in science and engineering (e.g., acoustics and electronics) and other disciplines. It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of, for example, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction."

No need to shout, Duffy (was that about 90db SPL?). Decibels may be confusing to the average person, but it is a very useful tool in the technical fields. And like any good tool it can be misused by some and misunderstood by others.

One key point to understanding, is that a decibel always represents a ratio. Thus 0dB is a ratio of 1, a positive dB is a ratio greater than 1, and a negative dB is a ratio less than 1. And there's no decibel value for a ratio of 0 since the log of 0 is undefined (or negative infinity).
 
I also think dbm specifications are a very useful and easy to use tool that can save a lot of mathematics.

However one complaint I have is it's often used without stating the impedance and one has to assume by context if they mean a 600 ohm system, 50 ohm system, 72 ohm system, etc.

Lefty
 
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Have I ever mentioned my opinion of the db?
 

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Thanks for all your help guys and gals.

I now understand why dB is a useful scale and how to calculate decibal increase/decrease.

im still confused as to how to apply it to the signal and in the circuit.

For example, i have been asked to make a low power (6w ish), efficient amp (because its battery powered). Im planning on using the LM48511 and i dont really have any problems creating the rest of the circuit. The only issue i have at the minute is i dont understand how to use the information about the input signal to get meaningful values. Without that i dont know if i need a pre amp etc etc.

One more (silly) question, what would happen if the 600 ohms wasn't there?

Thanks again.
 
Yeah, answer THAT, db aficionados! Suppose the resistor wasn't-freaking-THERE? Be a bit like trying to estimate voltage when all you're given is an awkward power rating on a specific load that's no longer there. Oh wait - it IS trying to estimate voltage from an awkward power rating on a specific load that's no longer there.

Decibels suck.
 
If the 600Ω resistor is not there, the voltage would not dissipate any power, and the dBm pseudounit would not be the one to apply.

The then applicable pseudounit would be the dBu if wanted to express in other than the real unit Volts.
The u notes unloaded, or unterminated.

If 0.774 Volts RMS are present with no resistor presence, that is called the 0dBu reference.

Edited: added---> You have been asked to make a 6W (output - not dissipation) audio amplifier.

Your input is 1 milliwatt (0dBm) . The output is to be 6 W. The power gain of the amplifier is then to be 6000.
6 ÷ 0.001 = 6000
If the amplification is set by calculating with input resistance and feedback resistance as on an operational amplifier, then you know the ratio has to be 6000.

If that IC alone can achieve such gain, you do not need a preamplifier.
If by itself that IC cannot amplify 6000 times the power, you need another stage(s)

By the way, that IC is not rated to reach 6 Watts at recommended operation...

Playing with my audio analyzer, set to 0.774 VRMS (0dBm) lifting the 600 Ω input load, the level rises to 1.48VRMS ; but I do not know the output impedance of the instrument. Added: Checked its manual, It is 600Ω
 
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duffy said:
DECIBELS SUCK.
Click to expand...

I can't help but wonder what the dB ever did to you.

Did you fail a test because of it?

Did it break in your home and steal all your stuff?

Did it seduce your girlfriend?

.... What happened?

The dB is your friend now. Go on. Make up with it. You'll be much happier.
 
Externet said:
Playing with my audio analyzer,

1 set to 0.774 VRMS (0dBm)

2 lifting the 600 Ω input load,

3 the level rises to 1.48VRMS ;


but I do not know the output impedance of the instrument. Added:

Checked its manual, It is 600Ω
Click to expand...

Bravo!
These three steps and a bit of simple maths is how output impedance is measured.


Just lay off the decibel, OK. If you understood it you would realise how usefull it is.

JimB
 
I like dB:
They make working out the gain of a system much easier because all you need to do is add the numbers up!.

The great thing about dB is that a huge range of numbers can be represented using small range of numbers - no more five or six digit numbers to worry about.

I can't see anyone who understands dB not liking them, the only reason I can think of is ignorance.
 

We were always taught at college that the only people who talk about decibels are those that don't know what they are on about
 
Nigel Goodwin said:
We were always taught at college that the only people who talk about decibels are those that don't know what they are on about
Click to expand...
You should have gone to a better college with lecturers who understood decibels themselves.

JimB
 
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