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Old 7th October 2003, 09:58 PM   #1
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Default How to calculate +/-db as a percentage of the source signal.

When attenuating a source signal, I understand that 100% of the signal = -0db, 0% of the signal = - infinity db. I wish to calculate exactly much % of the source signal is 1db, or 2db, or, -50db...

Whats the formula?
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Old 7th October 2003, 10:13 PM   #2
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http://www.phys.unsw.edu.au/~jw/dB.html

:)ensen.
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Old 7th October 2003, 10:14 PM   #3
moamps is offline moamps  Croatia
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Default Re: How to calculate +/-db as a percentage of the source signal.

Quote:
Originally posted by Brian Guralnick
[B]When attenuating a source signal, I understand that 100% of the signal = -0db, 0% of the signal = - infinity db. I wish to calculate exactly much % of the source signal is 1db, or 2db, or, -50db...
Hi,

20 log x/y where y is reference level (voltage, current, ..)

10 log x/y for power

Look here
http://www.rane.com/par-d.html
"decibel" and "0dBV,u,m"

for example for voltage

-40dBV=20 log x/1V = x=0,01V or 10mV

then you can express this value in %

regards
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Old 7th October 2003, 10:25 PM   #4
joensd is offline joensd  Germany
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Working alot with these values its nice to know some and you can easily calculate the "rest".
For example +6dB means ~twice the signal, -6dB ~half the signal.
So +12dB is ~4 times (2*2) the original and +18dB ~8 times(2*2*2)

+20dB is 10 times and so +40dB 100 times (10*10).

Its that easy.

Cheers
Jens
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Old 7th October 2003, 11:18 PM   #5
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Default Re: Re: How to calculate +/-db as a percentage of the source signal.

Quote:
Originally posted by moamps


Hi,

20 log x/y where y is reference level (voltage, current, ..)

10 log x/y for power

regards
Thanks a million.

So, If im reading this correctly, calculating for my linear 256 step attenuator:
------------------------------------------------------------------
a volume of 0 = ' 20*log( 0/255) ' = -(infinity) db.
a volume of 1 = ' 20*log( 1/255) ' = -48 db.
a volume of 2 = ' 20*log( 2/255) ' = -42 db.

...

a volume of 253 = ' 20*log(253/255) ' = -0.06 db.
a volume of 254 = ' 20*log(254/255) ' = -0.03 db.
a volume of 255 = ' 20*log(255/255) ' = -0 db.
------------------------------------------------------------------
Did I calculate this corectly?

I see the importance of a log pot. Arrrrrrg, I need to change the resistor wireings on my circuit...
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