How to calculate +/-db as a percentage of the source signal.

[Brian Guralnick]

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...

What’s the formula?

[purplepeople]

http://www.phys.unsw.edu.au/~jw/dB.html

:)ensen.

[moamps]

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

[joensd]

Working alot with these values it´s 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).

It´s that easy.

Cheers
Jens

[Brian Guralnick]

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...