milivolts to Decibel equivalent

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milivolts to Decibel equivalent

Postby tvcleaner » Thu Feb 24, 2005 12:38 am

I have configuration instruction stating to set an Input Audio Gain to 125 milivolts. The "updated" equipment only allows me to set this parameter to On, Off, or -1dB to -48dB. The input impedance should be 600 Ohms.
Is there a method for calculating a dB equivalent for the original 125mV setting requirement?
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Postby Guest » Sun Apr 10, 2005 1:02 am

There is a method, but you need to know the reference level. The level in decibels is a ratio (using logarithms) of a voltage or power to a reference level voltage or power. "log" below is "common" or "base 10" logarithm.

dB = 10*log (P/Pref) or 20*log(V/Vref)
These two are consistent because P = V^2/R
Common references for audio gear are dB re 1V, or db re 1mW.
Into 600 ohms, 1 mW is 0.7746 v

so 125 mV is 20*log(0.125 V/ 1 V) = =18.06 dB re 1 V
or -15.84 dB re 1 mW.

There is no way for me to guess which reference level is used; perhaps you can find it in the equipment specs. Either number will get you close.
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