dB Gain -- was RE: [K3] [Elecraft] FLx GN Adding Db gain

[Jim Brown]

Lots of questions. See some answers interspersed. 

On Tue, 11 Nov 2008 18:22:50 -0800, Dick Dievendorff wrote:

>I think I usually get that right (along with kHz), but I get confused when I
>see dBi, dBm, seemingly dBx for all X.  I presume it's a measure of loudness
>relative to X, so a dBi is gain relative to an isotropic radiator, and dBw
>is what with watts?  

All dB is relative -- that is, A is x dB louder or quieter than B. 

>And is a dB of voltage equivalent to a dB of power or an audio dB of
>loudness?   

The equation for ALL dB numbers is dB = 20 log (V1/V2) or 10 log (P1/P2), 
where V is voltage and P is power. When you append a letter to it, the letter 
defines some reference condition that could be voltage, power in some 
impedance, acoustic loudness, etc. MANY references have been defined. dBm 
indicates dB referenced to 1 mW. dBmV is dB referenced to 1 mV. dBu indicates 
a reference to 0.78V  dBV is referenced to 1 volt.  

 What is a VU meter measuring?  Voltage?

It's actually measuring power if it's a true VU meter. 0VU is 1 mW in 600 
ohms, and a VU meter has a carefully defined dynamic characteristic -- that 
is, the meter movement has its ballistic characteristics defined so that it 
indicates subjective loudness -- sort of. The standard for the VU meter goes 
back to the early days of broadcasting. VERY few modern meters calibrated in 
VU actually have this characteristic, which isn't cheap to build. AND -- most 
VU meters are calibrated so that a reading of 0 dB is +4VU or +8VU. Not only 
that, but a true VU meter includes rectifiers that combine with the 
ballistics to do a lot of smoothing so that they are reading the average 
power. Actual peaks of audio are typically 10-14 dB hotter than the indicated 
reading. Are you confused yet?  :) 

>And why are our ears logarithmic anyway?

That's the way God made us. 

>And every time I see a recording studio meter in a movie or TV clip, it's
>seemingly always dancing to the pin, way over into the red.  Why do they do
>that?  

Broadcasters have long been in an eternal search for loudness. If the meter 
moves below the red there isn't enough processing, and someone is going to 
get fired. :) 

What does the red mean? 

In today's world, not much, because broadcast stations, like hams, use LOTS 
of signal processing -- compression, peak limiting, and other tricks -- that 
prevent those peaks from being much greater than the average voltage. If 
there were no processing, the red range of the meter would indicate that the 
waveform was getting close to clipping on peaks. 

73,

Jim K9YC