[GreenKeys] Far OT: audio inputs

[Sam Hallas]

gil at baudot.net wrote:
>  balanced (pro-audio) signals is +4 dBu. The unit dBu, is a voltage
>  level with respect to 0.775 Vrms. A nominal +4 dBu signal is 1.23
>  Vrms from + to - terminals (+4 = 20log(1.23/0.775)).  Why 0.775V ref?
>  I have no idea.

I think you almost answered your own question, Gil. 0.775v is 0 dBm.

>  An older reference is dBm, which is 1 mW into 600-ohms --

I wouldn't say that in a disparaging way just because it has a longer 
history. dBm is still the most commonly used reference level in 
telephony. Also the formula you give is the same as for dBm - equivalent 
to 10log(P1/Pref) where P1= measured power and Pref = reference power 
(1mW).  So what's the significance of dBu? The Bel and deciBel are 
measures of power ratio. So setting the reference point as a voltage 
rather than a power seems anomalous.  To satisfy my curiosity I went to 
Wikipedia....

The article on Decibels <http://en.wikipedia.org/wiki/Decibel> states 
that dBu is a measure of voltage and not of power with no reference to 
the impedance. It doesn't explain why audio engineers choose to ignore 
circuit impedance.

Whoops! I'm starting to drift away from the original topic, which was 
off-topic anyway!. I'd better stop before I get onto moaning about the 
direct to memory card audio recorder I bought recently. The so-called 
'line input' is overloaded by not just the output of a professional 
mixing desk, but by a domestic CD player. Needs about 10 dB of 
attentuation to make make it work properly.

Cheers,
Sam