On Sat, 18 Mar 2006 17:54:15 GMT, "don" wrote:

dbm is as stated a reference of two like power values to a 1mW reference
however the impedence does not need to be 600 ohms, it can be any value of
ohms as long as both power values are based on the same impedence


No, the impedance does not need to be the same, and there are not two
power values, but one - specified as dB with respect to one milliwatt.
Impedance does not appear anywhere in this figure.

dbu is not unloaded but db(micro) it is as above but with a reference of
1microWatt it is not actually a u but the greek character mu


dBu is indeed dB (unloaded). It is a relic of 600 ohm line audio
systems and is the voltage that would have produced 0dBm in 600 ohms,
but since we now run into high impedances instead, must be specified
otherwise - hence dBu.

dbv would be a ratio based on two voltage levels and a reference of 1 volt

dBV, actually, not dBv.

power db caclulations are 10 log Pout/Pin
Voltage db calculations are 20 log Vout/Vin

dbFS is "decibels full scale". It is an abbreviation for decibel amplitude
levels in digital systems which have a maximum available level (like PCM
encoding). 0 dBFS is assigned to the maximum possible level. There is still
the potential for ambiguity, since some use the RMS value of a full-scale
square wave for 0 dBFS, and some use a sine wave.

No, no ambiguity, dB below full scale does not depend on wave shape,
merely how many digital levels remain unused.

this is treated the same as voltag calculations because it is based on the
signal to noise ratio.

It has nothing to do with signal to noise ratio - it is all happening
at the other end of the scale.

d

Pearce Consulting
http://www.pearce.uk.com