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

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

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

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.

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

"David Peters (UK)" wrote in message
...
On 12 Mar 2006, Serge Auckland wrote:


Back in the days of analog recording 0 db was 1 volt. Since
this was analog the 0 db level could be and was often exceed.
With the advent of digital CDs the 0 db level became the
maximum level, which can not be exceeded because of the digital
format, and is supposed to be 2 volts. Since these are maximum
levels the average will much lower. Microphone levels are lower
and vary widely.

Not quite. Firstly, a dB is a relative level, not an absolute,
so without stating the reference, a figure of "xdB" is
meaningless. Originally, 0dB was referenced to a power of 1mW
into a load of 600 ohms, and was referred to as 0dBm. Later, the
same voltage level, but unloaded, that is, without reference to
a 600 ohm load became 0dBu (that is, unloaded) Note that the
voltage level is the same in both cases (0.775v, or 1mW into 600
ohm) There was a strange semi-standard evolved of referring to
1V rather than 0.775v and that was 0dBv.

Digital outputs are referred to maximum digital output (when all
the bits are 1) and that is called 0dBFS (0dB Full Scale). It
has NO analogue equivalent, as analogue can keep getting bigger
without limit, digital can't get any bigger than when all the
bits are 1. In Digital-Analogue conversion, a number of
different conversion levels have become more-or-less standard.
The EBU (European Broadcasting Union) have defined 0dBFS digital
to mean +18dBu analogue after conversion. The USA prefers that
0dBFS = +24dBu because that provides 20dB headroom above 0VU. A
few dissidents prefer +25dBu as that's 1dB better than
+24...........

CD players have evolved a standard output of 2v analogue for
0dBFS, but as far as I'm aware, there is no official standard
for this.


I can't say I understand all of what you write but the parts I do
understand are very useful to me. Thank you for posting.

Are there any web sites or documents which explain this sort of thing
for a beginner: rigorously but not going too fast.