"Stewart Pinkerton" wrote in message
...
On Wed, 02 Nov 2005 17:24:40 GMT, "Jim Gregory"
wrote:

"Stewart Pinkerton" wrote in message
. ..
On Wed, 02 Nov 2005 00:11:55 GMT, "alan ralph"
wrote:

Hi,

At a radio rally, I bought a ALC/AGC module for £5 which seems to be for
audio use. On the supplied data sheet is shows the input and output as
+4dBu. (Unity Gain) With an ALC range of -16 to +14 dBu and headroom
18dB.

Does anyone know what a dBu is ?

It's an old pro-audio industry reference voltage of 0.775 volts rms,
being the voltage required for 1mW into 600ohms. The U stands for
unloaded or unterminated, to show that it's a voltage rather than an
actual power level.

Your quoted value of +4dBU is 1.223 volts rms.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

I had thought dB with the U or u suffix stood for a level at unspecified
impedance, whilst dB followed by an m stood for a level referenced to 1
milliWatt (i e,
term'd 600ohm).

Yes, which is exactly what I said.

U is sometimes used in Europe for denoting voltage, yet measured using the
SI term V.

I think you'll find that V was a standard term, somewhat before SI
standards were thought of................. :-)

Incidentally, AF tone at +4dBu, generated from a low-Z source, equals the
popular
datum of 0VU (just below red zone on a rectifier-driven VU meter with its
series resistor in place).

Actually no, 0VU is 0.775 volts rms regardless of source impedance,
didn't you read the first part of your *own* post?

Usually on critical analogue inputs there is 4-6dB of headroom beyond this
point.

Depends on the recording medium, doesn't it?
--

Stewart Pinkerton | Music is Art - Audio is Engineering

No No No That's a howler! 0VU certainly does not = 0.775V
A needle poised at -4VU is read from applying tone at 0.7746V rms (normally
'0' level in UK) to a standard VU meter.
[Bell Labs volume units, the U here is nothing to do with unspecified.]

And if the tone source Z is over 400 Ohms, the VU meter's build load will
impact (reduce) the true level.
Ideally a VU meter should be harnessed via an isolating driver amp.

Critically, available headroom above a stated reference level applies only
to certain downstream audio equipment (not necessarily recorder/s) that lies
unprotected by "brick-wall" limiters.
Jim