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From article <50146@apple.Apple.COM>, by malcolm@Apple.COM (Malcolm Slaney):
> What does 0VU really mean?  I assume it is power related but I'm not sure of


Here is a short article a friend of mine wrote about VU and audio calibration
in rather lay terms but I think it answers the question.  Please direct 
comments to the author.


------------------------------ cut here -------------------------------


	Recording Standards - Background Information & Terminology
	----------------------------------------------------------

	by Brian Minert (hellgate.utah.edu!uplherc!wicat!meph!minert)


Most of our present-day audio standards work was done over 50 years ago,
and we owe much of the work to the telephone and movie/recording industries.

dB Measurement
--------------
For example, while Harvey Fletcher was working for old Bell Telephone,
he developed technology for carrying phone conversations to long distances
by using repeaters at specific intervals along the way.  The length of the
interval was determined by the audio level, and when the audio power was no
longer high enough, it had dropped "1 Bell" and had to be boosted by a repeater!
It was soon found that dividing the unit into 10ths became easier to work with,
hence the deciBell (dB) was born.  

As a unit of measurent, the dB is RELATIVE, NOT ABSOLUTE.  For example, a 3 dB
increase indicates doubling the power, while a 3 dB decrease represents reducing
the power by half.

Test Milliwatt
--------------
In order to test and calibrate various instruments, a standard known as the
"Test Milliwatt" was adopted in 1939.  It is essentially a 1000 Hz tone across
a 600 Ohm resistor such that 1 milliwatt is dissipated by the resistor (That
calculates to a voltage of about .7746 Volts RMS).  Again, this was a telephone
company standard that grew to wider application.  The great value in the Test
Milliwatt reference is that it defines an absolute power level.

dBm Measurement
---------------
Now we have the relative measure of power, the dB, and an absolute power
reference level, the Test Milliwatt.  Industry chose to measure power levels
in dB, using the Test Milliwatt as the reference.  Enter dBm!  When measuring
a Test Milliwatt, the result is 0 dBm, or 0 dB referenced to the Milliwatt
(Test Milliwatt).  Thus measurements in dBm represent absolute levels, and
therefore are of the proper form to be used as calibration and recording
standards.

Power Level Calibration
-----------------------
The history of audio calibration involves the term "100% modulation", or
"how much power can I get out of this thing?".  High-end power is therefore
used, while still avoiding major distortion.  Please be aware that a CALIBRATION
standard is NOT the same as a RECORDING standard.  Calibration is performed on
the recording equipment, after which the recording takes place at the level
APPROPRIATE TO A SPECIFIC MEDIA/TECHNOLOGY.  For example, reel-to-reel mag
tapes are often recorded at +4 dBm, while digital CVSD might be recorded at
-10 dBm.

						Recording Standards - Page 2

Distortion
----------
I just used the term Distortion without defining it.  Here goes!  When the
original analog waveform shape is altered from its' true form, it is called
distortion.  Some examples include:

	Clipping Distortion - the waveform is amplified so high that the
			signal peaks attempt to go beyond the amplifier
			voltage range.  The result is a plateu (flat top)
			effect on a sign wave.

	Phase Distortion - electronic components such as capacitors and
			inductors respond differently at different frequencies.
			When such effects are prominent, the audio signal
			is slightly delayed in a frequency-dependent manner.
			Proper design and proper connections between
			equipment avoid phase distortion.


Noise Floor and Distortion Threshold
------------------------------------
How is the appropriate record level decided?  Each technology has a level of
silence.  Examples include a blank tape, uLaw PCM FF pattern, etc.  When
"silence" is played, there is always "hiss" that can be heard and measured at 
some low level.  That level is known as the Noise Floor.  Floor noise is 
caused by imperfections in the power supply, amplifiers, etc.  For good audio 
equipment, the noise floor will be far below -70 dBm. 

In reversal, there is some maximum input level that can be tolerated, above
which clipping (sine wave peaks are flattened on the top) or other types
of distortion become significant.  This is called the Distortion Threshold.
For example, telephone levels must be limited to +6 dBm, and even that is
unpleasant.

Signal-to-Noise Ratio
---------------------
We are now left with some number of dB between the Noise Floor and the
Distortion Threshold.  It is between these two levels that usefull recording
takes place.  This number of dB between the two levels is called Signal-to-
Noise Ratio.  For most technologies, this full range cannot be used.

Dynamic Range
-------------
For most technologies, there is a "lowest record level".  In PCM, that is heard
by alternating between FF/7F (silence) and FE/7E (almost silence).  The highest
level is heard by using values of 00/80.  The difference between these two levels
is the dynamic range, meaning the range over which there can be level change.
Dynamic range is almost always less than the Signal-to-Noise Ratio, because
recording levels need to keep well away from the Noise Floor and Distortion
Threshold.

						Recording Standards - Page 3

dBm Recording Level and Headroom
--------------------------------
Recording Level (in dBm) represents the average power level seen during an
audio sample (typically of 10 seconds length).  A Record level is chosen
for a particular media by choosing a level far enough above the lowest record
level (which is already well above the noise floor) to record quiet sounds,
yet leaving much of the available dynamic range above it to safely record the
peaks naturally associated with human speech.  The area between the Record
Level and Distortion Threshold is called Headroom.  The actual value chosen
for recording a particular media or technology is then based on experience
and best judgement.

VU Meter
--------
One of the most popular instruments ever used while recording audio is the
VU meter.  It was in use as early as the 1920's, and the movie industry was
using it by the 1930's.  It is a mechanical device meant to respond in a manner
similer to the human ear, and displays an indication somewhere between peak
and average for complex (speech/music) waveforms.  VU is dimensionless, where
units of VU refer only to the VU itself.  It is a relative measure only, and
is used in the following manner:

	In a recording studio, the calibration engineer will adjust calibration
	(calibration = "hidden and not to be touched") levels FOR THE
	PARTICULAR MEDIA TO BE RECORDED ON, such that the proper dBm level is
	being recorded on the particular media when the VU meter is "bumping"
	on 0.  The calibration engineer then locks the cabinet.

	The studio recording technician now uses the studio for recording audio.
	He adjusts mixer inputs such that 0 VU is his goal, based on judgement
	and experience.

Again, VU meters themselves display only relative measurements, not absolute
levels.  When properly calibrated to a particular media, it will indicate the
safe recording zone for studio work.  As you can see, it is totally impossible
to discuss level standards in terms of VU, because any standard can be met at
0 VU by proper studio calibration!