On Sun, 21 Nov 2010 08:46:10 -0500, Mike Rivers
wrote:
On 11/20/2010 3:09 PM, Randy Yates wrote:
(for
dBFS). Some people say it's a peak (instantaneous) measurement, yet I
see meters that use it for RMS measurements. I'm afraid the truth is
that there is no universal meaning for it like there is for dBm, dBV,
and several other dB units.
You're still not getting it, Randy. 0 dBFS has a precise
definition. What it doesn't have (and you seem to object to
"not definitions") is a magnitude, either voltage or power,
that relates the maximum digital number that a system
component can deal with to a physical property that can be
measured. You don't MEASURE dBFS, you look at the number
represented by the bits at some time and that's it.
If you were to take a bunch of samples of program material
over time, represent them as dB relative to digital full
scale, and plug them into the general RMS formula, you could
indeed come up with an RMS value for that set of numbers.
But what would be the value of that information? It will
always be below zero, but you can't just crank up the level
until your "RMS dBFS" is closer to zero unless you don't
care about clipping or you're working with a known,
continuous waveform.
So you're getting engineering answers. We tend to be
practical folk, and use concepts that are physically
meaningful, not purely theoretical.
I'm not comfortable with the concept of "instantaneous power." Rather, I
think we have to just concede that the "dB" sometimes breaks tradition
and works with instantanous quantities rather than power.
Initially dB referred to power because the Bel was a measure
of acoustic energy (which becomes power when related by
time). But it's always been a ratio to a given reference.
The Telephone Company (tm) defined a Transmission Unit as
the amount of attenuation in a mile of cable that could just
be detected by an average listener. This was important in
the days when you had to talk louder when making a long
distance call. It turned out that 1/10 of a Bel was about
equivalent to a Transmission Unit, so the deciBEL became a
useful measure.
As commonly used today, dB without any modifiers is usually
understood to be sound pressure level referenced to a
specific pressure in Pascals.
I think this is exemplary of one issue. This may be true in your
area of work, that dB without a modifier has to do with sound
pressure, but in communications dB without a modifier is generally
representative of a unitless scale factor in a system. e.g., an
amplifier that increases the signal power by a factor or ten has 10dB
of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm
comes out.
Since logarithms convert multiplication to addition, any application
of a scale factor in a signal chain, due to gain (e.g., amplifier) or
attenuation, (e.g., cable loss), can be represented in dB (without a
modifier). Antenna gain has this characteristic, but then it gets a
modifier (usually dBi) to indicate which sort of antenna provides the
reference gain.
So dB without a modifier is usually representative of a dimensionless
scale factor in the signal chain.
But not always.
We have "units" like dBA,
which means sound pressure measured through a bandpass
filter of a known transfer function. We have the "20"
formula for dB as a ratio since power is the product of two
physical quantities (voltage and current) where voltage is
only one, so we make then numbers work by compensating for
the "squared" term in the power equation.
If you have a dollar and I have fifty cents, you can say you
have 6 dB more money than I have (or maybe 3 dB more
spending power). If a TV station increases its power from
50,000 watts to 100,000 watts, that's a 3 dB increase. If
the digitized value of a sample is 1 bit smaller than
another sample, that's half the value, so we say that its
amplitude is 6 dB lower. If 1111111111111111 (that's 15 bits
plus the first bit representing the sign) is full scale,
then 111111111111111 is -6 dBFS.
--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson
http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com