Do you really like the way HDTV looks?

"Steve Maki" wrote:

Sure - you (or your gear) must know that *during* the measurement -
but
after that, the 5 dB figure stands on it's own, not connected to any
absolute level.

Maybe I misunderstood the argument....

You understood perfectly. This argument is getting ever more ridiculous.

My basic and still standing point is that there is only one definition
of dB, and it is always a way of expressing a ratio.

Sometimes, the dB number is referenced to a known, standard, power or
voltage or other level. Examples are dBA, dBm, dB SPL.

Other times, the dB figure is merely used as a way of expressing the
ratio. But in either case, the dB figure is computed the same way, and
in either case, it can be converted to an abolute quantity when a
reference level is made available.

There are no two ways of computing dB.

I tried to point this out by saying that the only way to verify an
unreferenced dB spec, e.g. a speaker response spec, is by supplying a
reference.

I think everyone has pretty much agreed to all of the above. Just saying
the same thing with different turns of phrase.

Bert

"Michael A. Terrell" writes:

Some people just don't get that you have to to have a starting point
to get a valid gain or loss reading.

That's funny - I never heard anyone say that here.

What I did hear someone here say is that a gain or loss specification
in dB is useless without an absolute reference level.
--
% Randy Yates % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC % and kiss her interface,
%%% 919-577-9882 % til then, I'll leave her alone."
%%%% % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr

Randy Yates wrote:

"Michael A. Terrell" writes:

Some people just don't get that you have to to have a starting point
to get a valid gain or loss reading.

That's funny - I never heard anyone say that here.

What I did hear someone here say is that a gain or loss specification
in dB is useless without an absolute reference level.


Are you saying that a measured, level can not be used as a reference
for a gain/loss measurement? When used as such, it is your "absolute
reference level" for measurement in dB, not dBm.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

"Michael A. Terrell" writes:

Randy Yates wrote:

"Michael A. Terrell" writes:

Some people just don't get that you have to to have a starting point
to get a valid gain or loss reading.

That's funny - I never heard anyone say that here.

What I did hear someone here say is that a gain or loss specification
in dB is useless without an absolute reference level.


Are you saying that a measured, level can not be used as a reference
for a gain/loss measurement?

How could you possibly get that from what I said? No, I am not saying
that. Of course it can.

I am making the following two assertions:

1. A dB value, by itself, cannot be translated to an absolute power
level. You must add more information in order to translate a dB value
to an absolute power level. Specifically, you must also know the
absolute power of the reference level. A dB value, by its lonesome
little self, has no way to be converted to an absolute power level.

2. There are many instances where decibels are useful without knowing
the absolute power of the reference level.

When used as such, it is your "absolute reference level" for
measurement in dB, not dBm.

What about the instances in which dB is used without knowledge
of any absolute power level, such as, e.g., the specification
of the frequency response of a speaker?
--
% Randy Yates % "Watching all the days go by...
%% Fuquay-Varina, NC % Who are you and who am I?"
%%% 919-577-9882 % 'Mission (A World Record)',
%%%% % *A New World Record*, ELO
http://home.earthlink.net/~yatescr

In article om "Steve Maki" writes:

Michael A. Terrell wrote:

If I measure 5 dB loss at some frequency in a piece of coax, is not
that a useful measurement regardless of what power level my generator
was operating at?

How do you know it is a 5 dB loss without knowing what the generator
output is?

Sure - you (or your gear) must know that *during* the measurement - but
after that, the 5 dB figure stands on it's own, not connected to any
absolute level.

Or, you just do the readings with an instrument that does not have
absolute calibration. i.e., something that can only do relative readings.


Alan

(Alan) writes:

In article om "Steve Maki" writes:

Michael A. Terrell wrote:

If I measure 5 dB loss at some frequency in a piece of coax, is not
that a useful measurement regardless of what power level my generator
was operating at?

How do you know it is a 5 dB loss without knowing what the generator
output is?

Sure - you (or your gear) must know that *during* the measurement - but
after that, the 5 dB figure stands on it's own, not connected to any
absolute level.

Or, you just do the readings with an instrument that does not have
absolute calibration. i.e., something that can only do relative readings.

Whoa. There is no such thing, if the readings are in "dB" (i.e., not
"dBm" or "dBW' or whatever).

If you're actually measuring some real, physical thing to come up with
a dB value, e.g., the loss in a cable, then you must have an absolute
reference level somewhere. In the cable loss case, the reference level
is the power input to your cable in the measurement and the referred
level is the power level measured at the output of the cable. In other
words, you can't just measure the power coming out of the cable and
pull a dB value of your hat (or any other place).
--
% Randy Yates % "And all that I can do
%% Fuquay-Varina, NC % is say I'm sorry,
%%% 919-577-9882 % that's the way it goes..."
%%%% % Getting To The Point', *Balance of Power*, ELO
http://home.earthlink.net/~yatescr

On Sat, 30 Sep 2006 16:47:52 GMT, Randy Yates wrote:

If you're actually measuring some real, physical thing to come up with
a dB value, e.g., the loss in a cable,

The input and output-power are for comparison (or the actual measurement).
In the case of cable-loss, I do not really care if I use 10 mW or 1 GW as
input-power. I only want to know how many dB's I loose on the
frequency-axis, so that's the only important parameter which I need to
know.

cheers

-martin-
--
"Every picture defines its own look, and that definition begins with the
director's intentions for the script ... I have Ingmar Bergman to thank
for letting me experiment with a kind of cinematography that utilizes
true light where possible. ... He was intensely interested in light and
how it can be applied to create a given atmosphere."
- Sven Nykvist, passed away 20-9-2006

Farewell Sven. You are a great inspiration. May you guide our lights
from heaven!

Martin Heffels t writes:

On Sat, 30 Sep 2006 16:47:52 GMT, Randy Yates wrote:

If you're actually measuring some real, physical thing to come up with
a dB value, e.g., the loss in a cable,

The input and output-power are for comparison (or the actual measurement).
In the case of cable-loss, I do not really care if I use 10 mW or 1 GW as
input-power. I only want to know how many dB's I loose on the
frequency-axis, so that's the only important parameter which I need to
know.

As a READER of the specification, that's correct. As a DEVELOPER
of the specification, you MUST make a real power measurement in
order to come up with a dB.

A dB is not power. In order to convert any measured power into a dB, you
must have a known reference level.

Conversely, you cannot convert a dB into power unless you know the
reference level.
--
% Randy Yates % "The dreamer, the unwoken fool -
%% Fuquay-Varina, NC % in dreams, no pain will kiss the brow..."
%%% 919-577-9882 %
%%%% % 'Eldorado Overture', *Eldorado*, ELO
http://home.earthlink.net/~yatescr

On Sat, 30 Sep 2006 18:23:55 GMT, Randy Yates wrote:

Conversely, you cannot convert a dB into power unless you know the
reference level.

I know, but it only matters in certain cases to know what reference-level
you are working with, like for instance sound-levels. With cable-loss the
value of the reference-level does not really matter.

-m-
--

Martin Heffels t writes:

With cable-loss the value of the reference-level does not really
matter.

It doesn't really matter when you're USING the dB specification, as
for example, when you're scoping out cable types in a catalog. But it
does matter when you're DETERMINING it, e.g., when you're the
technician on the bench measuring that cable in order to determine the
value of loss that should go into that catalog, simply because you
can't compute dB with just a single power level but rather must use
two power levels - the reference power and the referred power.
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% % 'Waterfall', *Face The Music*, ELO
http://home.earthlink.net/~yatescr