Do you really like the way HDTV looks?

Paul Ratcliffe wrote:
On Wed, 27 Sep 2006 17:06:10 GMT, Albert Manfredi
wrote:

"Richard Harison" wrote in message
...

A previous posted point of mine was that SPL is also measured in
db.
After that it gets a little vague as to source. What =0db?

Normally, by convention, SPL is referenced to 20 micropascals. So
0 dB
would work out to that SPL.

0 dB is 1.
0 dB SPL is 20 micropascals.

Can you not spot the difference?

Most of the problem with this is due to sloppiness - dB is used
where
dB SPL is meant.

Back before you all got into a high school math contest, the original
thread was " Do you really like the way HDTV looks? "

I say YEAH

BTW everybody gets a 'B'.

GG

Randy Yates wrote:

Well, Albert, maybe I'm wrong. Please enlighten me and show me what
power level 5 dB translates to.

As I explained previously, dB always specify a ratio. When you are
given the reference, they can always be converted to watts (or volts,
or SPL, or whatever). This is always the case.

5 dB is a ratio of 3.16:1.

If I say 5 dBm, I'm referring this by convention to 1 mW. No need to
specify the reference, as long as we all agree on the convention used.
So 5 dBm = 3.16 mW.

If I say "this amplifier tests 5 dB louder than this reference 50 watt
amp," can you not tell me what the power rating of the amp being tested
is? It's pretty easy.

Bert

"Albert Manfredi" writes:

Randy Yates wrote:

Well, Albert, maybe I'm wrong. Please enlighten me and show me what
power level 5 dB translates to.

As I explained previously, dB always specify a ratio.

Well then what are we arguing about? Whether you call it "a ratio" or
"relative," it's one and the same.

When you are given the reference, they can always be converted to
watts (or volts, or SPL, or whatever). This is always the case.

No argument there.

5 dB is a ratio of 3.16:1.

Approximately, yes.

If I say 5 dBm, I'm referring this by convention to 1 mW. No need to
specify the reference, as long as we all agree on the convention used.
So 5 dBm = 3.16 mW.

Approximately, yes.

If I say "this amplifier tests 5 dB louder than this reference 50 watt
amp," can you not tell me what the power rating of the amp being tested
is? It's pretty easy.

You're repeating yourself. That is the point you made in the second
quoted section above.

Did I ever say that if you were given a dB value AND the reference
power level you couldn't find the referred power? No, I did not.

What I did say is that a dB specification can be useful without ever
knowing anything about the absolute power of the reference power.

So, we've discovered in this exchange that you agree a dB is relative,
i.e., it is a ratio. This is a major step forward.

The next thing necessary to straighten your head out is for you to
see that a dB specification, without an absolute power reference,
is useful.

Let's modify our speaker response example as follows. Let's say at 1
kHz, 1 meter on-axis, 1 atmosphere, 50 percent relative humidity,
phase 0 of the moon (all dark) with 1 electrical watt fed into the
speaker I measure and find 80 dB SPL.

I then measure at 50 Hz, 1 meter on-axis, 1 atmosphere, 50 percent
relative humidity, phase 0 of the moon (all dark) with 1 electrical
watt fed into the speaker and find 75 dB SPL.

I can now faithfully say that at 1 meter on-axis, 1 atmosphere, 50
percent relative humidity, phase 0 of the moon (all dark) with 1
electrical watt fed into the speaker, the power level at 50 Hz is -5
dB from the power at 1 kHz.

So I'm pretty sure in this situation you would agree that it makes
sense to specify that the 50 Hz response is -5 dB from the 1 kHz
response since you have all the pertinent information, including
the absolute reference power.

Now, if instead I state that at 1 meter on-axis, 1 atmosphere, 50
percent relative humidity, phase 0 of the moon (all dark), the power
level at 50 Hz is -5 dB from the power at 1 kHz, would this -5 dB be
meaningless? (Note that I am now omitting the actual amplifier power
of 1 W.)

No, it would not. This is because, to a reasonable approximation, a
speaker opeerating at reasonable power levels is reasonably linear, so
its response at, say, 1 W of amplifier power is going to be the same
as its response at, say, 100 mW of amplifier power.

Therefore we conclude that WITHOUT KNOWING AN ABSOLUTE POWER LEVEL
REFERENCE, A DB SPECIFICATION CAN BE USEFUL. QED.
--
% Randy Yates % "How's life on earth?
%% Fuquay-Varina, NC % ... What is it worth?"
%%% 919-577-9882 % 'Mission (A World Record)',
%%%% % *A New World Record*, ELO
http://home.earthlink.net/~yatescr

Paul Ratcliffe wrote:

On Mon, 18 Sep 2006 23:16:19 GMT, Michael A. Terrell
wrote:

I repeat, in engineering, a dB figure is always given with a
reference.

No it isn't. It depends if you are specifying an absolute level or a
relative one.


"Relative" is a reference. It isn't the industry standard, "Start
here" reference, but it is definitely a reference. If I know that I have
- 10 dBm going into a +8 dbM stage of amplification, I know to expect a
-2 dBm output if everything is done properly. If you don't know the
input level, how to you measure the gain?


I have never seen 3dBW used.

You've never done any sat. comms. then. Just because you've never seen it
doesn't mean it doesn't exist.


Now that is DAMN FUNNY, Paul. I built the equipment at
Microdyne/L3-Com. One piece is aboard the ISS, another complete system
is at Wallops Island, Virginia at the NOAA facility to received the LEO
sats for weather data. How about the pair of earth stations we built for
the Italian government? One fixed site, and one mobile for their part
in the European Space agency? I'll let you in on a little secret,
Paul. The equipment is tested, aligned and calibrated to microvolts
sensitivity, has multiple IF and video bandwidths.

The last "Sat Comm" equipment I worked on was to prepare the
Microdyne/L3-Com RCB2000 DSP based dual diversity telemetry package for
the production floor. That included translating the engineer's notes
into real, repeatable test procedures.

I also engineered for United Video, a MSO CATV company back in the
'80s. I used to do the power calculations from the bird to earth, add
the antenna gain and the LNA gain, then subtract the power divider and
cabling losses to make sure that there was going to be a large enough
margin in the signal path so we didn't have to go back and make
modifications as the bird aged.

Have you used a Fireberd to do BER measurements on a FQPSK data
stream for receivers used to track deep space probes? How about
calibrating a Diversity combiner to track within .01 dB between channels
over the entire AGC range? Aligned a set of 12 different,
electronically selectable Salen-Key video filters, like those in the
Microdyne 700 series receivers?

BTW, I still have a Microdyne "C" band signal generator in my shop at
home. I used to repair a lot of cheap, non digital sat receivers for a
side business. Have you ever tackled a repair on something like a
Collins Rockwell receiver from the late '70s? They look like Rockwell
took one of their telemetry receivers, and simply replaced the
mechanically tuned LO for Sat operations. They ran hot as hell, had no
fans, and by the time they were couple years old, Rockwell didn't want
to fix them. My record was seven repairs in one day, at the United
Video system in Cincinnati, Ohio. I received 10 by UPS on a friday
afternoon, and shipped seven out on Monday. The other three were
scrapped because the LO modules were burnt, and Rockwell wanted $15,000
to fix one of them, for a $3000 receiver.


As far as your dbW, I really can't see a need for it, if you truly
understand the hardware. It looks like a marketing creation for people
who have no idea what they are talking about.


In the last 35 years I have worked in RADAR, Microwave
communications, CATV systems, radio and TV broadcast engineer, while in
the military.

Later on, I owned a communications company where we did everything
from simple sound systems to complete commercial two way radio systems.

When my health started to fail, I went back to broadcasting, on TV
station had a 5 MW EIRP signal on a 1749 foot tower. The other was as
engineer of record for a low power local station with 1 MW EIRP on a
small 300 foot tower in the Florida panhandle. When i could no longer
climb ladders, I went into manufacturing at Microdyne. We only had one
product I didn't work on, and that was because it was a mature product,
the TSS-2000 Telemetry test System that allowed larger installations to
fully test their equipment to make sure that it was in calibration.

My health was so bad when they moved the plant out of Florida that I
ended up on 100% disability. I really miss having $1,000,000+ worth of
test equipment on my work benches.

Now, you were saying something about "Sat Comm"?

--
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

"Randy Yates" wrote:

Well then what are we arguing about? Whether you call it "a ratio" or
"relative," it's one and the same.

Mostly, because you prefer to transmit than to receive?

Here's the offending quote from you yesterday:

Conversely, dB is NOT power. *You cannot translate a dB to an
absolute power level in Watts.*

(Emphasis mine.)

Obviously, I think you will now agree, this is untrue. Just as you do
with dBm or dB SPL, or any other, once a reference is made available,
you can certainly translate dB to an absolute power (or other) level. As
I showed you, and you agreed, when comparing an amp's output to that of
a reference amp.

QED.

Bert

"Albert Manfredi" writes:
[...]
Here's the offending quote from you yesterday:

Conversely, dB is NOT power. *You cannot translate a dB to an
absolute power level in Watts.*

(Emphasis mine.)

Obviously, I think you will now agree, this is untrue.

The statement is still true. It is true now, it was true yesterday,
and it will be true tomorrow. I do not agree that the statement is
untrue.

[...] once a reference is made available,
you can certainly translate dB to an absolute power (or other)
level.

But then you do not have "dB." You have "dB and a known reference
level."

QED.

To the contrary. I think you have a logic problem.

Would you agree that it is possible for a female to be pregnant? I
think it's safe to say yes, and I agree. However, I do NOT agree that
the statement "If you're a female, then you're pregnant" is true.

This is directly analogous to the assertion "If you have a known dB
value, you can convert it to an absolute power." That assertion is
untrue. It is *possible* to convert it in certain circumstances, but
it is not sufficient that given dB one can compute an absolute power.
--
% Randy Yates % "My Shangri-la has gone away, fading like
%% Fuquay-Varina, NC % the Beatles on 'Hey Jude'"
%%% 919-577-9882 %
%%%% % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

Albert Manfredi wrote:

Randy Yates wrote:

Well, Albert, maybe I'm wrong. Please enlighten me and show me what
power level 5 dB translates to.

As I explained previously, dB always specify a ratio. When you are
given the reference, they can always be converted to watts (or volts,
or SPL, or whatever). This is always the case.

5 dB is a ratio of 3.16:1.

If I say 5 dBm, I'm referring this by convention to 1 mW. No need to
specify the reference, as long as we all agree on the convention used.
So 5 dBm = 3.16 mW.

If I say "this amplifier tests 5 dB louder than this reference 50 watt
amp," can you not tell me what the power rating of the amp being tested
is? It's pretty easy.

Bert

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?

Steve Maki

Steve Maki wrote:

Albert Manfredi wrote:

Randy Yates wrote:

Well, Albert, maybe I'm wrong. Please enlighten me and show me what
power level 5 dB translates to.

As I explained previously, dB always specify a ratio. When you are
given the reference, they can always be converted to watts (or volts,
or SPL, or whatever). This is always the case.

5 dB is a ratio of 3.16:1.

If I say 5 dBm, I'm referring this by convention to 1 mW. No need to
specify the reference, as long as we all agree on the convention used.
So 5 dBm = 3.16 mW.

If I say "this amplifier tests 5 dB louder than this reference 50 watt
amp," can you not tell me what the power rating of the amp being tested
is? It's pretty easy.

Bert

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?

Steve Maki


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


--
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 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.

Maybe I misunderstood the argument....

Steve Maki

Steve Maki wrote:

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.

Maybe I misunderstood the argument....

Steve Maki


No, you've got it right.

Some people just don't get that you have to to have a starting point
to get a valid gain or loss reading. The marking on the dial isn't
always accurate, and I did become rather spoiled with the Boonton 9200
on my bench, along with the Fluke true RMS meter with the "relative"
button. Some video cards for telemetry receivers with DC to 40 MHz
bandwidth needed the digital attenuator tested to make sure it tracked
to less than .1 dB error from 0 to -63 dB.


--
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