Digital power???

Jon wrote:

(snip)

> Oh yeah, dB are technically related to power levels in the first place 
> (10*log10) and voltage decibels are a 20*log10 afterthought.  I assume 
> the dB I am using on screen are related to 20*log10, because they are 
> referring to amplitude levels.  

Well, the 20*log10 gives (relative) power in dB from an 
amplitude (voltage or current).

 > Hmmm... but the "average RMS power" is
> expressed in dB(FS?) as well.  Ok I think this is making some sense...

It was my understanding that there was no such thing as RMS 
power until the FTC set the requirements for audio power 
amplifier measurements.  Not only did they define RMS power, but 
they require that an amplifier (or receiver) model number not be 
twice the watts/channel of the amplifier.  (So as not to deceive 
people.)  There are some 200W/channel amplifiers with a model 
number of 401 just to get around that law.

RMS voltage or current give the same average power as a DC 
voltage or current and a given resistor.  In that sense, there 
is no meaning to RMS power.

-- glen

Tim Wescott <tim@wescottnospamdesign.com> writes:

> Randy Yates wrote:
>
>> Tim Wescott <tim@wescottnospamdesign.com> writes:
>>
>>>[...]
>>> Both abbreviations and acronyms should be written out with all caps
>> Proof by assertion? I've given a legitimate, formal case where
>> they're
>> not, so such a statement without any justification sounds a lot like
>> braggadoccio.
>
> Yes, by god!  You're too used to mathematics where that's Against the
> Rules -- this here be Style!  I mean, if e e cummings can get away
> without caps in his _name_, for crying out loud, surely I can make
> such assertions?
>
> Perhaps you should re-read my post, ask yourself where the words
> "laser", "radar" and "snafu" came from, and note the case of the
> letters that I used.

So, you mean this was a joke? Man, I'm getting slower every year. 
-- 
%  Randy Yates                  % "Though you ride on the wheels of tomorrow,
%% Fuquay-Varina, NC            %  you still wander the fields of your
%%% 919-577-9882                %  sorrow."
%%%% <yates@ieee.org>           % '21st Century Man', *Time*, ELO
http://home.earthlink.net/~yatescr

Tim Wescott wrote:

> Both abbreviations and acronyms should be written out with all caps
> -- although some acronyms get into lower case laser-quick; you have
> to keep your radar screens in tune to find all the cases but even
> then you can run into the occasional snafu.

those aren't acronyms anymore.  they've become words and therefore can
be written in lowercase.  as for how an acronym like RMS can become
lowercase without the capacity to become a word ("erms?") here is one
potential reason:

>> "rms" is written in lower-case because that follows the general
>> accepted practice as outlined in the Abbreviations Dictionary, 9th
>> ed. (Ralph De Sola, Dean Stahl and Karen Kerchelich; CRC Press
>> ISBN0-8493-8944-5, 1995) and other sources. Quoting the CRC book:
>> "American as well as British and Canadian publishers appear to be
>> following the trend to capitalize only those letters normally
>> capitalized: proper nouns and important words in titles. They reserve
>> lowercase letters for abbreviations consisting of adjectives and
>> common nouns." This is the way it appears in the IEEE reference
>> above. Other popular examples are "rpm," and "mph."

doesn't explain why we don't also have "thd+n", "imd", "snr", "cmrr",
and "cd-r".  so i think it's safe to assume that the language is just
screwy and we can do whatever we want.

-- 
Include "newsgroup" in the subject line to reply by email (or get dumped
with the spam).

glen herrmannsfeldt wrote:

> Well, the 20*log10 gives (relative) power in dB from an amplitude 
> (voltage or current).

well dB isn't a power, it's a ratio.

but since it was originally defined as a power ratio, the formula needs 
to be changed to make it consistent with amplitudes?

still don't understand digital "power".  :-)  it's on the tip of my 
brain, though...

> RMS voltage or current give the same average power as a DC voltage or 
> current and a given resistor.  In that sense, there is no meaning to RMS 
> power.

"RMS power" is just an incorrect term for "average power" which is 
determined using an RMS amplitude.

-- 
Include "newsgroup" in the subject line to reply by email (or get dumped 
with the spam).

Jon <u035m4i02@sneakemail.com> writes:
> [...]
> "RMS power" is just an incorrect term for "average power" which is
> determined using an RMS amplitude.

Jon, I believe this terminology may be a remnant of the audio era
circa the 60s and 70s in which some audio power amplifier manufacturers
would quote "peak power" instead of "RMS power," thereby artificially
inflating their power ratings. 

Note also that, when measuring power in the real-world, it is always
appropriate to ask what the averaging time is.
-- 
Randy Yates
Sony Ericsson Mobile Communications
Research Triangle Park, NC, USA
randy.yates@sonyericsson.com, 919-472-1124

Jon wrote:

(snip)

> "RMS power" is just an incorrect term for "average power" which is 
> determined using an RMS amplitude.

Well, the FTC version had to do with amplifier marketers using 
peak power, which may be the maximum power it could put out for 
a very short time will self-destructing.   I believe, then, that 
RMS power has a time duration that it must be measured over.

While one cycle would normally be enough, I believe the FTC 
requires something like 20 minutes.

-- glen