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From: "Phil Allison"
Subject: Re: Audio noise in diff amps
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Thu, 19 Dec 2002 23:43:34 +1100
NNTP-Posting-Date: Thu, 19 Dec 2002 23:31:39 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Woodgate" wrote in message
> I read in sci.electronics.design that Phil Allison
> wrote (in ) about 'Audio
> noise in diff amps', on Thu, 19 Dec 2002:
> > ** For decades the term "dBm" was used in audio for reference signal
> >levels. It only rarely meant a true power level but rather a voltage
> >referred to 0.775 volts rms or 1mW at 600 ohms.
> > Seems this confused some outside the club so the "dBu" was
> >invented to remove the ambiguity.
> No, Phil, it confused people INSIDE the club, and caused spats like you
> are indulging in at present.
** Only a pom could say that. Here I am being abused and attacked by
Mr Pearce who cannot see HE is in error. Nor can he see his error with the
dangerous de-earthing advice either.
(Do you really mean power, or just level
> relative to 0.775 V?)
** Of course the figure is voltage level. 2.4uV is a voltage level
isn't it ?
The "noise power" of all value resistors is the same - why would
ANYONE post that figure and then say it was for a 22 kohm one ??
This why 'dBu' was adopted by CCIR (later ITU-R)
> and by AES in due course, even though it doesn't gel with the rules for
> symbols in IEC 600027.
> The r.f. people sill use 'dBm' (which should be 'dB(mW)' according to
> IEC 600627, and is then *obviously* a power level). Some of them still
> have to cope with reconciling levels in dBm with both 50 ohm and 75 ohm
** If only *you* could convince Mr Pearce that "dBm" has another meaning
Because I cannot.
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