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Subject: Re: dB volume
From: Fred Maxwell (rockbox_at_anti-spam.org)
Date: 2004-03-19


Uwe Freese wrote:

> I would also prefer a dB display going from -something dB to 0dB.

So are you recommending that Rockbox report the +12dB point as 0dB? Or
did you want Rockbox changed so that the highest value of 0dB is what we
call 93% now?

I'm confused about all of this. A decibel is a logarithmic expression
of the ratio between two signal power, voltage, or current levels.
Assuming that P2 and P1 are power in watts, the decibel rating can be
calculated as:

dB = 10 log10 (P2 / P1)

I understand that the wattage can be measured at the headphone output,
but how is it measured internally?

Note: Decibels can be calculated in terms of the effective voltage if
the load impedance remains constant: dB = 20 log10 (V2 / V1). Since
the headphones vary wildly in impedence, that calculation is out the window.

> I don't think this is so unusual - all the modern amplifiers /
> receivers (including mine) show volume in dB.

While typical mid-fi consumer products do that, high-end audio
components and professional products generally do not:

http://www.adcom.com/preamplifiers/gfp750.htm
http://www.atma-sphere.com/products/mp3.html
http://www.legendaudio.com/prod02.htm
http://www.ys-audio.com/symphonies.htm
www.metaxas.com/pages/products/ opulence.html
http://www.tascam.com/images/popups/TASCAM_AV452Front_Rear.jpg

Regards,
   Fred Maxwell

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