XPost: rec.audio.pro, comp.dsp   
   From: mrivers@d-and-d.com   
      
   On 11/20/2010 3:09 PM, Randy Yates wrote:   
      
   > (for   
   > dBFS). Some people say it's a peak (instantaneous) measurement, yet I   
   > see meters that use it for RMS measurements. I'm afraid the truth is   
   > that there is no universal meaning for it like there is for dBm, dBV,   
   > and several other dB units.   
      
   You're still not getting it, Randy. 0 dBFS has a precise   
   definition. What it doesn't have (and you seem to object to   
   "not definitions") is a magnitude, either voltage or power,   
   that relates the maximum digital number that a system   
   component can deal with to a physical property that can be   
   measured. You don't MEASURE dBFS, you look at the number   
   represented by the bits at some time and that's it.   
      
   If you were to take a bunch of samples of program material   
   over time, represent them as dB relative to digital full   
   scale, and plug them into the general RMS formula, you could   
   indeed come up with an RMS value for that set of numbers.   
   But what would be the value of that information? It will   
   always be below zero, but you can't just crank up the level   
   until your "RMS dBFS" is closer to zero unless you don't   
   care about clipping or you're working with a known,   
   continuous waveform.   
      
   So you're getting engineering answers. We tend to be   
   practical folk, and use concepts that are physically   
   meaningful, not purely theoretical.   
      
   > I'm not comfortable with the concept of "instantaneous power." Rather, I   
   > think we have to just concede that the "dB" sometimes breaks tradition   
   > and works with instantanous quantities rather than power.   
      
   Initially dB referred to power because the Bel was a measure   
   of acoustic energy (which becomes power when related by   
   time). But it's always been a ratio to a given reference.   
   The Telephone Company (tm) defined a Transmission Unit as   
   the amount of attenuation in a mile of cable that could just   
   be detected by an average listener. This was important in   
   the days when you had to talk louder when making a long   
   distance call. It turned out that 1/10 of a Bel was about   
   equivalent to a Transmission Unit, so the deciBEL became a   
   useful measure.   
      
   As commonly used today, dB without any modifiers is usually   
   understood to be sound pressure level referenced to a   
   specific pressure in Pascals. We have "units" like dBA,   
   which means sound pressure measured through a bandpass   
   filter of a known transfer function. We have the "20"   
   formula for dB as a ratio since power is the product of two   
   physical quantities (voltage and current) where voltage is   
   only one, so we make then numbers work by compensating for   
   the "squared" term in the power equation.   
      
   If you have a dollar and I have fifty cents, you can say you   
   have 6 dB more money than I have (or maybe 3 dB more   
   spending power). If a TV station increases its power from   
   50,000 watts to 100,000 watts, that's a 3 dB increase. If   
   the digitized value of a sample is 1 bit smaller than   
   another sample, that's half the value, so we say that its   
   amplitude is 6 dB lower. If 1111111111111111 (that's 15 bits   
   plus the first bit representing the sign) is full scale,   
   then 111111111111111 is -6 dBFS.   
      
      
   --   
   "Today's production equipment is IT based and cannot be   
   operated without a passing knowledge of computing, although   
   it seems that it can be operated without a passing knowledge   
   of audio." - John Watkinson   
      
   http://mikeriversaudio.wordpress.com - useful and   
   interesting audio stuff   
      
   --- SoupGate-Win32 v1.05   
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