XPost: rec.audio.pro, comp.dsp   
   From: spam@spam.com   
      
   On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates    
   wrote:   
      
   >John O'Flaherty  writes:   
   >   
   >> On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates    
   >> wrote:   
   >>   
   >>>kludge@panix.com (Scott Dorsey) writes:   
   >>>   
   >>>> In article , Randy Yates   wrote:   
   >>>>>   
   >>>>>If dBFS is defined as   
   >>>>>   
   >>>>>  dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave),   
   >>>>>   
   >>>>>where XRMS is the RMS value of the digital data stream, and you're   
   >>>>>generating a "digital square wave," then you are wrong. The digital   
   >>>>>square wave can go to +3dBFS as defined above.   
   >>>>   
   >>>> dBFS has not got a damn thing to do with sine waves or reference levels   
   >>>> or anything in the analogue world.   
   >>>   
   >>>Again, I'm not asking how it's not defined, I'm asking how it is   
   >>>defined.   
   >>>   
   >>>You guys have danced around this one all day. It's getting humorous.   
   >>>   
   >>>> It has ONLY to do with how far a digital level is below the point at   
   >>>> which the digital value reaches full scale (all bits on).   
   >>>   
   >>>If you know what it means, and you're literate, then you should be able   
   >>>to come up with a precise definition. I haven't seen one yet.   
   >>   
   >> The problem is that dB is defined as a unit of power, usually applied   
   >> to signals with some time duration.   
   >   
   >YES!!! Thank you, John!   
   >   
   FFS. The dB is NOT a unit of anything. It is a ratio expressed in log   
   form for convenience (to avoid huge numbers). Nothing more and nothing   
   less.   
      
   >> Obviously, a square wave at full scale of a converter has more power   
   >> than, say, a sine wave or a 1% duty cycle signal at full scale. So,   
   >> how can one define dBFS so it represents how the figure is actually   
   >> used?   
   >   
   >Not a bad question, but I was hoping there was _THE_ definition.   
   >Apparently there is not. And this is really the crux of the issue (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.   
   >   
      
   This is getting increasingly ridiculous. FS is simply the point at   
   which you hit the ceiling. There is no more. You have limited. dBFS is   
   the ratio of the present signal to that ceiling. The relevant   
   measurement is instantaneous - this very next sample is the one you   
   have to care about. Normal good practice would suggest that you keep   
   between 10 and 20dB below to allow for the unexpected. If you do that,   
   you won't be troubled by either overload or noise.   
      
   >> How about "a signal at 0 dBFS is one whose instantaneous power   
   >   
   >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.   
   >   
   There is no problem with instantaneous power. Instantaneous energy is   
   the one you can't be doing with. That final sentence of yours is   
   simply gibberish.   
      
   >> reaches but never exceeds the instantaneous power associated with full   
   >> scale of the converter"? Modifying your formula above, dBFS = 20 *   
   >> log_10(peak signal voltage / converter maximum voltage)   
   >   
   >That is essentially what I wrote last night. Thanks for your input, John.   
      
   How many people have slightest idea what power their converters will   
   handle? Everybody thinks in terms of voltage, which is the fixed term   
   in non-matched systems such as audio gear.   
      
   d   
      
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