On 2010-02-03 01:59, ypatios wrote: > hello > > i understand that for sound level representation in the digital domain, a > special dB scale is used: The dB full scale (or dBFS). In this scale 0dB > represents the maximum possible magnitude of a sample before clipping (e.g.: > |x(n)|=1=0dBFS). > I noticed the scale on the vu object and it seems it has an analog look, > meaning it has some 12dB "headroom" above 0dB. > Now, in most audio software (editors like audacity, plug-ins etc..) level > meters go usually to 0dB max, and there is just an indicator to notify when > clipping occurs. > > So what is the use of the analod style scale of vu?
the dBFS has the implicit assumption that there is a "full scale". this makes total sense when you have a fixed-point arithmetic, where no value can go beyond a certain maximum point. e.g. if you have a 8bit signed signal, the values will always be in the range of [-127,128], you will never ever encounter a value of (say) 270, simply because it cannot be expressed in 8bit. now in Pd we are not using fixed-point math (well, in PDa you do; but not in "proper" Pd) but floating point math, with the agreement that values in the range of [-1.0,+1.0] are the equivalent of "full scale". this has really only a meaning when Pd interfaces with the real world: e.g. many (but not all!) audio APIs (ALSA, MMIO,...) will take the audio data as fixed point (e.g. 16bit), which has an absolute "full scale". [-1,+1] will be mapped to the audio APIs full range. therefore if you send values between [-2,+2] to the [dac~], you will notice clipping (because the conversion process between fixed pint and floating point will have under/overruns). within Pd you are free to do whatever you want with your signal: e.g. you can run your internal signals in a range [-1e5,+1e5], and it will all be just the same (as long as you take care to scale the signals back into the standardized range before your [dac~]). there is no "clipping" in Pd [1]. this means, that you can have signals louder than 0dBFS, and this is the reason why the vu-meters show a little bit of "headroom". the notion of "headroom" is obviously owed to the normal use of the meters with sort-of-normalized signals. fgmasdr IOhannes [1] for the sake of simplicity it is; obivously even floats have their limits, and there is a maximum at about 840dBFS. you probably don't want a vu-meter that has a range of almost 1000dB where you are usually using the lower 10%!
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