On 19 Mar 2006 09:37:00 GMT, John Phillips
wrote:
On 2006-03-18, Don Pearce wrote:
On Sat, 18 Mar 2006 17:54:15 GMT, "don" wrote:
dbFS is "decibels full scale". It is an abbreviation for decibel amplitude
levels in digital systems which have a maximum available level (like PCM
encoding). 0 dBFS is assigned to the maximum possible level. There is still
the potential for ambiguity, since some use the RMS value of a full-scale
square wave for 0 dBFS, and some use a sine wave.
No, no ambiguity, dB below full scale does not depend on wave shape,
merely how many digital levels remain unused.
This puzzled me.
The first quote (from don, not Don) is the opening part of the DBFS
entry in Wikipedia - see http://en.wikipedia.org/wiki/DBFS. I think it
is correct at least up to the final sentence about ambiguity. Then it
becomes at least ambiguous itself.
The actual ambiguity seem to be whether, when a waveform is said to
have amplitude x dBFS, you mean the peak amplitude of the waveform or
its RMS amplitude. Thus I think the fundamental ambiguity is not as
stated in the Wikipedia article about whether you use a sine or square
wave as reference.
Like Don (not don) I always assumed with dBFS you implicitly meant the
peak value of the waveform because of the nature of its representation
in a system having a waveform-independent overload level of 0 dBFS.
I had to think about this a bit when doing some FFTs (which usually work
in power/energy terms) on quantized signals. Maybe some people are more
comfortable to think of waveforms in power or energy terms however they
are represented, even when power or energy is probably no longer relevant.
Think of it this way:
By how many dB would you need to increase the signal level to hit the
limit of the ADC?
That is how many dB below full scale you are, and it ties in perfectly
with my definition. You don't concern yourself with what shape the
wave is - merely how tall it is. So yes, it is the peak-to-peak
amplitude that determines this, not the RMS. The former can be derived
from the latter for known wave shapes, but not for music.
d
Pearce Consulting
http://www.pearce.uk.com