Questions on Levels
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:
(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.
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