PStamler wrote:
On Nov 18, 8:39*pm, Randy Yates wrote:
* 1. If you backtrack to 0 dB from this wikipedia article
* *http://en.wikipedia.org/wiki/Line_level
* the reference levels for dBu and dBV are 0.775 VRMS and 1 VRMS,
* respectively. Is this correct? Are the reference levels I've given
* here exact?
No and yes, respectively. The reference level for dBu is actually
0.77459666924148337703585307995648V, according to my calculator --
that's the voltage equivalent of 1mW into a 600 ohm load. The
reference level for dBV is exactly 1V.
* 2. Is there a standard for converting from dBFS to dBu? If so, *
what is it?
No. There's an informal standard used in the film industry and in many
broadcast applications of +4dBu = -20dBFS, but it's never been
codified as an official standard. The informal standard, though, is (I
believe) based on an rms scale -- in other words, a tone that would
read 0 VU on a VU meter calibrated to +4dBu would be -20dBFS. Correct
me if I'm wrong on that last bit, but that's what I think is the case.
The optimum reference headroom (the difference between reference level
and full-scale level) would vary according to the application and the
equipment used. With lots of bits to spare, 20dB would be fine, but if
you were recording to 16-bit CD standards, you would have thrown away
20dB of your potential dynamic range and dropped your signal/noise ratio
to around 45 dB for average levels. (Digital noise sounds horrible, much
worse than analogue noise, so you really do want to keep clear of it.)
For something like A.M. broadcasting, a reference headroom of around 12
dB is more common and that is also quite satisfactory for 16-bit
recordings provided you are prepared to control the gain intelligently
on the very loudest passages. If part of the chain is in mono, there is
the further complication that two stereo channels can add to give any
level from much lower to 6dB higher, depending on coherence and phase.
For professional use, the system of marking the beginning of a recording
with a burst of reference tone (or a series of squeaks if equalisation
is involved) is the simplest way of ensuring the correct playback or
copying level.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk