Mike Rivers writes:
On Nov 18, 9:39Â*pm, Randy Yates wrote:
Â* the reference levels for dBu and dBV are 0.775 VRMS and 1 VRMS,
Â* respectively. Is this correct? Are the reference levels I've given
Â* here exact?
Exact for dBV, close enough for engineering purposes for dBu.
Right. As Paul said, dBu reference is the voltage corresponding
to 1 mW into 600 ohms. I knew this - just temporarily forgot (
).
Â* 2. Is there a standard for converting from dBFS to dBu?
No. This is a great point of confusion. You can't directly convert
them because they describe different kinds of units. It's like
converting the number of apples in a bushel to the number of potatoes
in a bushel.
Well, yeah, I didn't mean that way. What I meant to ask is if
there is a standard way to map the full-scale output of a DAC
(or input of an ADC) to a specific voltage voltage level.
There are a few conventions, however, but mostly there's things that
you choose for yourself, not a standard to which a manufacturer
adheres. Back when people had VU meters and digital recorders had
readable meter scales and an adjustable input level control, there was
often a mark typically somewhere between -16 and -20 dBFS on the
digital meter that was the recommended point corresponding to 0 VU.
And 0 VU correspondings to +4dBu?
But it's rare to see that any more. If you're working with 24-bit
converters and material with fairly high dynamic range, I'd recommend
0 VU (at whatever level that represents) provides a record level of
-20 dBFS. But it's up to you.
That's the same translation Paul mentioned: +4dBu = -20 dBFS.
No one has yet answered the question about whether it's FS sine
or FS square.
One other question: is there a specification on the absolute maximum
signal voltage magnitude for a line level output?
No specification or standard, but since most gear you'll find today
operates from a +/- 15 volt power supply, it's rare to find a maximum
output level before clipping much higher than +24 dBu. There are
exceptions, of course.
Thank you, Mike.
You'll find some discussion of these issues in the Gozintas and
Gozoutas and Meter Madness articles in the Technical Articles section
of my web site. http://mikeriversaudio.wordpress.com
OK, great. Thanks for the pointers, Mike.
--
Randy Yates % "Midnight, on the water...
Digital Signal Labs % I saw... the ocean's daughter."
% 'Can't Get It Out Of My Head'
http://www.digitalsignallabs.com % *El Dorado*, Electric Light Orchestra