[sdiy] doh!

[media.nai at rcn.com]

>My M-Audio sound card, when asked to put out a 0 dB (relative to +4)
>>signal, outputs a 10 volt peak to peak sine wave.
>
>So by somebody's measure, 10 volts peak to peak is +4 dB.

Don't let "+4" confuse you.

That would mean by someone's measure, 10Vp-p is 0dBu.

>Any ideas?

Yes, digital meters employ a different system.  Digital meters are offset
because anything above "zero" is clipping.  This is obvious if you use an
analogue console with a digital recorder.  Based on what you say, that
would indicate the maximum output of your soundcard is around 20dBu.

For analogue:

dBu is dB referenced such that .775V into 600 Ohm is 1 mW, and Decibels are
logarithmic so

dBu = 20 log (Vrms/0.7746) and

Vrms = .707Vpk  (Vpk = Vp-p/2)

So any signal below approximately 1Vpk is shown as a negative value on an
analogue meter, while every reproducible signal on a digital meter is shown
as a negative value.  I guess it makes zero seem more absolute :)

Yes, I'm aware that most digital meters allow a certain amount of "overs".

>> I recently said that -/+5V or 10Vp-p is about 20dBu.
>>
>> It isn't.  It's about 13dBu.
>>
>> 10Vpk (or 20Vp-p) is about 20dBu.