[Ardour-Users] Ardour input level issues

[John Rigg]

On Wed, Oct 24, 2007 at 06:39:54PM +0100, Steven Chamberlain wrote:
> Isn't 0dBFS 'supposed' to be the same as 0dBu in a 'normal' setup (hence
> the reason for having standard line levels of +4dBV and -10dBV)?

No. 0dBFS is full scale in the digital domain. Standard analogue line
level is +4dB, but analogue full scale (ie. clipping) is usually
well over +20dBu on modern equipment. 

0dBFS is the clipping level of your AD and DA converters. Ardour can
use higher levels internally because it uses floating point, but your
sound cards can not because they use integer samples.

> I assumed that the Delta 1010 trying to 'cheat' in some way, sacrificing
> accuracy of its ADCs for very quiet signals, in order to avoid
> distortion when a sound source is allowed to exceed 0dBu.
 
The Delta 1010 has no concept of levels. It's the metering in the
software that shows levels relative to digital FS.

> > Steven, I suspect that by adding those amplifier plugins you're
> > wasting the top 20dB of resolution in your Delta 1010s, and effectively
> > using only the lower 20 bits or so.
> 
> Yes, I believe it would throw away 20dB of dynamic range on the inputs
> and outputs.  I was *hoping* it wouldn't be important considering the
> SNR of either my desk or sound card, but I'm open to suggestion here.

You are almost definitely throwing away 20dB of dynamic range. Low
level detail like reverb tails will be lost, despite the `low'
noise floor in your gear. Why pay for high resolution and then throw
it away?

The important thing in both the analogue and digital domains is
to avoid clipping. The number of dB displayed on your meters
is just an aid to help you do that.

> My intention was to have the meters in all of my audio software match up
> with the meters on my desk.  I simply found it annoying to have the
> meters on my desk not match the meters in Ardour and other audio
> software.  Is that sensible?

Not very. There is very little connection between the analogue
levels on your desk and the levels displayed in software as they
use completely different references. You need to think of those
dB readings as levels relative to something (where the `something'
is different in the analogue and digital domains), and not as
absolute levels.

FWIW I've been using three Delta 1010s with an analogue mixer
for quite a while without any level problems. I just make sure
that analogue signals are a sensible (non-clipping) level on the mixer's
meters, and that the digital signals are a sensible level (also
non-clipping) on Ardour's meters.

Hope this helps.

John