Trying to understand Zero VU, etc.

buzzsmith · Post by **buzzsmith** » Thu Oct 11, 2007 1:36 pm

I••™m the first to admit that I••™m more of a musician/arranger/producer than a tech guy but I••™d really appreciate a technical (maybe for Dummies!) answer to this whole ••œWhat is 0 db question••

sdemott · Post by **sdemott** » Thu Oct 11, 2007 6:06 pm

Let's start by defining the differences between all the different dB types - that may help.

dBU = referenced to .775 volts: 20*log(voltage/.775)
dBv = referenced to 1 volt: 20*log(voltage/1)
dBspl = referenced to .0002 Pascals: 20*log(spl/.0002)
dBm = referenced to 1mW: 10*log(power/1mW)
dBfs = referenced to your bit depth: 20*log(sample level/2^bitdepth)

You see dB all on it's own means nothing - it's a comparison to a reference and you must define the reference (u, V, spl, m, fs).

so - the question now is: what is each digital device's reference level. There are a few loose standards: -20dBfs = 0 VU = +4dBU, this is the original AES suggested reference. Maybe it's -18dBfs (the EBU standard) or -14dBfs (another loose standard used by some manufacturers). This should be labeled somewhere in the manual or on the unit itself.

The task would be to find the reference point of the 2408 & 24i. Let's assume it's -18dBfs. So you would send a 1k sine wave (though uncorrelated pink noise is even better) at -18 out of Atmosphere and the Mackie should read 0 VU...and an analog sine at +4dBu should be coming back in at -18dBfs.

The caveat with the Mackies is that they are not referenced to +4dBu, nor are they referenced -10 dBv. They seem split the difference (which is why you where getting a +10 and not a +12 with Atmosphere at full output). For the sake of simplicity, you can ignore that and setup as described above.

As far as the OctoPre - I'm a little fuzzy on the exact setup/routing you have in place. But, if I understand you, you have a line level signal going out to the OctoPre. Is that correct? Is that coming from the Mackie or a different source?

At least it's a start...

buzzsmith · Post by **buzzsmith** » Fri Oct 12, 2007 6:04 am

sdemott wrote:As far as the OctoPre - I'm a little fuzzy on the exact setup/routing you have in place. But, if I understand you, you have a line level signal going out to the OctoPre. Is that correct? Is that coming from the Mackie or a different source?

SD...

(Thanks for the dB defs, BTW)

In my test, yes, it was line level coming from the Mackie's Conrol Room Outs (volume fully clockwise)

In real life, though (the Mackie is only used for monitoring and headphone sends), it's just a straight condensor microphone to any input on the Octopre. As mentioned, even with the Octopre's trim (not really a trim...if I'm reading the specs correctly, it really is a gain boost) set fully counterclockwise, if the drummer really "pops" the snare, the Octopre LEDs show no or minimal activity, but the meters within DP and the waveform will show some clipping.

http://www.focusrite.com/product/octopre/spec/

I'll try your more sophisticated tests later today.

Thanks!

Buzz

monkey man · Post by **monkey man** » Fri Oct 12, 2007 11:24 am

Well said, Steve.

sdemott · Post by **sdemott** » Sun Oct 14, 2007 11:26 am

buzzsmith wrote:
sdemott wrote:As far as the OctoPre - I'm a little fuzzy on the exact setup/routing you have in place. But, if I understand you, you have a line level signal going out to the OctoPre. Is that correct? Is that coming from the Mackie or a different source?
SD...

(Thanks for the dB defs, BTW)

In my test, yes, it was line level coming from the Mackie's Conrol Room Outs (volume fully clockwise)

In real life, though (the Mackie is only used for monitoring and headphone sends), it's just a straight condensor microphone to any input on the Octopre. As mentioned, even with the Octopre's trim (not really a trim...if I'm reading the specs correctly, it really is a gain boost) set fully counterclockwise, if the drummer really "pops" the snare, the Octopre LEDs show no or minimal activity, but the meters within DP and the waveform will show some clipping.

http://www.focusrite.com/product/octopre/spec/

I'll try your more sophisticated tests later today.

Thanks!

Buzz

I'm not familiar with the OctoPre, but from the specs, the 1st LED (signal present) illuminates at -20dBfs...but, that isn't giving any indication of the reference of that to the overload LED (+22 dBu). If we assume that it is +4 dBu what makes sense to me is this:

If you are seeing any metering on the OctoPre you're getting a healthy signal (0VU, +4 dBu) leaving about 18 dB of headroom. If you are getting this with the gain @ 0, either [a] you have a hot mic (sensitivity somewhere in the 40mV/Pa range) or the OctoPre is applying significant gain at the full left position. If the case is the former, try a less sensitive microphone. If the case is the latter, you may find that the OctoPre is not a suitable drum/percussion preamp. The omission of an output level on the channels (at least that's how it look from the secs) makes the OctoPre very limited in it's usefulness as a pre for loud/transient sources.

I'd be curious to hook an OctoPre up to a familiar metering device (like my Metric-Halo) and feed it some signal to see what it's really doing. Do you have any metering on your inputs that you can trust to see what is really coming out of the OctoPre?

buzzsmith · Post by **buzzsmith** » Mon Oct 15, 2007 3:35 pm

Steve:

Definitely agree on the lack of an output trim...this will be the last time!

As far as mics go, it seems to not make a diffrence if they are simple SM-57s, Rodes NT5s, AKG 414's, etc.

It may definitely be a complete hardware incompatibility issue, but before I throw in the towel completely, I, of course, have...

••• posted here
••• posted at GearSlutz
••• contacted Focusrite
••• MOTU Tech Support (very cryptic, seemingly non-interested responses such as..."As I mentioned previously, the 24i's inputs and outputs are referenced to +4dBU, which makes it compatible with most professional-quality audio gear. Perhaps you should contact Focusrite for further assistance")

But I do have an audio tech friend who's coming by Wednesday and bringing his measurement tools and other bag of tricks.

With no progress after that, it's on to eBay!

(I just hate it when 2 devices according to specs should not have a connectivity issue, but they certainly do.)

Buzz

sdemott · Post by **sdemott** » Wed Oct 17, 2007 11:20 am

I hope you get some resolution. My instinct is saying the OctoPre is not performing to spec., but that's only going on what you've posted. So take it for what it's worth.

Good luck.

buzzsmith · Post by **buzzsmith** » Wed Oct 17, 2007 12:13 pm

Well, the tech guy came by and the only way to get them to be truly compatible would be to put inline output pads on each of the 8 outs before they hit the 24i.

I figure that's about 350.00 I'm *not* spending!

Looks like I'll be going in a different multiple mic-pre direction.

Buzz