Output Limiter Levels on SD 302? at DVinfo.net

Peter Moretti · February 10th, 2008, 03:17 AM

I'm going through my 302 settings and I see the default level for the output limiters is +20 dB.

This seems extremely high to me. What setting do you recommend?

I also wonder about the purpose of output limiters if the input limiters are also engaged. Wouldn't the input limiter stop the signal from overloading?

I'm aware of the axiom: "Two is better than one." But is something more subtle (or obvious, LOL) going on here?

If you choose to limit the input, do also you need to limit the output? (Maybe in the case of input coming from the Mix In connection, which might bypasses the input limiters.)

Steve House · February 10th, 2008, 06:32 AM

Quote:

Originally Posted by Peter Moretti

I'm going through my 302 settings and I see the default level for the output limiters is +20 dB.

This seems extremely high to me. What setting do you recommend?

I also wonder about the purpose of output limiters if the input limiters are also engaged. Wouldn't the input limiter stop the signal from overloading?

I'm aware of the axiom: "Two is better than one." But is something more subtle (or obvious, LOL) going on here?

If you choose to limit the input, do also you need to limit the output? (Maybe in the case of input coming from the Mix In connection, which might bypasses the input limiters.)

Remember the VU scale is relative so +20dB means it's 20dB above 0VU and doesn't really refer to an absolute voltage. If tone at 0VU on the mixer is set to -20dBFS on the camera or recorder, setting the output limiter on the mixer to +20dB means that any excursion that would attempt to drive the camera over 0dBFS and cause clipping on the recorded signal would be clamped off. Some workers do prefer a little more safety margin and set the limiter threshold down a bit to +17dB or +18dB. As for the input limiters, they're to prevent high SPL signals from overloading the mixer's preamps. They come before the mixer adds any gain. With low settings of the input gain and fader, a loud signal could overload the inputs without causing an excessive signal on the output.

The SD mixers meters are calibrated so that 0VU metered = 0dBu output. If you really get picky about setting everything up strictly according to SMPTE, +4dBu analog should align to -20dBFS digital with nominal peak levels hovering at -12dBFS. That means that if one goes by the book one should align 0VU tone on the SD meters to -24dBFS on a digital recorder and that in turn means that the mixer's +20dB limiters will prevent any signal excursions from exceeding -4dBFS, a good safety margin to guard against any possible clipping. But the full 24dB "headroom" of the SMPTE alignment isn't really needed for most sources and current practice is to use 0VU = 0dBu = -20dBFS or even a little hotter in order to take advantage of the better S/N ratio obtained at the higher average level. If you choose to go even higher, say setting average levels at -12dBFS as some people do, remember that even peak reading meters take time to respond and brief peaks can be higher than the meter indicates. With tone at -12dBFS you'll see voice peaking at -2dBFS or so - it might look like you're still okay but due to the meter ballistics the REAL peaks of the waveform may be much higher, high enough to cause audible distortion.

Just to make it even more confusing, EBU standards call for recording 6dB higher with 0dBu analog to be aligned to -18dBFS digital, with nominal peaks falling at -10dBFS.

Bill Ravens · February 10th, 2008, 08:29 AM

Great answer, Steve...

It occurs to me, the possibility that some people may not understand that when you refer to units of dBfs, you're speaking digital, in which 0 dBfs is the absolute max. The SD302 is an analog mixer and the units used are VU, which are analog units. In the analog world, absolute MAX is somewhat of a nebulous term, generally around +18-+20 VU as you described.

Chris Leone · February 10th, 2008, 12:08 PM

can anyone recommend a website or book perhaps that attempts to simplify all of this for us newbies?

Steve House · February 10th, 2008, 12:17 PM

Quote:

Originally Posted by Chris Leone

can anyone recommend a website or book perhaps that attempts to simplify all of this for us newbies?

"Sound for Digital Video" - Tomlinson Holman
"Great Sound for Digital Video" - Jay Rose

February 10th, 2008, 03:17 AM	#1
Peter Moretti Trustee Join Date: Mar 2007 Location: Sherman Oaks, CA Posts: 1,259	Output Limiter Levels on SD 302? I'm going through my 302 settings and I see the default level for the output limiters is +20 dB. This seems extremely high to me. What setting do you recommend? I also wonder about the purpose of output limiters if the input limiters are also engaged. Wouldn't the input limiter stop the signal from overloading? I'm aware of the axiom: "Two is better than one." But is something more subtle (or obvious, LOL) going on here? If you choose to limit the input, do also you need to limit the output? (Maybe in the case of input coming from the Mix In connection, which might bypasses the input limiters.) Last edited by Peter Moretti; February 10th, 2008 at 04:02 AM.

February 10th, 2008, 08:29 AM	#3
Bill Ravens Join Date: Nov 2001 Location: santa fe, nm Posts: 3,264 Images: 10	Great answer, Steve... It occurs to me, the possibility that some people may not understand that when you refer to units of dBfs, you're speaking digital, in which 0 dBfs is the absolute max. The SD302 is an analog mixer and the units used are VU, which are analog units. In the analog world, absolute MAX is somewhat of a nebulous term, generally around +18-+20 VU as you described.

February 10th, 2008, 12:08 PM	#4
Chris Leone New Boot Join Date: Feb 2008 Location: New York, NY Posts: 12	can anyone recommend a website or book perhaps that attempts to simplify all of this for us newbies?