AC130: Audio Channel Limiters???

C

ChrisHarding

Well-known member
Hi Guys

My AC-130 manual seems to be very vague about specs regarding the audio limiters on Channel 1 and 2 !! It shows a little diagram with a vertical line at -12db and another line at 0db. Does anyone know when the limiter actually kicks in when you are manual audio?? and at what level...the diagram seems to indicate that any signal higher than -12db is attenuated down to -12 ... I admit I do like my audio level at weddings to be around the -6db level. Does the limiter "set itself" on a high level and then keep everything below that level.

Who knows exactly how the limiters work and do they do an "auto adjustment" or just clip the waveform...If you run a test in auto the waveform with loud DJ music most definately looks like it has been given a "haircut" the camera seems to chop the top peaks of the signal but doesn't do anything to the bottom part of the waveform.

I would love to have some clarification on the limiters from someone who knows?? What do most shoot on..I have been, for now, just running manual on both XLR's with limiters off as I would rather have some peaks to contend with during wedding vows than have everything dropped too low!!! I sometimes will have a bit of wind noise (often from the officiant's mic) and if the limiter "sees" that as an excess level I don't want it to then drop my mic level down based on that one wind spike!!!

Some clarification would be very nice and I can see why Barry has a book out for the 130/160...the manual is really poor nowdays!! (My little Lumix FZ50 still camera has a way better written manual!!)

Chris
 
I can't answer specifically for the AC130/160, but thought some general info might be of some help.

Unlike an ALC, a limiter still requires that you set the level manually. It just takes care of the peaks. If the signal is continuously needing to be limited, the result will be a very compressed signal that will tend to "pump". Most limiters start operating at around -6dB, but from what you say, it sounds like the limiter in the 130 starts at -12dB. That doesn't mean it will clip the signal to -12dB, but rather it will progressively decrease the gain as the level to the input of the limiter rises above -12dB. With the fact the we have 16 bits of data and pretty good noise reduction in most modern NLEs available to us I don't have any problem erring slightly on the low side. A clipped signal is unusable as far as I'm concerned.

One thing to be aware of is that it's possible to clip the signal in the pre-amp before the limiter, in which case you'll just end up with a distorted mess anyway.
 
as the post above said- limiter is for unusual spikes in audio. it keeps it from over modulating into useless audio, but if you rely on it too much -- in general all your audio is useless. the limiter doesn't affect audio when it is 'low'.

as Mike also previously said - it is easy to raise the volume, in post, to the proper level vs bring down slightly distorted audio that was limited by the camera.
 
Thanks Guys

The auto setting on the 130 is really poor and clips audio quite badly. I put the audio limiters on during a 2 hour Realty shoot this afternoon (I use a lav mic via transmitter/receiver for a running narration) and purposely lifted the Channel 1 gain from -40db to -50db..the audio track was clean and perfect, except, because I left the level control where it was (intentionally) all the audio was around the -2 to -3db mark ..way too loud and I had to pull the track down by 4db...the interesting thing is the level was never clipped at all, despite being way too high so the limiter certainly doesn't come in at the -12db line at all.... my audio at some stages was peaking on the camera meter into the red so maybe it clips anything over 0db????? The audio waveform in the NLE certainly didn't show any sign of clipping even at these high levels!!! Do the same exercise with a channel switched to "auto" the the top of the waveform looks like it has been given a haircut...all the waveform peaks are brutally slashed...very strange situation!!!

I do remember many years ago my Panny GS500 had a "Manual + AGC" setting and that used to only clip audio that was over +8db!!!

Your input is much appreciated!!

Chris
 
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