the real nitty gritty on audio levels during acquisition

[Josh Bass]

What is the real deal? I would aim for not getting anything higher than -12, but the show I shoot for is on public access and has to boost their levels on the final output. It's easier to record to loud and then lower it in post than record to softly and raise it in post, right? Am I technically okay as long as I don't go past 0?

[Dean Sensui]

Josh...

"0" in digital audio is the absolute limit, beyond which anything gets mercilessly chopped off. "0" in analog is about 9 or 10 dB below the absolute limit, depending on the particular recording system. The reason for having different standards is baffling to me. But it means that the analog equivalent of a "nominal" reference point in digital is somewhere around -12 or -14 in order to provide "headroom" to accomodate transient peaks.

I generally shoot to keep the average max level somewhere around -14 and leave headroom for the occasional peak.

When editing I set my reference tones at -14 and try to keep all the peaks at -14 with nothing going much beyond -12.

Then when making the Betacam master, -14 becomes the "zero" reference level which means that none of the transient peaks exceed +2 dB. Anything above that seems to trigger the broadcaster's limiter and the audio levels take an undesirable drop for a significant time.

When recording make the most of your dynamic range by setting levels high enough (but not too high), then adjust slightly downward. Trying to compensate for low levels would increase the amount of noise in the track. But recording too hot means clipping highlights or getting distortion.

Quite a tightrope to walk!

Dean Sensui
Base Two Productions

[Ryan Martino]

"But it means that the analog equivalent of a "nominal" reference point in digital is somewhere around -12 or -14 in order to provide "headroom" to accomodate transient peaks."

dean:

your evaluation is totally on track. i would like to add some thoughts about the analog/digital comparison that might help someone somewhere. there is one interesting variable regarding this comparison that few people bring up:
the way analog tape naturally compresses and/or soft limits the audio when the input level rises above zero. yes, analog tape recorders have headroom over zero, but they distort and "clip" the signal above that point just like digital recorders do. the difference is that the artifacts of analog tape distortion are generally very pleasing to our ears, while digital clipping is just plain unacceptable. that's why when they started making digital recorders they didn't bother with putting in "headroom".
now, where it gets really interesting is that tape doesn't just leave the signal alone until it peaks above zero and then clip it. it distorts on an upward slope, so the closer the level gets to zero, the more tape distortion will occur, and above zero it gets really out of control. ...out of control in a pleasing way, of course! and of course, you can push it too far and it sounds like ***, but used subtly it's a nice effect. i'm sure this is why the pros still love nagra recorders. - 'cause they record onto tape...

SO: the upshot is that if you compared the same audio recorded at the same level on a digital recorder vs. an analog recorder, you would find that although the peak levels would be the same, the analog audio would sound "hotter" or more even, richer, etc.... because the tape distortion has produced a compression effect, leveling out the dynamic range of the audio. the digital audio would be technically just as loud, but only on the peaks. therefore, you would perceive it as being much softer and at the same time, have a harder time controlling the dynamics of it. all this makes analog recorders more "forgiving" then digital recorders.



JOSH-

i would think that yes, technically, you are o.k. with digital as long as you don't go over zero. you can always lower the level in post, right? but dean is right, there is no real harm in recording things lower. don't worry about noise floor in the digital realm. it's a relative non-issue.


i may have gotten a bit off track here, but i hoped my meanderings might help with the context of analog compared to digital... sorry if this is all irrelevant. i read these forums daily, and since i'm a relative newbie to DV, i don't get the opportunity to contribute very often. (yet...) but sound is the one place so far where i try to contribute.

-ryan martino

[Robert Aldrich]

A trick I've been using when recording sound to my PD150 is to send the sound from one mike (Sennheiser 416) to both channels, then set channel one on auto (agc) and channel two to -12 db using a 1 khz tone, so that if the agc channel blows out I still have something useable on the other channel to fix it with in post.

So far I haven't seen the real world end use of this (no comments from editors either way) but what the hell, if ther's really trouble, they can fix it in post but when you've only got 77 DB audio range on the PD150, no mixer, recorder, or sound man you have to try to make it work somehow!

[Josh Bass]

Noise floor not an issue? I've always got some kind of hiss, hum, growl, or something else in there, just waiting to surface when I raise the levels. Every once in a while a get a signal so clean that I can jack it up a lot in post and you can't tell, but very rarely. It's very much an issue when you have no budget.

[Eivind Vaa]

I agree with Josh, that noise due to a raised level in post often is very much an issue. Using a PD150, I always try to make sure that the sound recorded lands somewhere in the area from around -12 to -6. This would be the "important" sound, often dialogue. This way, I don´t have to adjust much in post, other than the occasional peak or blowout. In my opinion, the most important thing is to monitor the sound as you record, if this is possible. Set the level as high as you can, and use headphones to make sure you don´t get dropouts or unwanted noise due to peaking or other factors. It can often be difficult to monitor sound, in noisy enviroments, or if the headphones doesn´t deliver a good enough signal, so checking the meters on the camera is always a good idea.

[Ryan Martino]

josh and eivind -

when i was talking about noise floor, i was only talking about the recorder and the media itself. i just meant that in comparison to analog, noise floor problems are a relative non-issue for digital recorders. i should have been more clear, as i was only talking about noise that the recorder and media might introduce on their own. i didn't include all the other factors because, of course, there are a hundred places that noise could be introduced into the recorded audio signal. bad cable or cheap mics or wacky grounding, etc. can screw with us regardless of what we're recording the audio on to!

so if you wrestle with that on a regular basis, i would say definately record as hot as possible without clipping.

didn't mean to ruffle anyone.

-martino