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-   -   General advice on monitoring sound level (https://www.dvinfo.net/forum/all-things-audio/66364-general-advice-monitoring-sound-level.html)

Douglas Spotted Eagle May 7th, 2006 12:09 PM

Quote:

Originally Posted by Fred Retread
Strictly speaking you could say that's true, and there are other applications for limiting output level. For videographers though, the reason is almost invariably to prevent clipping, and the limiters marketed to us for that purpose do not themselves clip, flatten, fold, bend, staple or mutilate audio signals to any significant degree, including pure sine waves, when operated within their design range. That would be absurd. That's the door I came in.

Limiters marketed to any market do indeed fold, spindle, mutilate, bend, whatever, because that's exactly what they're designed to do. They're not designed to prevent clipping, they're designed to hold a specific level at all times. There is no "videographer's limiter" and a "studio engineer's limiter" etc. They're all the same, just used differently in various contexts. I'd never use a really hard limit when recording a bit of dialog, I'd prefer to use softer compression starting earlier, because I don't like recording hard limits unless that's really, really, called for. If you record this way, you're married to it for ever. If you've got a dialogic that calls for extreme measures, then you'll want a compressor prior to the limiter so it's not quite so harsh. BTW, a limiter is just a compressor with more strict behavior. It's like a parent to the compressor's child.
Maybe you want a hard limit at 0dB, and you're feeding it +9 in the analog world. Perhaps you want it to limit at -6dBfs, and you're feeding it -2dBfs. You're going to get two extremely different results and audio qualities from both of those scenarios. Limiters have nothing to do with distortion overall, and they'll distort the signal by their very existence. You're associating "distortion"="bad" when in truth, distortion is anything that changes the paramaters of the audio from its orginal dynamic form. Color correction "distorts" the original image the eye sees, but it likely (hopefully) is a pleasing distortion.
If you're feeding the limiter a very dynamic signal, it may prevent truncation of the signal, but the waveform is still flattened out much like a truncated waveform due to the ceiling placed by the limiter. That will present itself audibly, as "distortion" because of the way your ear will respond. But it's not truncated. It can actually be somewhat repaired in post to make it more ear-palatable, where truncated audio really can't be.
Either way, it seems the thread is pretty far off track at this point.

David Ennis May 7th, 2006 01:22 PM

Quote:

Originally Posted by Douglas Spotted Eagle
...it seems the thread is pretty far off track at this point.

You're right, Douglas. This was not the initial poster's subject. I see that it turned this way from the third post and pretty much stayed there. Sorry I didn't notice. Your reply to my last post was kind of strong, so please just permit me to say that I know that compression, limiting, equalization, etc., constitute amplitude distortion. And I trust that if I had left out the firgurative excesses and simply said that units like the DXA-8 and MixPre, which tout their limiting functions in their advertising only in terms of preventing clipping, do not themselves clip audio when operated within range, you wouldn't disagree.

A. J. deLange May 7th, 2006 02:48 PM

Watching the posts fly by I thought back to my generic advice on controlling dynamic range which I posted a day or 2 ago: "Turn off the @#*&^ AGC and set the gain so the noise floor is at the LSB" and wondered how it might apply to this industry. So I looked at the specs for a Shure SM81 which I suppose is a typical mic. It's noise floor is 16 or 19 dBSPL (depending on the weighting) and the maximum it can handle is +136 dBSPL. Using the lower noise floor that's 120 dB difference which is 20 bits worth. Given a complex (other than single tone) load an A/D needs 2 1/6 bits head room and 1 bit for the sign bringing us up to a total of 23 1/6 bits. Thus a 24 bit A/D (i.e. one that really delivers 24 bit performance) can handle anything this mic can do to it with out clipping and with almost a whole bit to spare. Now I would never go into a sound field at +136 dBSPL (that's 6 - 16 dB above the threshold of pain) but I understand it does get that bad at rock concerts. A 16 bit A/D will have 48 dB less dynamic range and would, set up this way, start to clip at 27 - 37 (don't forget that last 5/6 bit) dB below the threshold of pain.

For 16 bits I guess I'd set the LSB at the background room noise level and guessing that this is a good 10 -20 dB above the threshold of hearing pick up 10 to 20 dB out of the potentially problematic lost 27 - 37 that way. Clearly a second mic with 20 dB less gain would cover a good part of what's left over.


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