MIC ALC, is it redundant recording Digital Audio?

[bob furlong]

(not that we would ever want to, of course, but...)

Overmodulated audio signals recorded onto a Firestore (P2 format) will clip at 0dBFS.

Wouldn't the MIC ALC (auto level control) on the HXV similarly clip the signal at 0dBFS before sending it to the digital recorder?

Wouldn't the two signals be the same?

Or, is there a reason that it's better to have the clipping done by the ALC?

Also, does the ALC 'push' (increase gain) the quiet parts (decrease the dynamic range) like other ALCs do?

 

[MrBill]

Would be intersted to know too.

 

[Barry_Green]

Quite a bit of misunderstanding here. The ALC on an HVX or DVX is not an automatic level control, it's a limiter (automatic limiter control).

So -- no it won't "push" the quiet parts, it has no effect on the quiet parts at all.

It kicks in when the sound starts to get really loud, and it internally attenuates the signal in the analog domain before it gets digitized. This is very important -- it's compressing the signal, but not clipping the signal. Ideally you won't find any distortion at all. It kicks in when the sound level exceeds about -6dB and tries to compress to where it doesn't allow anything to exceed -4.5dB (IIRC).

And the firestore is getting its signal after the digital conversion, so there's absolutely no part that the firestore is playing in this whole process. The audio will be identical, whether recorded on tape or firestore or P2.

 

[ullanta]

To add a little -

Digital clipping chops off the "top" of a waveform, making it into something more like a square wave than a sine wave the toips are "flat" like the mountain in Close Encounters. Square waves are ugly to listen to.

A limiter retains most of the curvature of the waves, just scales it down (in effect, lowers the gain on the loud parts), enough to avoid digital clipping.

In digital audio it's much MORE important to prevent clipping than with analog - analog degrades gracefully when pushed past the limit (some even consider iot a pleasing effect for many purposes); digital audio becomes ugly and pretty much unusual as soon as the limit is crossed.

 

[wseng]

Quite a bit of misunderstanding here. The ALC on an HVX or DVX is not an automatic level control, it's a limiter (automatic limiter control).

So -- no it won't "push" the quiet parts, it has no effect on the quiet parts at all.

It kicks in when the sound starts to get really loud, and it internally attenuates the signal in the analog domain before it gets digitized. This is very important -- it's compressing the signal, but not clipping the signal. Ideally you won't find any distortion at all. It kicks in when the sound level exceeds about -6dB and tries to compress to where it doesn't allow anything to exceed -4.5dB (IIRC).

And the firestore is getting its signal after the digital conversion, so there's absolutely no part that the firestore is playing in this whole process. The audio will be identical, whether recorded on tape or firestore or P2.

I'm using a DVX 102B (pal, south east asian). I've put the ALC to 'ON' state all the time, but it's very easy for me to get distorted sounds. I shoot people in scenes screaming, shouting very loud. Is this because of the built-in Mic's limitation of the frequency or my mic is having some problem.

Another problem I noted that this kind of distortion sometimes occur even when I monitor the audio levels; it show in the green side not even going into the reds. Normally I will adjust the levels to 1 or 2 dots to the red in normal conversation, which I believe is around -6db.

I've included a sample of audio file I extracted from my recording. Please help to determine what is the problem with it... If I remember properly previously I don't have to repair those noise problems in adobe audition, but as currently almost every shoot I have to do it to correct those cracks, pops and other distortion in audio.

Thanks