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-   -   HVX audio (https://www.dvinfo.net/forum/all-things-audio/87707-hvx-audio.html)

Marco Leavitt February 27th, 2007 01:52 PM

HVX audio
 
I have a shoot this weekend and I've never connected to this camera before. Is there anything I should watch out for? Also, I've heard the preamps are pretty good on this camera. Can I leave 20 dB of headroom? Leaving 12 dB never seems like enough. I'll be connecting with line inputs from an SD302 mixer. I believe we'll be recording to MiniDV.

Ralph Keyser February 27th, 2007 03:34 PM

The preamps on the HVX200 are OK given the class of camera. If you have an SD302, the preamps on it will be orders of magnitude better, so just plan to use the 302 preamps and run line level to the HVX.

I don't know what kind of shoot you have coming up, but I usually run my peaks as high as I can on these types of cameras. I'm often just a couple of db below 0 dBFS on the camera for my peaks. The concept of "headroom" is really different for digital audio, and it doesn't buy you anything by not using all the bits available provided that you don't run out of bits and end up clipping the waveform. The limiters on the 302 are really good, BTW, so you can depend on those if one of your actors gets enthusiastic on a take.

Marco Leavitt February 27th, 2007 03:48 PM

Well, by headroom I mean peaking two or three times a sentence at -12 or higher (on the camera) with the limiter at +8 on the mixer. I don't like slamming into the limiter too hard, but it always seems to happen at some point. Sounds really nasty to my ears, but other people don't seem to notice. I'd much rather maintain average peaks at -20 on the camera and raise the limiter on the mixer to +16.

Ralph Keyser February 27th, 2007 04:32 PM

For this class of camera, I usually tone up 0dBVU on the mixer with -12dBFS on the camera and set the limiter at +12dB on the mixer. If it's really dramatic material, then I may drop back to -20dB on the camera with the limiter at +18 or +20. The audio circuits on these little cameras just aren't very quiet, so I'm always looking to get as much signal to noise ratio as possible out of them.

Really though, it's entirely up to you. If you're more comfortable with your 0 point at -20dBFS, then you should run there. What you need to know is how does it show up for your post folks. If they have to add a lot of gain to get the source tracks to the level they need, then you should consider more gain on the production side. If they're happy, then you're doing great.

Marco Leavitt February 27th, 2007 04:52 PM

Interesting. I always took it as gospel that you need at least 4 dB of overshoot. You're not clipping digitally with tone at -12dBFS and limiter at +12dB? It's something I've been tempted to try.

Yeah, I guess that was my question are the preamps quiet enough for that extra 8dB of headroom. If I can buy another 4 dB by raising the limiter, maybe that's the way to go. Makes me kind of nervous though.

Douglas Spotted Eagle February 27th, 2007 11:24 PM

Quote:

Originally Posted by Ralph Keyser (Post 632951)
I don't know what kind of shoot you have coming up, but I usually run my peaks as high as I can on these types of cameras. I'm often just a couple of db below 0 dBFS on the camera for my peaks. The concept of "headroom" is really different for digital audio, and it doesn't buy you anything by not using all the bits available provided that you don't run out of bits and end up clipping the waveform. The limiters on the 302 are really good, BTW, so you can depend on those if one of your actors gets enthusiastic on a take.

Unfortunately, many folks don't quite grasp this very often...but it's the correct method, IMO.
If you don't use all the bits, it's like trying to upscale low resolution video. You just can't.

Marco Leavitt February 28th, 2007 08:12 AM

This has been an interesting thread. Is this limitation inherent to 16 bit recording? I had thought that the noise floor was the main limiting factor. If I was recording double system sound to a quality recorder, say the SD702t, in 16 bit mode, would I still be limited to 12 dB of headroom?

Tim Gray February 28th, 2007 11:37 AM

It's not a limitation of 16 bit audio per se, but of audio with low numbers of bits.

The usual number is 1 bit = 6 db. So 12 db down on 24 uses 22 bits. Its a bit more complicated than this since a 24 bit converter doesn't necessarily give you 24 bits of useable audio.

24 bit audio has 16777216 quantized levels, while 16 bit has 65536. If you are 20db down on a 16 bit signal (kind of like 13 bits), you are only using 8192 quantized levels, which when you turn it up on your NLE, makes the noise floor of your audio rise.

Do the same on 24 bit audio, (20db down roughly equal to using 21 bits), and you get *2097152 quantized levels, still more than 16 bit has and definitely more than 20 db down on 16 bit has. Since many delivery formats are still 16 bit, you still have a lot of wiggle room.

In my opinion, 24 bit capture is quite awesome for both movie sound and music production. All the above of course is completely ignoring the noise of your mics, recorders, preamps, and environment. If any one of those things (or the combination) raises your noise floor significantly, than it doesn't ultimately matter that much if you are recording in 16 bit or 24 bit.

If you have a 702t, record in 24 bit.

Marco Leavitt March 6th, 2007 10:30 AM

Just to update this for anyone interested:

Well, we went through the shoot. Very nice camera, but it's got some audio quirks. I had previously read on the Internet that the audio levels can shift for no apparent reason. It certainly has nothing to do with the gain knobs getting bumped.

I was a little skeptical about it, but no joke. It is for real.

We had to check tone after nearly every shot. The levels can change between takes by as much as 4 dB! They usually drift under, but sometimes it's over.

It happened every time the camera was powered up, and at other times for no apparent reason, so watch for this. Camera operator was really cool about verifying the levels when tone was sent to camera so the mixer didn't have to keep running back and forth.

Scary stuff.


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