Audio spikes on a Varicam

[Clayton Harper]

Anybody ever experienced audio spikes/distortion on the Varicam?

Sound mixer friend of mine recording first to DAT and then to tape observed distortion on recorded DVCHD tapes on later playback from the camera and another tape machine. The audio monitor did not send out distortion or clipping and apparently everything was set properly on camera.

The same audio rig worked the next two days after that shoot on a Betacam rig with no problems.

The spikes happened to sounds at -15db and above.

I have seen audio dropouts and macroblocking from dirty heads and humidity on the Varicam but never this weird distortion at a certain sound level.

Ideas?

[Mark Crabtree]

My Varicam is usually very clean but of course does not have the headroom of a good mixer. Most mixers have the ability to output both line and mic, so of course you want to be sure you are matching the levels both on the ouput of the mixer and on the input on the Varicam. Your tone should be set on the Varicam at minus 20, that is the digital version of zero. So set tone at zero on the mixer and minus 20 on the Varicam. Set impedence out of the mixer to line and impedence in of the Varicam to line also. And since headroom on the Varicam is not as great as a good mixer has,
you might want to set tone at minus 25 and tell your audio guy to watch the hots.

[Clayton Harper]

My Varicam is usually very clean but of course does not have the headroom of a good mixer. Most mixers have the ability to output both line and mic, so of course you want to be sure you are matching the levels both on the ouput of the mixer and on the input on the Varicam. Your tone should be set on the Varicam at minus 20, that is the digital version of zero. So set tone at zero on the mixer and minus 20 on the Varicam. Set impedence out of the mixer to line and impedence in of the Varicam to line also. And since headroom on the Varicam is not as great as a good mixer has,
you might want to set tone at minus 25 and tell your audio guy to watch the hots.

What you describe is exactly what went down. Tone was at -20 and the impedence was set to line.

This work was done by a veteran mixer with a crazy amount of studio and field recording experience.

So weird...

[Paul Hazlett]

sometimes its the obvious, does the playback deck have variable out?
check the tape on headphones from the deck, cant hurt...

and try checking the tape in the cam if thats possible. how are the levels and
does it sound good in the phones from camera?

[Clayton Harper]

Phaz,

Tapes were checked on the camera and a deck. They are 100% for sure distorted. It seems like the distortions were not audible in the monitor but may have clipped on camera somehow.

I am supposing that maybe the things were peaking a little bit and the operator maybe wasn't watching the VU meters and now that the tapes are jacked he's playing dumb.

[Cail Young]

Not very relevant to the problem, but out here people pretty much use -12dBFS for the 0dBu equivalent, not -20. I know that -20 will give you more headroom but perhaps SNR has something to do with the choice of -12?

[TimothyD]

Sounds like it could just be transients. Meters are not very sensitive to extremely fast changes in amplitude. What was the sound you were recording, or did it do this on a wide range of sounds that day?

I don't know anything about Varicam's in particular, but I do know that heat plays a major role in how well A/D converters work. (I have heard that you cannot actually capture true 24 bit recordings unless the A/D's are kept at 0 centigrade. 20 bit yes, 24 no, so supposedly all 24 bit recordings only really have something below 24 and above 20 in resolution)

Anyway, I have no idea if hot converters could cause the problem, but that is a thought. What about the tape mechanism? Maybe try some experiments at higher than -20 with a different brand of tape? Who knows man, good luck with that.

Maybe you should stop down at the bar and see if Grady has heard of any of the other caretakers having problems like that;)

Peace,

Tim

[ericyoung]

Sounds like it could just be transients. Meters are not very sensitive to extremely fast changes in amplitude. What was the sound you were recording, or did it do this on a wide range of sounds that day?...

Well that's true of VU meters, but actually PPM meters were designed with exactly this problem in mind. They have an extremely fast rise time to catch transients, but a slow fall time, so that you can see how high it got before it dropped.

[tj williams]

IMHO If the sound is of a type which you would expect to be at the same level such as steady talking and suddenly it gets distorted then maybe you have a technical problem with the deck, or maybe the mixer just bumped the gain. If the nature of the sound is for instance a sudden bump up in volume ie closer to the mic (less ambient) or just louder performance than what went before then what you have is a very experienced mixer who missed. A big indication here is that you have not been able to duplicate this effect on your Varicam again, even an intermittant problem should show up more than once a day. We had a series of tiny stars in the video when the HDCam got hot for a while....Ahhhh the joy of the intermittant....