Mixpre level alignment with cameras lacking -20db mark

[Tom Morrow]

I need to figure out a quick procedure for aligning levels between my SD Mixpre and random cameras that don't have the -20db mark on their level displays.

For those who don't know, the SD Mixpre outputs a -20db tone, and the normal procedure is to simply send the line level XLR signal into the camera, and adjust the camera audio gain until the -20db mark is hit.

Recently I've shown up on shoots with my mixpre, but found the camera that the DP brought doesn't have a -20db mark. So far the two cameras with this issue were the Panasonic AF100 and the Red One. With the AF100 I think I got passable results just leaving the camera gain dials centered, but with the Red One I got clipping; I googled and found this document which indicates that the Red has a lower level input than pro line level, more like consumer line level:

Audio Performance of the Red Camera (Red One)|Sound Notes|Sound Devices, LLC

The Red doesn't have gain dials (as far as I know; its fixed gain). I don't have motivation or budget to build the 1kOhm pad cable that they suggest, and I'm hoping for a more general procedure that will work on any camera.

I was originally thinking I would

1. Generate a full scale (+20) signal on my Mixpre (either by blowing on the mic or with an external tone generator fed into the mic input). Since I have the limiters set at full scale this should be relatively easy.

2. Then turn camera gain dials (on AF100) or add switchable XLR pads (for the Red) to get that aligned with 0dbFS on the camera level display.

But then I remembered the camera operator saying that the AF100 has built in limiting, and realized the camera limiting could foil that strategy.

I'm curious if anyone has suggestions for how to quickly do this on the morning of a shoot.

[Jeffery Magat]

You should never encounter a 'random' camera. It only makes sense to read the manual of the camera you'll be working on, and where is what deep inside the menus.

Secondly, you shouldn't be depending on the RED audio. That's just the main rule most soundies have adopted. Whenever I do record audio to the RED it's just a scratch track. I always eye for shooting tone in the middle of the meter. I find anything that goes passed yellow sound like crap. Double-system always!

I worked with the AF100 last year, but the last thing I remember is that the marker on the audio meter is actually -12dBFS and there are 6 hash marks between that line and the top of the meter. So perhaps each hash is 2dB and countdown for the supposed -20dBFS hash mark? I really can't remember what I did.

Sometimes you can get the camera to send out a test tone in their colour bars mode, and sometimes you can see it show up on the meters which may give you an idea where the -20dbFS hash is.

If you really don't have time to figure it out... just get it as close as possible.

[Steve House]

Your budget doesn't cover $2 for a couple of resistors to build a pad?? Come on!

Note the article points out there is a faint -20 mark on the Red by virtue of a bolded segment.

To generate a full scale tone, there's a better way than blowing in the mic. There are a number of tone tone generators on the market that plug into a mic input I have one called the GTC Tone-Plug that is built into an XLR plug, runs off phantom, and works really well. Behringer's CT-100 cable tester has a tone output and is only $20. Pretty fundamental piece of kit for anyone doing sound.

[Rick Reineke]

"For those who don't know, the SD Mixpre outputs a -20db tone"
-- Yes and no.The MixPre's (default) reference tone is displayed as 0VU on the meter which equates to a 0 dBu output level on the balanced outs. The subsequent recorder is then set to -20dBFS.

"The Red doesn't have gain dials (as far as I know; its fixed gain)"
-- Yes, but unlike the previous REDs, the Epic has a software based level adj. fader. furthermore, the two (2) 1/8" TRS input jacks (front, under the lens with the standard audio package) can be used balanced or unbalanced.. (though the manual and on camera menu is vague). I used a G2 (receiver @ mic level) to send a scratch track which worked out OK, however the Sennheiser 1/8" to 1/8" cable shorts out the input. This still indicated an input connection, which the camera op was under the impression that's all that's needed.. Not so. In this case, the TRS plug needed to have the ring tied to ground or use a TS plug.
That said, I would not rely on or want production to use the on-camera audio, despite it being touted as 24bit.

-- My two cents.. Colored meter displays on cameras can sometimes be misleading. In my experience on a old DVX, the camera op radically turned down the level, without my knowledge and despite the control knobs being taped down, because the level was "in the red". Which on that cam, started at -12. This was not the first instance of cam op 'audio level' confused by the color of the meters.

-- Yes, whenever I'm going to encounter an unfamiliar camera, I download the manual, read and copy the audio section. Chances are, the cam op won't know..

[Tom Morrow]

I suppose I will concede that "read the manual" is a good suggestion. I just heard a lament from a cinematographer that every time he shows up to a shoot with a new camera he has to spend half a day reading the manual to get up to speed. And with the pace of progress, just about every shoot has a new camera :-(

Your budget doesn't cover $2 for a couple of resistors to build a pad?? Come on!

I'd love to see the camera op's face when I haul out a soldering iron and tell him I'll get the audio straightened out on his camera!

To generate a full scale tone, there's a better way than blowing in the mic. There are a number of tone tone generators on the market that plug into a mic input I have one called the GTC Tone-Plug that is built into an XLR plug, runs off phantom, and works really well. Behringer's CT-100 cable tester has a tone output and is only $20. Pretty fundamental piece of kit for anyone doing sound.

I think we have a different idea of Full Scale... in my world full scale is +20dbu, which Sound Devices likes to align with 0dBFS. The CT100 puts out only +4dbu, and I'd guess the GTC doesn't put out any more. This is the difficulty with full scale... 20dbu is 21.9 Volts peak to peak, hard to generate with batteries (although today's efficient DC/DC converters should make it possible).

[Richard Crowley]

Originally Posted by Tom Morrow

I'd love to see the camera op's face when I haul out a soldering iron and tell him I'll get the audio straightened out on his camera!

Seriously, you know that Mr. House was not suggesting modifying any camera. A padded adapter and/or patch cable should be part of any decently-equipped sound kit. $2 is a significant exaggeration of the actual cost (which is actually pennies).

I bought a bunch of these to use for making fixed pads. $3.75
CA-301-PR
You can buy a dozen of these and resistors for what a single store-bought fixed XLR pad costs.

You can calculate the value of the resistors at Rick Chinn's website...
Uneeda Audio - Build your own attenuator pads

[Tom Morrow]

That was a great link on building attenators! It's rare to find info on calculating the values. I've got my Shure switchables 15/20/25 which should cover most situations. Someday I'll pick up a 5 and 10 so that I can deal with anything.

As far as generating a fullscale signal from my mixpre, I suppose I could plug one of those tone generators into a mic input to avoid having to blow across a mic.

[Steve House]

Originally Posted by Tom Morrow

I...

I think we have a different idea of Full Scale... in my world full scale is +20dbu, which Sound Devices likes to align with 0dBFS. The CT100 puts out only +4dbu, and I'd guess the GTC doesn't put out any more. This is the difficulty with full scale... 20dbu is 21.9 Volts peak to peak, hard to generate with batteries (although today's efficient DC/DC converters should make it possible).

The Tone Plug generates a mic level signal to send into the mixer's mic input. What comes out the other side to send on to the camera depends on where you set the channel gain faders and the mixer's master output fader. You want +20dBu on the mixer's outputs all you need do is crank up the gain.