Adjusting Sound devices 302 from line out to mic
 

Hello,

I've just been sorting out my new DS 302 mixer. In most conditions I'll be able to use it at the default line-out setting. I will when using a mic attached to the camera have to change the setting to mic-out (my camera's XLRs are both either mic or line). I've spent today playing around with the menu that allows you to increase attenuation. My question is this: how much attenuation is appropriate for mic-out? I see that the cable pad Sound devices sells for this purpose is 40dB and I suppose that settles the issue. But I was told by the dealer who sold me the mixer to attenuate to 50dB and I have read an opinion somewhere else that I should set it to 35dB.

Does anyone have an opinion on this matter?

thanks.

John

In an ideal world, I would want to attenuate enough so that you do not overload the inputs on the other device taking into consideration that you would want to minimize the gain needed on the other device.

You can try to see what works best for you. I would start with -35db and set the gain low in the other device. I would set up the 302 to output tone for these tests.

Be sure to monitor the audio in the other device (I assume it is a camera) with headphones. (The meters in the camera may look normal, but your input could be overloaded thus causing distortion.)

If you get distortion, or high meter readings, try making adjustments. First lower the gain on the camera to see if that works to eliminate the distortion. Then, if necesary, increase the attenuation on the output of the Sound Devices 302 mixer, say from -35db to -38db or -40db or more.

If you do get audible distortion in the headphones, adjust so that the distortion goes away, then lower it some more to be sure that it is not distrorting, but you just can not hear it.

After you get the levels correct with tone, then turn off the tone, and record some samples of the actual sound, if possible. In any case, record some tests, then play them back to ensure that everything is working properly.

I hope some other experts will share their experiences.

If it helps any my camera is a Canon XH A1.

John

Why do you need to use the mic input on the camera, if a line input option is available?

While I'm sure the camera mic inputs are very good, they are not likely to be in the same class as the 302 - I can't see the point of using the high quality mic preamp of the 302, then attenuating the output, and 're-amplifying' using an inferior mic preamp.

I'd stick to line in - use the 302 mic preamps to their best advantage.

I think the XH-A1 is one of the Canon cameras that has a strange line-in - very low and noisy, and just padded down from the mic input. I know I've ended up going in mic level to an XH-L1 for that reason.

John, one of the great things about the 302 is that you can attenuate the outs in its setup menu. I stole this very complete explanation from another forum:

"What you need to do is turn off the mixer. Then turn it on while pressing the Peak/VU button. Now you are in the setup menu. -30 should be flashing on the left channel, which is the XLRs attenuation setup. This is where you change from line level to mic level. You basically add the left and right meter values to determine the attenuation. You will use the brightness button as the "down" button and the battery button as the "up" button. When you are in line level both the left and right meter will read 0. As you press the down button (brightness button)you will notice the left channel will start going down -2 dB all the way to -16 dB then after -16 dB the next increment will be that the right channel will go down to -30 and the left channel will go to -10 dB equaling a total of -40dB. If you keep pressing the down button the left channel will keep going down in increments of 2dB all the way down to -26 while the right channel will stay at -30. At that point you are at the most attenuation which will equal -56dB (-26 + -30). Mic level can be anywhere from approx. -20dB (high mic level) to -56dB (low mic level) Since I don't know what you are feeding I can't tell you what level of mic level you want. So your SD 302 XLR levels can be set either at 0 -2 -4 -6 -8 -10 -12 -14 -16 -40 -42 -44 -46 -48 -50 -52 -54 -56. Once you choose the level you want then press the peak/VU button and keep pressing it until the LEDs scroll back and forth telling you that you are now out of the setup menu. I hope this helps.

Andy"

Totally false. XH-A1 and standard line level delivers best audio I have heard in a prosumer camera. I can get over 90 dB S/N ratio with XH-A1 and SD302 with standard line in. SD302 feeding normal level signal, XH-A1 potentiometers turned ALL THE WAY OPEN. That is the proper way of doing it. Canon has built the line in in XH-A1 so that with potentiometers all the way open there is no amplification/attenuation and a perfect match with pro line level equipment. For some reason people just are afraid to turn the pots open all the way. See the pots as attenuators, not amplifiers.

When padding SD302 down to mic level out and using XH-A1 at mic level in, the S/N ratio drops to 79 dB.

The line in on the Canon XH-A1 and X1 cameras is rated at +6dBv which translates to +8dBu. The typical '0VU' point for analog gear traditionally was +4dBu. SD and other manufacturers building gear expected to interface primarily with digital devices have deviated from that older standard somewhat and are calibrating their meters so '0VU' corresponds to an output level of 0dBu.

Interesting your comment on the gain pots on the camera. Most of the time one expects unity gain to be at the centre point of the pot's range and running it full up increases circuit noise and hiss.

I have no data to back up my idea, but as I get over 90 dB S/N ratio with the pots all the way open, nothing is certainly adding any hiss there...

This whole problem stems from an ancient fear of turning any volume controll all the way up. With XH-A1 you can. Maybe it works as an attenuator?

How are you coming up with the 90db S/N ratio? If you send it tone with the Auto Level control engaged, what level does it set the signal to on the camera's meters? Then if you turn off the ALC, where do you set the input pots to produce the same camera meter indication?

No kidding Petri. I'm interested to hear more about this too. Oh, correction from above - the camera I worked with was an XL1.

Here is a copy of a post I made some months ago about this matter:
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We all know HX-A1 has a bit funny line in levels, with a standard line in levels (like from SD302) you have to set the potentiometers to full open, or just a hair from full to get decent levels. Some people have sounded concern that this can not be right and they rather use attenuation on the mixer and use mic levels in to be able to use the midrange area with the level pots on camera.

Test setup: Loud pink noise from a good stereo system, quiet room, Rode NT-1A super-quiet studio condenser microphone. First pink noise via SD302 to line in, camera pots open, level on SD302 just starting to hit the limiter set at +17 dBVU, then the mixer pot turned down to kill the input (to get the system noise floor). On the second sample the output levels pulled down on the SD302 so that when using mic in the camera pots are at midrange (optimum?). Same pink noise at same levels, also "quiet" recorded after the noise.

Result: The line level gave max peaks at -4.60 dBFS and minimum average RMS at -93.93 dBFS. An amazing dynamic range for a camera, approaching the theoretical limits of a 16 bit system. Almost 90 dB!

With mic levels the peaks were at -3,17 dBFS and with no signal average RMS at -79.21 dBFS. Thus the mic in was 10 dB noisier than the line level even with the pots all the way open with the later.

I will continue to use the line level in, and turn the potentiometers all the way south.

Both results are good, better than you can get with any Nagra...

Addendum: shot with SD, which uses uncompressed PCM audio.
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I remembered the S/N to be over 90 with line in, pots open, but it was slightly under 90dB. Still GREAT! Of course by riding the mixer hotter or without limiter would give another 4 dB giving a 94 dB S/N ratio. And in any case over 10dB cleaner than with mic in levels.

I have not tested the system with ALC, with a mixer there is really no point in that.

I'll agree there may be no point in shooting using ALC when you're using a mixer but engaging it a moment for testing purposes that will tell you what recording level the camera designer's consider 'normal' or optimum. If you put tone into the camera with ALC engaged, the camera itself will set the level and you can see where it falls on the camera's meters. That level is the alignment level the manufacturer used as his design parameter to define optimum operation. Is it -20dBFS?, -12dBFS?, something else? That's the level you should see on the camera meter when you 'tone up' to align the camera and mixer gain staging.