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Static and Very Loud Hum on Sound Devices 702 for Only 1 Setup


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Hi Guys,

I had a pretty bad sound issue today and as I am still learning the ropes, I wasn't able to figure it out on set. It happened only at one of my two recording sessions today. I could use any input to get to the bottom of this so I don't repeat it ever again.

I completed my first field recording session with no problems. Then when helping a couple friends on an unpaid pickup shoot after, all hell broke loose and my audio is largely un-usable.

This is what happened:

We were recording in a relatively noisy small restaurant. By noisy I mean that there were loud variables we could not turn off (commercial fridges, loud oven going in the nearby kitchen, etc). I was able to kill the AC which was very loud and helped.

I was running on my Sound Devices 702 with a Sennheiser Lavalier Mic plugged into channel 2 and a Rode NTG-2 plugged into channel 1. I was recording Mono tracks of each. I was running Phantom Power to the NTG-2.

Whatever I did to get closer to the two people talking, there was an extremely loud hum and on and off static on both lines. The lav can always act up, so I was trying to solve the problem on the shotgun mic. The hum was way louder than any fridge sound should have been, so loud that even with the mic about a foot away from the subjects mouth, it is hard to hear them over it.

I thought I had a bum cord somewhere. I tried not using the internal cord in the boom pole (no change). I tried switching my XLR cable (no change). I tried temporarily unplugging the lav (no change). I tried switching headphones (no change and now that I have listened to the audio, it is as crappy as I thought it sounded). I couldn't figure it out. The lav sounded even worse.

I went home and tested the mic, the XLR cords, and the boom internal XLR and everything seems fine.

I noticed one huge problem: I had lent my recorder to someone who uses a stereo shotgun mic, he changed setting number 28) Input 1,2: Linking, MS. And I hadn't noticed. He changed them to be linked. From what I understand (and I could be wrong) this is just somehow linking the gain knobs so that channel 1 also effects channel 2 (although upon testing this, it doesn't seem to make channel 1 the master control for both because no sound comes out of channel 2 unless the second gain knob is also turned up a little bit.).

What this seems to have done is totally screw my Lavalier channel. It sounds like it was clipping even when it didn't seem to be by the level. I don't see how having the second channel on with the Lav would then introduce static and a hum (two separate sound issues) into my first channel. They were recording two independent mono tracks.

Please help me get to the bottom of this. I can answer more questions if I have been confusing or vague anywhere in here. Thanks!

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Whatever I did to get closer to the two people talking, there was an extremely loud hum and on and off static on both lines.

Is it possible there was bad RF interference in the area that was so severe, it was getting into the mike preamps? If that happened to me, I'd try a different part of the room and see what happens. Or try to run the machine off AC and see if that changes anything.

I noticed one huge problem: I had lent my recorder to someone who uses a stereo shotgun mic, he changed setting number 28) Input 1,2: Linking, MS.

Do a Factory Reset -- page 60 in the 702 manual. Then go through every single menu item, one at a time, and confirm that it's where it needs it to be.

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Is it possible there was bad RF interference in the area that was so severe, it was getting into the mike preamps?

I thought this may be possible, but I doubt it because they shot here for the actual episode (this was a tiny pickup shoot) and I didn't hear any problems with the sound for the main shoot.

As for a different part of the room, it was a pretty damn small restaurant, I couldn't have been more than 10 feet from where they recorded the main episode a couple weeks ago. That said, maybe it was all these fridges and whatnot around. Could they cause this? What normally causes crazy RF interference? Broadcasting?

Or try to run the machine off AC and see if that changes anything.

Do a Factory Reset -- page 60 in the 702 manual. Then go through every single menu item, one at a time, and confirm that it's where it needs it to be.

Doing a Factory Reset for sure and then going through the Menu. Thanks for the AC power suggestion, I'll try that next time, but I really doubt it was my battery, I just recorded an hour before this one in a different location and had no issues.

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It sounds like inputs 1,2 were matrixed together for MS. If that's true, it's likely any Lav noise was mixed into track A and B to a degree to complete the MS encoding. What that mode does, is put input 1 in the center of tracks A and B then it takes input 2 (lav) doubles the signal and puts the normal signal on the Left of tracks A and B and then inverts the phase of the "copied" input 2 signal and places it on the right side of the stereo image recorded on tracks A and B.

You weren't burned in the first half of your day, probably because you didn't have anything going into input 2. Once you put the Lav into the equation it leaked into track A (boom) because of the MS matrix. It also sounds like you were getting a lot of RF noise on your wireless.

When you were troubleshooting did you unplug the Lav from the transmitter, or did you pull the receiver out of input 2 all together.

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It sounds like inputs 1,2 were matrixed together for MS. If that's true, it's likely any Lav noise was mixed into track A and B to a degree to complete the MS encoding.

When you were troubleshooting did you unplug the Lav from the transmitter, or did you pull the receiver out of input 2 all together.

Hey Dustin. Thanks for that response! In my gut, from your explanation I think you are right. That would make the most sense. Because the dials were synced in some way, when I cranked up my boom channel to get enough volume on it, it cranked my lav to blow it out and then that blown out lav sound was then mixed into the shotgun channel.

The only strange thing is I did totally unplug the lav from Channel 2 when testing and didn't notice a difference in the sound (that said I didn't record a test without it, so I don't have proof of that now). Maybe the channel 2 was just cranked way up and all out of wack and even without the lav it was causing noise...

I'm still perplexed, but starting to think it got turned on MS encoding.

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" he changed setting number 28) Input 1,2: Linking, MS. And I hadn't noticed. He changed them to be linked. "

The M-S setting is a huge issue...

part of the production sound gig is problem solving... (that is what separates the men from the boys)...

" on an unpaid pickup shoot after, all hell broke loose and my audio is largely un-usable. "

as this sounds like some sort of a production, the problem needed to be resolved before shooting.

(and before you say it: their bad for not letting you sort it out!)

and reminders: it takes years of experience to get years of experience + <tiger>

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I know this was an unpaid gig, like a student film, but you gotta be totally paranoid about shoots like that the day before they happen. I really go through my equipment from top to bottom and check every connection, every cable, every wireless feed, make test recordings, charge every battery, and make a big checklist to make sure I have everything I could possibly need (plus backups). It's good to make mistakes on freebies -- the key is to learn from them and never repeat them.

My joke is, I make new, more interesting mistakes on every shoot... never repeating the same old mistakes. In particular, I go through every user setup item by hand and check every single one of them to make sure it's OK, the day before the shoot. SD now has recallable user setups on the 788, but I don't think it's possible on the 702 or 744, and the setups don't change every single parameter (like track names), as I recall. The only exception would be if I have two back-to-back shoot days for different clients, but if the setup is the same, I still make a cursory check to make sure everything is OK. Those menus will kill you -- on all machines, audio and video, Sound Devices, Zaxcom, Fostex, Tascam, Nagra, Cantar, Sony, Panasonic, everything. One selection in the wrong place... kablooey.

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Thanks for the help guys. I guess the MS is what did it. I normally am all loony paranoid the night before and run checks, but these were strange circumstances.

I'm renting my sound gear to a colleague (that he is using on and off for the month) & I wasn't able to get the recorder back until 30 minutes before the shoot, so I blew it. I asked him about changes and went through everything he told me he changed, but that wasn't the right thing to do. In retrospect, even for this little unpaid gig/favor I should have run through the whole menu beforehand. Thanks for the help.

And I will look into user profiles. I haven't used that, but if it's possible it would be a good thing to check out.

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I wasn't able to get the recorder back until 30 minutes before the shoot, so I blew it. I asked him about changes and went through everything he told me he changed, but that wasn't the right thing to do. In retrospect, even for this little unpaid gig/favor I should have run through the whole menu beforehand.

Lesson learned. Never give yourself only 30 minutes to look over your gear and make sure it's in working order before the gig starts. That said, once you are familiar with it, it shouldn't take you 30 minutes to do that check and be certain that your gear is set the way you expect.

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  • 2 weeks later...
  • 3 months later...

Your "day before" prep of your gear (as well as running thru the shoot's particular issues in your mind) is just as important as what you do on the set. Production sound has far too many "fiddly bits" to be trying to figure them out on the fly--you'll have plenty of opportunity to do that anyway as you modify your approach to handle changes on the day. For myself, I'm just not a good enough improvisor to not go thru all the gear before every job, and repack the stuff so I have a mental picture of where everything is. A checklist for those early morning pre-coffee departures is a really good idea.

philp

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