Freeheel Posted March 31, 2013 Report Share Posted March 31, 2013 So this is a weird one. Regular Sennheiser 50/30 Midside setup into a Sound Devices 552. Mics are being matrixed by the mixer and output as stereo. I've been doing a fair bit of b-roll in small towns lately and at least 1/2 the time when a loud Harley motorcycle passes by, the right channel of the mixer will drop out completely for a second. It will also happen with big (loud) diesels. Just cuts out completely on one side. 552 Level meter is fine on the left and drops to nothing on the right. As soon as the noise passes the right channel comes back. I've had this rig for a few years and have never had a problem with it except for this. It's happened a few times in the past but I seem to be in locations recording when loud vehicles pass more often lately and it's getting annoying. So the sounds that trip it are low frequency and rhythmic and it doesn't matter which side of the mic they are on. It's always the right side that goes. Which is also weird since I would imagine that if its some sort of microphone overload on the MHK 30, the mid signal would stay up - how can one side of a dual mic midside rig fail? But then again, who has ever heard of a single side of a stereo pair (momentarily) failing in a 552 because a Harley drove past? Any Ideas? I have tried different cables... Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
JackHenry Posted March 31, 2013 Report Share Posted March 31, 2013 Limiter turned on for that channel? Quote Link to comment Share on other sites More sharing options...
Freeheel Posted March 31, 2013 Author Report Share Posted March 31, 2013 Limiters are on - the audio actually cuts out to nothing though - not sure what you are getting at? Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
Miles Anderson Posted March 31, 2013 Report Share Posted March 31, 2013 it sounds like a phase thing Quote Link to comment Share on other sites More sharing options...
Freeheel Posted March 31, 2013 Author Report Share Posted March 31, 2013 Hey Miles, I agree it's a possibility, but I'm really puzzled as to how a phase problem (and yes, mid/side is all about phase relationships) cuts out only one side. I could see wacky effects if the figure 8 diaphragm was forced into oscillation by the rumble, but I would think that it would effect both channels.... also, if it's a phase cancellation - it's perfect cancellation on one side only, which I can't see how that would happen with two microphones creating the stereo image. cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
Ray Collins Posted March 31, 2013 Report Share Posted March 31, 2013 Any chance you are overdriving the mic? Maximum Input Sound Level for a 50, SPL 134 (142) dB at 1 kHz Harley's shake the earth and you can feel the sound in your bones. Quote Link to comment Share on other sites More sharing options...
Philip Perkins Posted March 31, 2013 Report Share Posted March 31, 2013 I think you have a mic pre/limiter issue, and should send your mixer in to SD ASAP. They need to look at it for you, it will not heal on its own. (I've tried to get my stuff to do that--didn't work.) philp Quote Link to comment Share on other sites More sharing options...
pscottm Posted March 31, 2013 Report Share Posted March 31, 2013 maybe try it w/o the mid/side matrix engaged to make sure it's the matrix causing the prob? i'm with philip, sounds like the multed-out-of-phase side of the matrix is collapsing for some reason. bad digital routing? very odd indeed. Quote Link to comment Share on other sites More sharing options...
Freeheel Posted March 31, 2013 Author Report Share Posted March 31, 2013 Ray - The sounds that are making it cut out are definitely below the max SPL, Yesterday it was a diesel school bus at 20 feet that made it cut. (happened twice in 2 separate passes) It's always roaring engines though, I've recorded waterfalls and massive hydroelectric turbines with this system and never seen this problem in those circumstances. Phil and Pscottm, I agree with you, but feel I need to do more work on this as due diligence. The channel(s) involved have never failed in this way with wirelesses or regular mono mics. The matrix is definitely a suspect - I don't think the inputs are.. I'll have to do some testing - hopefully my old diesel Landcruiser is loud enough to use as a test sound source, otherwise I may have to procure an old Harley with straight pipes. It's always so frustrating to troubleshoot such wacky problems - I was hoping that someone else had discovered some weird endemic flaw in the MKH 50/30 SD 552 chain. Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
Ray Collins Posted March 31, 2013 Report Share Posted March 31, 2013 Perhaps a damaged mic capsule? Quote Link to comment Share on other sites More sharing options...
Freeheel Posted March 31, 2013 Author Report Share Posted March 31, 2013 Hey Ray, I don't see how a damaged mic capsule could take out one side of the stereo output. If the Mid MKH 50 was the problem it would leave the side MKH 30 on both sides. If the MKH 30 was having an issue it would have to take out both sides since the figure eight mic is creating the left and right channels from the same diaphragm. I do appreciate the all the comments though, they are helping me think about elements of the chain in different ways. Thanks Brent Calkin Quote Link to comment Share on other sites More sharing options...
Neil Hurley Posted March 31, 2013 Report Share Posted March 31, 2013 Hi Brent, My guess.It may just be M/S doing its thing so to speak with regard to sum and difference. In an M/S recording as I understand it, right ® is the result of M-S (ie the difference between what the mics are hearing at any one point in time) , In this case because the low frequency sound waves are so big the signal could be arriving at both mics at the same time. That is to say the Mid and Side signals are in fact the same and only signal at that moment in time. Left (L) will add the signal to itself ( M+S) to give the sum,which results in double the original signal. Right ® will subtract the signal from itself (M-S) to give the difference which in this case results in zero or no signal! which is what you are getting. Good Luck!Neil Quote Link to comment Share on other sites More sharing options...
Freeheel Posted April 2, 2013 Author Report Share Posted April 2, 2013 Hi Neil, I think M/S recording is a little more complex than the mathematical relationship in your post. L=M+S and R=M-S is a funky shorthand for saying that (Left = the mid mic + the side mic) and (Right = the mid mic + the side mic with phase reversed 180°) This means that mathematically, sounds coming in the right side are coming as a negative wave (since it's the "back" of the figure 8 mics diaphragm) When this wave is phase inverted it adds positive wave information to the right channel. So the mid and the right - with the same audio signal - don't actually cancel each other out. You did make me think and research further about how M/S signals work, so thanks for that impetus... Cheers, Brent Quote Link to comment Share on other sites More sharing options...
VASI Posted April 2, 2013 Report Share Posted April 2, 2013 Extended analysis to M/S Technique. Quote Link to comment Share on other sites More sharing options...
VASI Posted April 2, 2013 Report Share Posted April 2, 2013 Quote Link to comment Share on other sites More sharing options...
Freeheel Posted April 2, 2013 Author Report Share Posted April 2, 2013 Nice article Vasileios! Unfortunately, it doesn't shed much light on the specific problem I'm having, but good info nonetheless. As for the hint to RTFM, I've been using midside for years on 442's and now the 552. What I do need to try is running the M/S array into channels 1 and 2 to see if it makes a difference (I've been noticing the problem on 3 and 4 M/S linked) Unfortunately , the noise of my old mufflerless truck engine is NOT able to reproduce the problem, so I'll have to go hang out at the nearest motorcycle club and try to avoid being killed for having a pile of "surveillance" equipment with me... Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
VASI Posted April 2, 2013 Report Share Posted April 2, 2013 What is the RTFM? Your problem it's 3/4 linked channel from Sound Devices 552? Quote Link to comment Share on other sites More sharing options...
Freeheel Posted April 2, 2013 Author Report Share Posted April 2, 2013 Ahh... I'm surprised you haven't seen that acronym before - it gets tossed around quite a bit on this site. It's short for "Read The Freakin' Manual". When you posted the appropriate pages of the manual, I thought it might be a polite hint to RTFM. And yes, I'm having the issue on channels 3/4. Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
johnpaul215 Posted April 2, 2013 Report Share Posted April 2, 2013 The motorcycle could be putting out a lot of subsonic sound pressure that is overloading something that is damaged, but not acting up under other operations. It also might not seem as loud as it is. Quote Link to comment Share on other sites More sharing options...
Miles Anderson Posted April 2, 2013 Report Share Posted April 2, 2013 that's what I'm thinking too Quote Link to comment Share on other sites More sharing options...
Freeheel Posted April 3, 2013 Author Report Share Posted April 3, 2013 That's possible and something I've been thinking about as the most likely problem. ( although subsonic usually refers to aircraft flying slower than the speed of sound - but I know what you mean. ) The low cuts are engaged on both the microphones AND the mixer, which should reduce infra-sonics in the signal chain. And I think that infra-sonics could potentially overload a mic diaphragm, but a failing mic diaphragm in either of the mics wouldn't necessarily have the symptom I am seeing. And actually, you've given me a great idea for testing this problem. The airport isn't far away and has a little park at the end of the runway. If a 747 at 200m can't recreate this problem in a repeatable way, I'm not sure what can. This way at least I can test different pairs of channels, and also test the same mics with different mixers. Cheers, Brent Quote Link to comment Share on other sites More sharing options...
Derek H Posted April 3, 2013 Report Share Posted April 3, 2013 I hate problems like this, best of luck! Quote Link to comment Share on other sites More sharing options...
Freeheel Posted April 3, 2013 Author Report Share Posted April 3, 2013 yeah, thanks, me also. Lots of variables and relatively high difficulty of triggering the problem + relative certainty that it will screw up at exactly the wrong time on set = grumpiness Cheers, Brent Calkin Quote Link to comment Share on other sites More sharing options...
VASI Posted April 3, 2013 Report Share Posted April 3, 2013 Maybe in input 3 or 4 they have some dust or other little garbage. Check it. Quote Link to comment Share on other sites More sharing options...
studiomprd Posted April 4, 2013 Report Share Posted April 4, 2013 " a little more complex " yes, it is... the amount of "S" that is added to L and subtracted from R controls the "spread"... but I do suspect that the cancellation you are experiencing is related to this... are you set at max separation ?? are you recording the M and S mic's, and decoding for in your cans (preferred)? check the individual tracks. " subsonic usually refers to aircraft flying slower than the speed of sound - " it frequently refers to sounds that are below the range of typical human hearing Quote Link to comment Share on other sites More sharing options...
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