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-   -   A-T 835ST - When to use the roll off? (https://www.dvinfo.net/forum/all-things-audio/26818-t-835st-when-use-roll-off.html)

George Ellis May 31st, 2004 11:39 AM

A-T 835ST - When to use the roll off?
 
OK, this is a sample of 1, but I think I have found the best time to use the low-frequency roll-off switch on my mic. I have been having a problem I have been trying to figure out. When I am recording a drum corps, I will have my 835-ST on a mic stand with a 25' 5-pin cable into my A-T supplied 5 to 2-3pin XLR into my DXA-8 Beachtec. The mini-jack is into my Sony VX2100. I check and recheck my levels to the pit, drumline, and Doctor Beat (the pit is all of the non-marching percussion including 'bells'). I have the audio on manual at 20-25% on camera. Limiters are on. I do not get any redline indication in the on camera audio levels. New, fresh battery.

The moment the high brass, aka trumpets, start to wail, I pickup a slight crackle. It was baffling. I thought I had a static ground out. I tape the connectors even.

As a last resort last night (after I had already shot what I really wanted to record), I tried the roll off switch to on (a 'bent' line). It seems to have disappeared.

Was I just lucky in this one instance or could the roll-off switch be the fix? I thought it being low-freq roll-off, it would be something like the bass drums (and however you spell timbani, err kettle drums) that would affect it. What am I missing?

Jay Massengill June 1st, 2004 09:23 AM

Your DXA-8 may be overloading the input on the camera due to the total energy of the loud passages, including extra bass energy. This can occur before the level controls and metering, so it would only be noticeable while listening to the loud passages and it may be most easily noticeable in the high end.
Engaging the bass roll-off is probably robbing enough energy from the signal to eliminate this overload. It's also possible that this low-end energy isn't even audible on your camera, so you may be able to get away with it with no ill effect on your audio.
You'll have to do some further experimentation with the DXA-8 levels, the limiters if they are adjustable, etc. to find the best combination.

George Ellis June 1st, 2004 04:17 PM

Thanks Jay. It might be. I think I am safely inside the range of the mic.

I will leave the roll-off on for awhile and see if that has cleared it up.

Bryan Beasleigh June 1st, 2004 04:43 PM

Sub sonic frequencies and extreme lows can overload a microphone very easily. There is also a paragraph that addresses this issue in the PDF from Audio - Technica. This could also be the reason they included the bass roll off. An outdoor drum corps is a difficult subject at the best of times.

http://www.audio-technica.com/prodpro/addinfo/AT835ST_english.pdf

George Ellis June 4th, 2004 10:19 AM

I am in post now and I think I may have found the contributing factor, wind. It was breezy. I has a Mike's Muff on, but the gusts were probably over 10mph but closer to 15. So, the wind noise may have help.

George Ellis July 11th, 2004 12:03 PM

A post-mortum. I think I have isolated this to the mic. The field is just too loud for it. If I am about 20-50ft away, the elements in the mic are the culprit. I found this out when my battery was low in my DXA-8 this weekend at Portland Head Lighthouse. I got the clicking static when I moved the camera. The Beachtec was not rejecting the noise and had low phantom voltage because of the almost dead battery. I could tap the mic gently to reproduce it. Pulled the Mike's Muff and foam cover off (man, do they reject noise!) and could physically see the elements rattling in time with the noise.

I guess my best bet is to figure out a way to dampen the volume to the mic to reduce its sensitivity. Otherwise, I need to make sure my mic has a little more distance from the source. In the event environment, it may be best to figure out how to dampen the input.

Bryan Beasleigh July 11th, 2004 02:13 PM

Buy an in line pad (attenuator)
AT adjustable pad

UniMute

B&H cheapy

For the two AT products browse the same section of B&H as the link takes you.

Douglas Spotted Eagle July 11th, 2004 02:17 PM

Stick in a -50dB attenuator, that should clear you up. Not expensive at all.

[edit] oops, Bryan was posting the same thing/same time. Great minds and all that...:-)

George Ellis July 12th, 2004 11:23 AM

DSE, -50? I did not see such a critter at B&H. Do you think I should pursue it?

Thanks guys. Otherwise, I will try the A-T solution. I will need two and I think the A-T may be the best bet with the selectable -10/-20/-30.

George Ellis July 12th, 2004 04:19 PM

Bring on the corps...
 
Just a slight mid-stream adjustment. I found Shure 50dB attenuators locally. So I will play with them tonight and try them Thursday. I will let you know.

George Ellis July 12th, 2004 05:27 PM

No joy. With the 50dB attenuators inline, I get 0 audio. Full gain everywhere and 0 noise except a camera hiss.

George Ellis July 13th, 2004 09:51 AM

Ok, Update... lost count.
 
The attenuators are back at the store. Beachtek responded too. I do not know a way of using them now. Harry at Beachtek said that 20dB would be about right, but I lose my phantom voltage through the pad. Bad, as the 835st does not have a battery. Those A-T pads support phantom, so I guess I will order 2 and try it. But he also says I may want to switch to a dynamic mic as I may just be too sensitive ;) .

Jay Massengill July 13th, 2004 10:35 AM

Yes, 50db is way too much. The AT -10,-20,-30 switchable pads will support phantom power and are very useful to have on hand for a variety of situations. I'd order a pair to test out at -10.
However, if the mic elements themselves are being totally overdriven, then these attenuators won't help. The distortion will be occuring before the attenuation.
If the distortion is occuring downstream from the mic, then the attenuators may help. It will depend on exactly where and how the distortion is occuring.

George Ellis July 13th, 2004 11:00 AM

Thanks Jay. If they arrive in time, I might be able to get some time before Spirit performs to do a level check. Usually, the groups before the top 12 corps are just not as loud, so it is always a guess until the blast comes. I may take a dish towel and hang it over the mic while on the mic stand and see if that helps as a last resort. The budget just does not include getting a new set of mics yet. I have to buy some other hardware for field reproduction first. sigh.

George Ellis July 14th, 2004 08:38 PM

Got the AT8202s today. They work. Live fire test tomorrow night.

Art Cohen July 17th, 2004 03:33 AM

I'm on the tip of my seat! How did it turn out?

I'm looking at buying sound gear to record a Marching Band and a Drum Corps with a GL2, so I'm very interested in what you come up with.

My best so far has been a mono DR4000 condensor....which was all I had on hand. It certainly beat the on camera and a Sony I bought to try.

Art

George Ellis July 19th, 2004 12:05 PM

I think it was a success. I ran about 25% manual onboard, the attenuators online at -20db, and around 55% on the preamp. While I was recording, I did not hear any of the noise. I have to check it out in post. I am pretty sure it is 'fixed' as I took the camera down and then put it up sans attenuators for the Retreat. During the America the Beautiful/Oh Canada, I could hear the problem again. I was able to do an audio check and calibration during Carolina Crowns performance to get my levels. During their blasts, I did not hear a peep of the crackle. I have to admit that I was busy framing during Spirit's performance, but I did not hear it.

I will post a clip on my webpage for a comparison when I can.

George Ellis July 19th, 2004 09:40 PM

Arrgghhh...

Ok, maybe next time? -20db was too much. The audio is not as bright as it should be. -10 next. Hope I get a chance.

David Ennis July 20th, 2004 09:58 AM

Musings
 
Troubleshooting a problem that only appears intermittently is a nightmare. Ask any repairman. You come up with a theory of the problem that correction "A" should fix. So you want to test the effect of "A" vs. "no A" on the problem. But since the problem comes and goes with no correction applied, the chances of getting a false indication about "A" are pretty good.

Crackling is definitely the clipping of peaks of waves somewhere in the sound reproduction chain. If it's because the 835ST puts out an electrical signal too strong to be handled by another part of the system, then attenuation will help. But if it's because the mic element is being overdriven by sound pressure, electrical attenuation won't help.

Although either or even both could be happening, from my armchair I woud put my money on the sound content overwhelming the mic. When waves mix, two interesting effects occur. The first is that new low frequency ("difference frequency") waves are produced. These would respond to roll off. The second is that wave amplitudes can add to and subtract from each other. From time to time, the chance combination of added waves may exceed the sound pressure input spec of the mic.

Yeah, I'm overly analytical. But the bottom line is that pulling back from the source would tend to correct either cause.

Also *really* watch out for what I said in the first paragraph.


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