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Old August 11th, 2008, 08:32 PM   #1
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Interference-like hiss when using the minijack connector

When I connect microphones to the FX7 minijack, in the background I can hear some really minor hiss that looks like interference.

It's really minor, nothing that would ruin audio.
But it's kinda bothering me.

Is this something related to impedance matching? Is there a way to avoid it?
How would I go about cleaning this in post? Would it be worth it?

I'm attaching a short audio clip I've extracted from a video.
I've applied 32db amplification so the noise becomes clear.

Thanks in advance.
Attached Files
File Type: wav interference_32dbAMP.wav (691.0 KB, 78 views)
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Old August 12th, 2008, 12:53 PM   #2
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That'll be either impedance or an improperly sheilded cable (most likely).
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Old August 13th, 2008, 12:37 AM   #3
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I have that both with a Hosa impedance matching cable and the mini cable out of my Juicedlink.

Could I solve this in anyway? Maybe a better mini cable?

Advice is appreciated, thanks a lot!
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Old August 13th, 2008, 08:16 AM   #4
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Pietro, what pre amp setting are you using on your Juicedlink box?

If you are using the high setting, then don't as it won't cut down much of the noise floor.

I am generally always using medium for normal shooting, or low for very loud situations).

I don't think it's your 1/8 cable, as it's not really long enough to be a shielding problem.

I will say that the internal circuitry in Sony cameras are notorious for being noisy.
Bu the Juicedlink box should really correct most of this, as it really cuts down noise in my FX1's.

BTW, this can easily be fixed in post, by either EQ'ing the noise out, or using come noise reduction software.
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Old August 13th, 2008, 09:44 AM   #5
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Sounds just like a cheap audio circuit with the gain cranked. Solution is to record decent level with out clipping. Means riding the faders and keeping on top of the mix. Not easy for a one man band but there you have it.
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Old August 13th, 2008, 10:03 AM   #6
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This is not uncommon with unbalanced audio. There are many opportunities for electrical noise to get into your audio when it's unbalanced.

What mic and cable are you using? Try others to see if it's either.

Regards,

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Old August 13th, 2008, 10:19 AM   #7
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Ty, I really think that it's poor circuitry in the FX7, typical of Sony cameras, and his gain is too high.
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Old August 13th, 2008, 10:41 AM   #8
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Yeah, I exagerated in post so you guys could hear it.

In the original sample I was using a JL with gain setted to high, trims at 50% with my AT897 running on phantom.

With the JL I use the provided cable and my sound is pretty clean aside from this minor issue.

I also hear it using a HOSA MIT-156 ( http://www.bhphotovideo.com/c/produc...w_to_High.html ) connecting my AT897 running on AAs to my FX7.

I'm posting a wave sample without gain applied in the software.

And a screenshot of some weird "spikes" in the spectral view, those squares are during silence.
If I select them with the Lasso tool and delete them the high pitched sound spikes go away, at least while it's silent.
Attached Thumbnails
Interference-like hiss when using the minijack connector-spikes.jpg  
Attached Files
File Type: wav test.wav (688.6 KB, 62 views)
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Old August 13th, 2008, 10:47 AM   #9
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Pietro, listening to the correct (non boosted) sample, that you posted, I am certain that the audio hiss you are hearing is the poor internal pre amps on the FX7.

You will most likely get this when you are using the built in mic as well as just a shotgun mic plugged into your FX7.

If you are using your Juicedlink box, then I would suggest NOT using high gain.

But rather Medium of Low preamp gain settings.
You should get a much cleaner audio by doing this.
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Old August 13th, 2008, 10:56 AM   #10
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You mean gain on the JL itself?

Well, I can't do that with the AT897 I guess.
It's too low output!

But I can use medium with the AT3031 (Cardioid)

And the camera gain is set to 8 dots (les than 1/3 of the total).

I suspected it was because of FX7 poor audio parts, anyway, it's very minor and I figured out a away to take it off when It's perceptible, thanks a lot!
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Old August 13th, 2008, 01:57 PM   #11
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I think it could be high frequenzy from a tv-set or computer monitor nearby. They send a 15 KHz sound to all things electronic within 4 meters.

If this is true, you might want to switch off any tv or monitor nearby.

If not, you could try to filter this out with a ferrite on the microphone cable. They were just switched around the cables. If you like, you can put them on any cable from your camera.

Just my thoughts

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Old August 13th, 2008, 02:51 PM   #12
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I've never seen ferrite rings for sale.

The only ones I've got were with motherboards but I threw them away everytime.

I'll try switching off all PCs and TVs here and unplugging them from the wall.

Thanks!

Edit: Bought a pack of 6 small ferrite rings on ebay.
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Old August 14th, 2008, 09:22 AM   #13
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The noise you have is mainly the noise level of the internal preamp in the FX7. This noise is affected by the impedance feedng it. It should be LOW. (Eg: if you short the inputs the noise will decrease.) If you use a "matching ??" transformer between the mic & the camera, you will raise the impedance that the camera input sees and it will probably be worse, even though the signal will be raised by the step up ratio.

Increasing the gain in the camera will allow more amplification of this so it will be further degraded, but if the source has very low impedance this will be minimised. Incidentally, the input impedance of most camera minijack mic I/P's is around 5 - 10K ohm so they don't "load" the preceding device.

Camera I/P noise is easily checked by making a 3.5mm test plug with a resistor of about 100 ohms connected between earth and each input (L & R) to approximately simulate the output impedance of the AT897. Try various gain settings on the camera.

I would suggest you make (or have made ) a cable that feeds directly from the mic XLR O/P & feeds both channels on the minijack. - XLR Pin 2 to both minijack i/P's. Common earth bridges XLR 1 & 3 to earth on minijack.

I don't know about the FX7 but on some camera minijack I/P's, if the channels are bridged, it senses mono & may cause other problems.
The simple way around this is to simply fit a 47uf BP capacitor between the inputs rather than just shorting them. Hope this helps.

R.C.
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Old August 14th, 2008, 02:21 PM   #14
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Quote:
Originally Posted by Ron Cooper View Post
The noise you have is mainly the noise level of the internal preamp in the FX7. This noise is affected by the impedance feedng it. It should be LOW. (Eg: if you short the inputs the noise will decrease.) If you use a "matching ??" transformer between the mic & the camera, you will raise the impedance that the camera input sees and it will probably be worse, even though the signal will be raised by the step up ratio.

Increasing the gain in the camera will allow more amplification of this so it will be further degraded, but if the source has very low impedance this will be minimised. Incidentally, the input impedance of most camera minijack mic I/P's is around 5 - 10K ohm so they don't "load" the preceding device.

Camera I/P noise is easily checked by making a 3.5mm test plug with a resistor of about 100 ohms connected between earth and each input (L & R) to approximately simulate the output impedance of the AT897. Try various gain settings on the camera.

I would suggest you make (or have made ) a cable that feeds directly from the mic XLR O/P & feeds both channels on the minijack. - XLR Pin 2 to both minijack i/P's. Common earth bridges XLR 1 & 3 to earth on minijack.

I don't know about the FX7 but on some camera minijack I/P's, if the channels are bridged, it senses mono & may cause other problems.
The simple way around this is to simply fit a 47uf BP capacitor between the inputs rather than just shorting them. Hope this helps.

R.C.
I'm using the Hosa cable (Hosa Technology | MIT-156 Low to High Impedance | MIT-156 | B&H).

So, can you briefly say why shorting (this word scares a newbie like me) the inputs would decrease noise?

What's I/P?

I don't know anywhere I could make such cables...

"I would suggest you make (or have made ) a cable that feeds directly from the mic XLR O/P & feeds both channels on the minijack. - XLR Pin 2 to both minijack i/P's. Common earth bridges XLR 1 & 3 to earth on minijack."

I would use this for what?

"The simple way around this is to simply fit a 47uf BP capacitor between the inputs rather than just shorting them. Hope this helps."

What about this one?

Excuse my ignorance, but there were lots of terms I never heard in my life there haha.
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Old August 15th, 2008, 06:47 AM   #15
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OK, Wait a minute.

1. I question that a transformer is needed here. In fact, that could be part of the problem. You don't need to match impedances as long as you are plugging low into high. By that, I mean a low impedance mic into a preamp with a higher impedance mic input is OK.

OTOH, high impedance into a lower impedance input is a problem. Want to keep that theory in your brain? Try this plumbing analogy. Small pipe (high impedance) feeding big pipe (low impedance) lots of slop, not a good match. Big pipe (low Z) feeding small pipe (high Z), works great.

2. There are "regular" cables that are built to connect a balanced mono source into an unbalanced stereo input.

3. There are special cables that have a blocking capacitor to prevent the DC voltage that some camcorders have at their mic input for powering some mics from getting into mics that don't want to see that voltage.

Without knowing the circuits' and mic's technical descriptions, it's impossible to tell which would work best.

Regards,

Ty Ford
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