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.

That'll be either impedance or an improperly sheilded cable (most likely).

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!

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.

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.

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,

Ty Ford

Ty, I really think that it's poor circuitry in the FX7, typical of Sony cameras, and his gain is too high.

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.

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.

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!

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

Stefan

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.

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.

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

Sorry if I scared you Pietro !

Shorting inputs (I/P's), is never a problem apart from losing sound temporarily, but it causes no equipment harm. However, shorting outputs (O/P's), is not recommended, although hopefully most low level outputs, - line, headphone etc. won't be harmed either, but don't try it on loudspeaker outputs.

I/P - Input. O/P - Output.

The input of an amplifier is basically a high impedance so that it does not cause loading on the preceding output which is connected to it. However, this high impedance, if left this way, will exhibit more noise than if it were shorted across.

If you have nothing connected & turn the gain up you will hear "some" noise which is the inherent thermal noise of the input devices, - transistors, resistors etc. This noise is present in ALL devices but it is the level of the noise that concerns us here.

In well designed input stages the noise will be much lower for a given amount of gain (amplification) than poorly designed equipment. My experience is that most video camera inputs are in the latter category regardless of brand. Frustrating, as it would cost less than a buck to rectify it. (Don't get me started on this one as noise is just the tip of the iceberg!).

However, in well designed equipment I/P noise is miniscule & in most cases inaudible if used correctly. BUT, as this noise is affected by the input impedance connected across it, it is desirable to feed the input from a LOW impedance. - Ideally the best noise figure would be if we could generate a signal across a short circuit and connect this to our input.

So in other words it is desirable NOT to have strict impedance matching in the interest of best noise performance. In other words, feed a low output impedance into a high input impedance. This is why most good (Pro) microphones will have an output impedance of around 100 - 200 ohms so they present a low impedance termination to the following input. Further if you try to "Match" the microphone output impedance to a similar following input impedance, you will "load down" the signal and it will need more gain to bring it back up, thus increasing your noise further. In low level audio such as this we are simply dealing with voltage NOT POWER, so hopefully from the above, you will see that matching is not necessary or desirable.

Re the 47uf BP, (Bi-polar, - non-polarised capacitor). I was merely referring to some consumer cameras that have a DC bias on their minijack inputs to power electret mics and also auto sense mono switching if only one channel is connected. Incidentally, don't fuss over "ferrite rings" as thay have no relevance here.

R.C.

So the ultimatum is...

These cams have lousy audio circuitry and you have nothing to do besides:

a) Buying a better cam
b) Completely replacing the cam audio parts
b) Going with a separate audio recorder

Right?

Not necessarily so Pietro.

If you want to bypass the cameras loud internal circuitry then it's best to knock the signal down before it gets to the camera, either via Mixer or XLR adapter box with active pre amps (like Juidelink box that you have).

As I said earlier, since you have a Juicedlink adapter, then use it on either the Medium or Low pre amp setting. This will knock the noise floor down and should give you considerably quieter audio.If you are running running your Juicedlink box on High pre amp, or using a straight mic feed into the FX7's mic port, then you will get noisy audio. Oh and also if you are using the JL box and only running one XLR input instead of two XLR feeds, then make sure that you turn down the unused XLR port all the way to '0'. As you mihgt still pickup some extra noise in the JL box, because you are knocking your audio well below the noise floor, lower then the internal pre amps.

But as stated another way to capture great audio is off camera using some of the excellent digital audio recorders available today. I use a Marant PMD620, Edirol R09, Zoom H2, and Edirol R-44 (4-track field recorder), in various situations. I tend to like to use my on camer audio for sync backup purposes only, but also like quiet audio, whcih is why I use the JL boxes on my FX1's.

But for my final video edits, I find that I mainly use my audio recorded from my recorders and dump them on my timeline in my NLE and sync up and edit the video as needed. Using teh captured audio in camera for sync purposes.

Michael, the problem is some electronic rambling noise that is only present when I hit record.

My guess is that the tape transport mechanism and the recording process interferes with the audio in some way.

However this "electronic high pitched rambling" is only perceivable in total silent.

And in total silent I can easily remove it by editing the spectral view on Adobe Audition.

Thanks to everybody in this thread.

Ty, I missed this statement of yours.

Since the FX7 mini-jack has this so-called PLUG-IN POWER, I think that might the problem.

Doing a search on B&H under camcorder XLR adapters I couldn't find anything but beachteks and the Hosa MIT-156 (XLR to Mini) that I already have and also gives me this problem.

So, could you point me to a cable with such a blocking capacitor?

Ciao Pietro,

Trew Audio - Cables, Connectors & Adapters - DV Cam Cables - DV Cam Mono Input Cable

Regards,

Ty Ford