Digital/Static-y noise from Microphone

[xchrisxtopher]

Hi there, long-time SOS reader as well as long-time lurker. Hopefully someone can help me with an issue I'm experiencing.

I have an Aston Spirit always at the ready to track vocals with. The Aston Spirit is connected via an ART P16 Patchbay to either an Apogee Symphony MkII, as well as a Symphony Desktop. Both interfaces are ADATed together.

The issue: During vocal sessions both myself and the vocalist have noticed static/digital noise come and go when the Aston Spirit is active. It hasn't proven unsurmountable. But it certainly casts doubts on sessions.

This I can confirm:

-This is not due to sample rate mismatches, as I've isolated each interface from the other.

- This issue occurs regardless of mic I have connected. I've tried the Aston Spirit, AKG p200, and Shure SM57.

- This issue occurs regardless of cable connected. Short or long, doesn't matter.

- I've connected microphones to the Symphony Desktop in standalone mode, as in not connected to a computer, another interface, and have completely moved the interface away from my desk. Listening only with the internal mixer.

- I've also unplugged every Phillips Hue bulb and hub in the space. Turned off rack mount units, etc.

The issue persists.

Here's an audio example of the issue of what's being captured:

https://www.dropbox.com/s/l7pcqnhlbe0zv ... g.aif?dl=0

Any help is appreciated!

[Hugh Robjohns]

This sounds like an interface issue to me.

First off, I'd check whether the clicking is affected by the buffer value. A short buffer (for lowest latency) is more likely to cause this kind of issue.

Secondly, I'd try different leads between the interface and computer just in case you have a dodgy or damaged cable.

[xchrisxtopher]

This sounds like an interface issue to me.

First off, I'd check whether the clicking is affected by the buffer value. A short buffer (for lowest latency) is more likely to cause this kind of issue.

Secondly, I'd try different leads between the interface and computer just in case you have a dodgy or damaged cable.

I appreciate the feedback Hugh!

To your first point - my buffer was set to 1024 in Logic Pro when I recorded the example. Which is the largest buffer size in Logic Pro.

Regarding #2 - I experience this issue with both the Symphony Desktop and the Symphony MkII. I suppose it's possible the MKII's thunderbolt cable *and* the Symphony Desktop's USB cable could both be damaged. I'll swap them each and try it out.

However, I'm listening right now through the direct mixer of the Symphony Desktop, which allows me to route the analog input directly to a headphone output (as does the MkII), and I still hear the digital noises. So a computer/DAW isn't required to perceive these issues.

I will say I spent some time with the Symphony Desktop completely disconnected from my Mac and DAW, monitoring as I am right now. And as I moved the microphone around the room, the noises are more persistent in some areas than others. Is it possible there is interference from elsewhere?

[Hugh Robjohns]

Is it possible there is interference from elsewhere?

Yes of course. It's a case of eliminating the most common things one by one, and then exploring the less common and more subtle possibilities, until you find the source.

So we can rule out buffers and computer cables, as well as specific mics.

I'm intrigued that the problem is worse in specific parts of the room. That's definitely something to explore further...

Listening to your audio clip, it might be mobile phone interference or some other strong radio frequency source — WiFi repeaters or that kind of thing....

[xchrisxtopher]

I'm intrigued that the problem is worse in specific parts of the room. That's definitely something to explore further...

I should clarify that the problem that gets worse based on mic placement is the digital whining that can be heard through my example file.

I do have a variety of bluetooth/wifi devices in my immediate studio space:

1) Wifi router

2) 7x Phillips Hue lightbulbs

3) 4x Phillips Hue Smart plugs

4) Mac keyboard (connected via Bluetooth)

5) Magic Mouse (connected via Bluetooth)

6) iPad (connected to wirelessly)

7) Mac mini (connected wirelessly)

8) iPhone

But again, disconnecting at least the Phillips Hue devices did not seem to make an impact.

[Hugh Robjohns]

Disable or de-power them all, and if the noise goes away then add them back one by one till you find the culprit(s).

And don't forget it could be devices in the rooms next door/above/below.

[xchrisxtopher]

Understood! Thanks for the help, I’ll report back.

[James Perrett]

That certainly sounds like radio interference to me though I'm not sure what sort. Do you have a decent ground in your system? Are their any radio devices in the rooms adjacent to your studio?

[xchrisxtopher]

That certainly sounds like radio interference to me though I'm not sure what sort. Do you have a decent ground in your system? Are their any radio devices in the rooms adjacent to your studio?

Thanks for your reply, James. I’m renting an office space within a building with 4 other units for my studio. I don’t know what exists in the other units, but I do know each unit has internet service. I assume the ground is okay (“assume” being the key word). All musical and computer equipment is connected to several Furman power conditioners.

As I mentioned above, I have Wi-Fi routers, Phillips hue lights and hub, plus Wi-Fi/Bluetooth connecting devices. All of which I’ll turn off and reintroduce as per Hugh’s suggestions.

[Eganmedia]

I know that noise all too well! My studio is across a parking lot from a big FM radio station. I knew RF was going to be a problem when I built the studio, and Fran Manzella (www.fmdesign.com) who designed the studio specified a serious grounding scheme to deal with it. It worked well until a few years ago when the station started licensing space on their transmitter tower to cell phone companies. The tower is absolutely full of cell phone antennas now. I have a handful of old Neumann mics which seem most prone to picking up that high frequency chatter. One of my favorite mics that has been in my collection for decades- a Neumann SM69fet stereo mic, is unusable in this location. Whenever I do remote gigs or work in other studios I can use it, but here it sits on the shelf. This Youtube video- https://www.youtube.com/watch?v=Qj7bdZTwdv0&t=1s shows a super simple mic cable wiring fix that worked perfectly for most of my mics that had the problem. Still no luck with the stereo mic though. Good luck. You can tear your hair out chasing down those kinds of problems. Ground loop hum is a common problem that can be remedied with good cables and a proper grounding scheme. RFI is much harder to deal with.

[Martin Walker]

Hi Eganmedia, and welcome to the SOS Forums!

What a helpful first post as well

[Hugh Robjohns]

This Youtube video- https://www.youtube.com/watch?v=Qj7bdZTwdv0&t=1s shows a super simple mic cable wiring fix that worked perfectly for most of my mics that had the problem.

That's hilarious. A lucky bodge, applied without understanding, fixing an issue that wouldn't have existed had he used a properly constructed XLR cable in the first place!

Microphones have a metal case (and grille) to provide a Faraday shield which keeps RF interference out of the high-impedance parts of the mic's circuitry. And that casing needs to be grounded via the connecting cable's shield.

The AES recommendation for the wiring of standard XLR cables calls for the cable shield to be connected to both the connector's shell tag and pin-1. This ensures a good ground to the mic casing, and hence no RFI.

If the cable shield is only wired to pin-1 in the XLR connector the mic's casing relies on getting grounded via a link inside the mic itself.

Now, in high-quality modern mics pin-1 of the male XLR in the mic itself is 'swaged' directly to the mic case to achieve the best possible direct ground path.

However, in some (many) vintage mics, boutique mics, and budget mics, this is rarely done because it's expensive and difficult to engineer properly. Instead, the typical arrangement is a simple wire connection between pin-1 and some part of the casing inside the mic.

Unfortunately, though, while that arrangement might provide a low-impedance ground at DC and audio frequencies, the short length of wire becomes an inductive aerial at radio frequencies ... and in our modern RF-swamped world that's a well-known way for unwanted RFI to get directly into the mic's circuitry.

With mics built like that, moving the cable shield connection from pin1 to the XLR shell contact will improve the mic case's RF grounding, thus reducing RFI. The mic's internal ground connection between the case and pin-1 preserves the phantom powering, of course.

However, EXACTLY THE SAME RFI improvement would be achieved simply by linking the shell contact and pin-1 in the cable's XLR — as per the AES recommendation, and as done in virtually every commercially manufactured XLR cable.

So the video's fix is not fixing a problem with the mic, it's fixing a rather embarrassing problem with the home-made mic cable....