Tech Question: cell phone mic inputs

Perhaps some of you who are into Podcasting have run up against this qustion:

What is the typical input impedance of cell phone mic inputs? They are Hi-Z and I suspect somewhere between 10KHz and 50KHz but haven't tested anything yet while I wait on cables to arrive. Has anyone had to resolve this issue before for Podcasting, call bridging, or recording? Can a cell phone accept a line level without distortion or overload?

Sweet...More detailed information on the web than I expected to find.

Here's a link if anyone is interested, apparently there is also more than one wring standard for the TRRS plug. If not working consistently with the same device it's necessary to have a switch or switchable XLR inline for the mic return.

Thank you Steve, (and Jeff)

I have a couple iRig2 units on order, which I will drop into the DJ Toy boxes but, I'm also trying to build an all-purpose solution for the big PA boards for call bridging with multiple cell phones in the absence of conferencing solution. That will be an XLR input to the phone fed from a line input so, if not all phones are consistent the solution has to be adaptable for wiring, level, and impedance matching.

For the DJ rigs I'll have to find a mix minus solution. My older high end DJ mixers had mix minus outs, but I've reduced DJ rigs to very discreet tools and the "Record Out" on most economical DJ mixers today are just parallel of the Main.

The whole project just proves I'm bored.

Rather than a recording interface for iPad or Android devices what I really want is a duplex solution to a live cell phone call during an event.

This device might be a ready solution: JK Audio Dapter Two but, I suspect I can build my own for about $20 or use existing inline audio transformers to accomplish the same thing at the mixing board. Ideally - I'd like a solution with a single lightweight TRRS cable to a phone at the podium or FOH so that the user experience remains unchanged, even though the call is conducted entirely through the PA system. I want a quick and easy way to add a remote caller into the presentation without need of a PC for conferencing, or the associated latency.

The first instance of this will be town meetings, where a key town official may not be able to attend directly because of quarantine or social distancing concerns. The other case is politicians who are scheduled to appear but, back out the last minute. It's unlikely I'm going to be given the personal cell phone number of a Governor/Senator/Congressman for example, so it has to work with any potential model of cell phone.

I have a land line bridge for up to two callers, but I'd like to be able to do this on the fly even when a phone lines have not been reserved in advance. I also think people have become accustomed to the instant connectivity of their cell phones. The kind of customer experience I want to deliver is to keep any of the technical details invisible and out of the conversation. I want to be ready to simply say: "Yes, I can do that, we'll have it on stage in a few minutes."

ittigger said:

Could you have a business account on a service like Zoom that you call into with your bridge? Would that work?

Click to expand...

Zoom is currently one of the best services from a latency standpoint, but the latency is still there. Plus, your adding a PC or network to the routing unnecessarily which takes up real estate and adds more choke points. The bridging issue is post phone jack so, even Zoom voice on the phone is no better than a direct call.

Skype would probably be better if you had to do it this way simply because (I think) Skype is now standard on the imaging of most corporate or institutional PCs.

djcrazychris said:

i love my iRig2... but i got an annoying buzz on my last 2 live streams... the forum discussed it and decided it was either the volume on the iRig itself or possibly the usb power cables for the tablet ... still not sure...

cc

Click to expand...

Even with the iRig2 you need an isolation transformer in most applications because the iRig2 doesn't have one internally. You probably wouldn't get a buzz using a cell phone but, you would from the floating ground system of a PC, and potentially the earth ground of the mixer.

If it's not a 60Hz buzz, then there's likely a mismatched impedance. You can add a level matching or impedance matching transformer to eliminate that.

Try to repeat the problem and then insert a direct box on the connections to the board. It's not a perfect solution but, it may reveal for you which connection(s) need to be isolated.

For VoIP I have one of these on my wishlist to replace the analog units. Still a little pricey for my application, even used.

Auto IP2

ittigger said:

You don't need a computer to use Zoom - it has a voice only capability. It doesn't have to be Zoom, which is why I said something like it - with a voice portion.

Click to expand...

It doesn't solve the duplex bridging problem from phone to FOH. The issue isn't getting audio out of the phone - that part is just as simple as plugging in an iPod. It's more complicated to get a reliable full mix back into the phone (of all types and brands) while subtracting the caller's own voice and rejecting residual noise from the PA at the external mic.

In practice - you are speaking into the podium mic while the caller is heard over the PA (just like a radio station studio.) Neither of the signals should be present in the opposing phone returns, and for quality audio all of the level and impedance matches need to be accounted for.

There's also a wiring issue of the TRRS connection having the Ground on R2 instead of the sleeve since the ground is shared by both speaker and mic, but audio systems expect it to be on the sleeve. Whatever solution I use has to account for the two different wiring standards. This is probably why when you read customer reviews of the iRig2 and similar devices there are a few that say it doesn't work or it's really noisy. Those users are likely using devices on the unexpected wiring standard.

ittigger said:

Which is why I suggested using your bridge as a piece of this. If you can grab and feed audio to and from your bridge, then this is easy.

Click to expand...

I think there's a part of this process that alludes you. A bridge is not required for a cell phone, but there are interface issues that have to be resolved to preserve the phones full duplex quality.

I'll keep that in mind if I ever have a Polycom issue....