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Cell Phone Recording Filter?

Hi,

I have a need to record some of my cell phone conversations for contracts. I have purchased a Wireless Phone Recording Controller from Radio Shack. The problem is that my phone interferes with something in the recording device that causes an annoying buzz to be superimposed over the conversation, making it difficult to listen to the recording. I am using a digital voice recorder, but have also tried an anolog microcasette recorder and experienced the same problem.

The radio shack device plugs into the 3/32" hands free headset jack of the phone and runs on 2 AAA batts. The hands free headset plugs into a

3/32" jack on the controller. The controller plugs into the 1/8" mic. jack of the recording device. Apparently, the radioshack device is an audio mixer.

The sound is not present on the headset or on the other end of the conversation. I only hear the buzz when I play back the recording. The phone produces this interference with the recording device, even if the two are not connected. That is, if I turn the recorder on as though I was going to dictate into the built-in mic. and make a call from my cell phone, without the two being connected in any way, the recorder records the same annoying buzz. No wires, no headset, just the raw devices.

I understand that GSM phones can cause havoc when in close proximity to speakers, and this is a motorola v220 gsm phone. So when I did same test above, I was able to eliminate the interference and the buzz sound by moving the cell phone 8 feet or so away from the recording device. But, even at that distance, with the phone connected to the recorder through the controller, the buzz reappeared.

The above experiment suggests that the interference can be eliminated, if I can prevent the signal from entering the recorder through the mic. wire.

Suppose I move the recording device far enough away from the cell phone, or shield it by placing it in a metal box. Now, can I eliminate the buzz that seems to be transmitted through the mic-in wire? Is there a filter, a choke or some other device I can place between the controller's mic plug and the mic-in jack on the recorder? If there is, where can I buy it and how would I go about installing it or building it if I have to? I have very little radio electronics experience. Or, should I just scrap the scheme I am using now for an alternative interface?

Please help.

TIA,

Jeff

Reply to
JBBMI
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Roger,

Thanks. By the way, its Jeff.

Sorry, I thought I was clear. I did try the box already. But, as I already explained, distance also works. In both cases, I can get a clean recording without wires. But once the wire is connected, I get the buzz. I either need a filter of some sort, or another interface. I suppose the mic. line from the controller to the recorder is transmitting the interference signal directly to the recorder.

I noticed that the buzz is not a dynamic frequency, but always seems to be the same note, possibly a scale higher or lower at times. It also pulses when connecting and then becomes steady during conversation. There are even times it is not too loud or audible at all. I think its a single frequency that is being pulsed to send data over the voice transmission.

Thanks, Jeff

Reply to
JBBMI

Get one of those ferrite rings found on computer cables and put the shielded input cable of the recorder inside the metal box a couple of turns around the ring, no need to solder even. If this is not enough, you can solder a small (220 -1000p) ceramic capacitor across the input jack of the recorder.

--
ciao Ban
Bordighera, Italy
Reply to
Ban

Roger,

Thanks so much for your help. I really appreciate the effort you gave to me. I will try the solution you provided and report back. Hopefully I can do this within a few hours.

The cord is not shielded. I could replace it if necessary.

Jeff

Reply to
JBBMI

Well, modern cellphones don't send an analogue signal, it's all digital data.

The screened box is likely to be the only practical answer.

Graham

Reply to
Pooh Bear

Cell phones are notorious for interfering with other electronic equipment. I doubt that any filter will help. In fact I expect you'll get that buzz even with no mic plugged into your recorder. Try and see. If the buzz is still there then its a non-starter.

American ideas about the sizes of mini-jacks are always good for a giggle too. The actual sizes are 2.5 mm and 3.5 mm btw. And yes I know that Rat Shack call them by the bogus inch sizes too.

Graham

Reply to
Pooh Bear

....

Joe, I think you can succeed.

Some kind of scheme involving a metal box will definitely work. Latch on to this basic fact: the interfering radio signal inside a completely enclosing metal box with no apertures and with tight or overlapping joins is known to be miniscule. In practice, even significant apertures in the box will likely be OK.

Step 1 is to put the recorder in a metal box and get a clean recording with no cable connected to the phone or recorder. Instead of a box, you might wrap the recorder in aluminium foil - leave a hole for the microphone ! The ultimate "box" which will always work, is a metal box with a screw-down metal lid - often diecast zinc or aluminium, but sheet metal can work. You may not even need a lid - just sit the recorder down on the bottom of the box. Step 1 is not finished until you have got that interference-free mic recording.

Step 2 is to see if you really need a filter on the cable, or if the box alone is enough. Connect the cable, put the recorder back in the box and try ! If the interference comes back, you need the filter. For the filter, you will need a capacitor, or a resistor and capacitor, or perhaps a ferrite donut. The filter part is messy, so I suggest you do Step 1 then let the group know how you got on.

Roger Lascelles

Reply to
Roger Lascelles

So get a ferrite core of some kind to thread the cables through, where they come into the box, like a ferrite bead, but bigger. And maybe more turns. Also, if the mic wire is shielded, ground the shield to the box.

Good Luck! Rich

Reply to
Rich Grise

OK...

RC filter did not work. Isolation transformer did not work. Have not found a ferrite core yet. If I do and it works I'll let you know.

Thanks to everyone for trying!!!

Jeff

Reply to
JBBMI

OK Jeff, Step 1 is completed - you have a metal box which eliminates the interference with no wire connected.

Now, for Step 2. With the recorder in the box, you connect the wire, and the interference comes back. A filter can definitely cure this.

It is not likely you can just buy the filter, because connection to to the metal box is part of the secret.

Here is my sketch

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The diagram shows a cross section of a hole drilled in the side or bottom of the metal box. If you are using an open topped box, the filter may not work if you just add the R and C as the wire passes over the lip of the box - you need the hole an inch or more away from the lip.

I am assumimg that the cable is a coaxial shielded type, but if not, connect to the box the wire which connects to the plug body.

The capacitor is a small ceramic disk type. The capacitor leads must be short - i.e. solder to the box and resistor right at the capacitor body.

The resistor is a small carbon or metal film type with a wire at each end.

The R and C values are not critical. 220pF might be a better value than the

1000pF on the diagram.

Another post by Ban has ideas I like and the ferrite ring requires no cutting or soldering. However, the filter through the box wall is the industial grade solution with a very high probability of working first time without further experimentation.

Roger Lascelles

Reply to
Roger Lascelles

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