Cell Phone Recording Filter?

[[email protected]]

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

 

[Roger Lascelles]

.....

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?

.....

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

 

[[email protected]]

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

 

[Ban]

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

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.

 

[Pooh Bear]

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.

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

 

[Pooh Bear]

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?

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

 

[Roger Lascelles]

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.

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
http://www.geocities.com/rogerlasau/temp/PhoneFilter1.bmp

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

 

[[email protected]]

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

 

[Rich Grise]

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.

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

 

[[email protected]]

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