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PostPosted: Wed Nov 07, 2007 11:45 am    Post subject: could anybody suggest a solution Reply with quote

could anybody suggest a solution for the following,!
in order to connect and communicate from a computer to mobile phone and opposite (phone to computer)

i need a system ;which should be like that : from a computer speaker output to be connected to the earphone jack (input of a mobile phone) and an output of an earphone jack to be connected to the input of a computer.
the purpose is to communicate (voice ) between computer and Mobile phone
could anybody suggest the solution for this or send the circuit diagram ?
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PostPosted: Mon Nov 12, 2007 11:40 pm    Post subject: Reply with quote

I 'think' you may be struggling here because of the audio bandwidth is tailored for speech and the IF filtering employed in the reciever... the available audio bandwidth would be less than that you get with a phone connected directly to the telephone line.

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Joined: 07 Jun 2007
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PostPosted: Tue Nov 13, 2007 8:52 am    Post subject: Reply with quote

The audio bandwidth is not a large problem.
An ordinary fixed telephone line typically has a bandwidth of 300 to 3400 Hz, exactly the same bandwidth is used for digital mobile phones (GSM etc). Obsolete analog mobile systems (NMT, AMPS etc) often have a somewhat narrower bandwidth, with the cutoff frequency around 3000 Hz instead.

But, in this case, the major challenge is matching the signal levels at the cell phone and at the computer end of the interface.

Those signal levels and arrangements vary a lot between different brands of cell phones, while most computer sound cards are relatively standardized.

Typically, you will find that the cell phone has an output that is intended for a earphone of 32 or 150 ohms impedance. In some cases, the output is balanced, in other cases, it is ground referenced. When the output is ground referenced, it might in some cases be necessary to have a DC blocking capacitor in series with it. Some cell phones have facilities, i.e. "answer button" that can be activated by temporarily shorting the earphone output to ground.
This output has to be matched to an input of the computer's sound card. If there is a line input, you can probably connect the signal directly there, through a capacitor as mentioned above. If the sound card only has a mic input, it is probably designed for an electret microphone which delivers just a few millivolts. That means that you must connect a voltage divider ahead of it. Note that some sound cards deliver supply voltage to the electret mic superimposed on the signal line while others use the ring of the 3,5 mm connector for a 2,5 V or 5 V supply.

The interface in the other direction will be approximately the same. But all sound cards have ground referenced outputs, in some cases there are headphone outputs as well as line outputs to choose between. No dc blocking capacitor is necessary at the sound card.
The sound input of the cell phone can usually accomodate either line level (some hundred millivolts) or electret microphone level (a few millivolts). The switching between the modes is usually done by connecting various DC loads (resistances) between the input and ground. The line level mode is used for comprehensive car kits etc, while the mic level mode is used for cheap handsfree kits.

So, anyway, you should first determine which model of mobile phone you want to use and then search the internet for documentation of its bottom connector, signal levels etc. Usually, there are tecnically oriented fan clubs and forums dedicated for most major cell phone makes.

A problem might be EMC issues, i.e. that the RF field from the cell phone's transmitter interfere with the sound card circuitry. Solutions might be small resistors and/or ferrite coils in series with all signals, decoupling with capacitors in the range of tens of pF to ground, putting toroidal ferrite cores around the cable to the cell phone, increasing the cable length and placing the cell phone well away from the computer. It can also help to make sure that the cell phone gets an as good as possible transmission path to the base station, so that it has a chance to be commanded from the base to decrease its output power.
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PostPosted: Mon Dec 03, 2007 5:45 pm    Post subject: Reply with quote

i think all you need is sotware to communicate with your computer. lots of them around. be carefull of what you wish you are opening up to stealing info.
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laith sameer kazim

Joined: 14 Dec 2007
Posts: 1

PostPosted: Fri Dec 14, 2007 11:17 pm    Post subject: Reply with quote

i see device for this purpose but i not rember the name it has connector that connect with phone inter face and has system like the semoll box has the connector for speaker and computer
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PostPosted: Wed Dec 19, 2007 5:46 am    Post subject: pc/cell Reply with quote

Check out the G3 abled cell phones,theres some new bluetooth options that might do what I think your tring to do..dls
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