Tech Notes
Hands Free Connections
The Matrix TelCell Module and BlueBox Cellular Port add the ability to send and receive audio through the Hands-Free port of many mobile phones. While this feature does not actually add any audio enhancement to the wireless phone call, it makes a useful back-up tool, and often the addition of professional microphones will make a substantial difference in audio quality over the cheap microphones generally found in phones. In this mode, calls can be placed to regular telephone hybrids (no Matrix or BlueBox is required on the receiving end of the call).
In Wireless Hands-Free mode, the mobile phone keypad is used to place the call. The Matrix must have the TelCell Module installed and be set to Hands-Free mode via the Dial->Tel/Cell->Cell Phone selection in the menu. This will route all send/receive audio from the Matrix mixer through the 1/8” jack on the module labeled Cell.
The BlueBox does not require any menu or configuration changes. Simply leave the BlueBox at the Main Status Display. You will place the call using your cellular phone’s keypad. This will route all send/receive audio from the BlueBox through the 1/8” jack on the side of the BlueBox labeled Cellular.
The jack labeled Cell on the Matrix TelCell Module or Cellular on the BlueBox will need to be connected to the Hands-Free port of your mobile phone. This is done differently for different makes of phone. For most phones with 2.5mm headset jacks, an 1/8”-to-2.5mm cable (Figure 1) will make the connection. Some phones may require a modified cable (Figure 2), included with new TelCell modules and available upon request to owners of BlueBoxes and past TelCell modules.
Other types of phones will require a modification and/or an adapter (see below). We have tested the following modifications with success, but please note that Comrex cannot take responsibility for your headset or phone when performing these modifications.
Motorola
Most Motorola phones have a 2.5mm jack built in for Hands-Free operation. To connect a Motorola phone to the Cell/Cellular jack, you must construct a cable with a 1/8” three conductor plug on one end and a 2.5mm three conductor plug on the other. As shown in Figure 1, wiring is 1-1 (tip-tip, ring-ring, sleeve-sleeve). However, the Motorola StarTAC requires an 1K ohm resistor between tip and sleeve, shown in Figure 2:
Figure 1
Figure 2
Ericsson
These phones use a specialized plug that attaches to the bottom of the phone as shown below. The easiest way to build this cable is to get an Ericsson Hands-Free set and use the cable, removing the microphone and earpiece. Wire the cable to an 1/8” plug as shown in Figure 3. The cables we tried consisted of braided wire with individually insulated strands. This wire is extremely difficult to solder. Our recommendation is to break apart the plastic housing of the microphone on the cable to find a small PC board. Unsolder the wires on the PC board and connect your 1/8” mini plug to the wire ends here, where the wires have already been stripped.
Figure 3
Nokia
Like Ericsson phones, Nokias use a specialized plug that attaches to the bottom of the phone as shown below. The easiest way to build this cable is to get a Nokia Hands-Free set and use the cable, removing the microphone and earpiece. Wire the cable to an 1/8” plug as shown in Figure 4. In order for the phone to switch to “Headset Profile,” a 1K ohm resistor is required between the tip and the sleeve of the 1/8” plug. The cables we tried consisted of braided wire with individually insulated strands. This wire is extremely difficult to solder. Our recommendation is to break apart the plastic housing of the microphone on the cable to find a small PC board. Unsolder the wires on the PC board, and connect your 1/8” mini plug to the wire ends here, where they have already been stripped.
Figure 4
Samsung
The Samsung line of phones uses a unique type of connector, which can make this interface especially challenging. Although it looks like a typical 2.5mm 3-conductor jack, the Samsung hands-free port is actually a 4-conductor jack with unique pinouts. We speculate this design was to allow operation only with Samsung headsets. Because this connector is so unique, the easiest way to interface will likely be to modify the headset included with your phone to make the required interface cable.
This headset, like many others, is made with twisted strand wires that are individually insulated. This makes soldering to the wires very difficult. We recommend scraping as much insulation as possible from the individual strands before soldering and using a very hot iron to melt away remaining insulation.
The connections between the Samsung 4-conductor headset plug, and the Comrex 3-conductor “Cellular” plug are shown in Figure 5. The Samsung cable consists of 2 pairs of wires, each with an insulated center conductor and an outer “shield.” Tie the 2 shields together and connect them to the “shield” conductor on the Comrex 1/8“ plug. The red inner conductor should connect to the tip connection on the Comrex 1/8” plug, and the green inner conductor should connect to the “ring” conductor on the Comrex 1/8” plug.
Note that if you “ohm out” the finished cable, the connections should be like this:
Figure 5
Other Brands
For other brands and models, an adapter can convert various types of headphone jacks to accept the 2.5mm cable supplied with your TelCell module (or available to BlueBox owners from Comrex, free of charge). A wide line of adapters is available at: http://www.cellularaccessory.com/audioadapter.html.
Please note that some cables may still require modification even when used with an adapter.
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