Connect a third party microphone to a Cisco SX20

Preface

The Cisco SX20 comes as a Quick Set including the codec, camera and microphone. It´s designed to use it as it is. In contrast, the C-Codecs Series (C40,C60,C90) are integrator codecs and able to integrate with external AV-equipment. I always recommend to use a C-codec if the SX20 was not sufficient.

For various reasons, however, it may still be necessary to connect an external microphone to the SX20.

About the microphone input

The SX20 has two mic inputs on the rear. It´s a 4pin 3.5mm mini jack with the following pinout:

It should be easy to connect an external microphone, but I found out the following:

Adapter Cable

Using a 3,5mm mini-jack for a symmetric microphone is very uncommon. Symmetric audio connections are using XLR or 6,3mm stereo plugs. There are some adapter cables from XLR-female to 3,5mm, but these are internally bridged to connect a symmetric XLR mic to a asymmetric device like a camera or pc. So it´s hard to get a ready made cable for this application.

Phantom Power

I have found that there is no phantom voltage, even though it is stated in the specifications. The 11V I could only measure between mic control and ground. So it will not be possible to simply connect an external mic, which requires Phantom Power.

Solution

To connect an external XLR-Mic which requires Phantom Power (like the Audioscience Ceiling Mic), do the following:

Get the right cable

We will need a cable with the following pinout:

Microphone Adapter Cable

There are several ways to get such a cable:

• do it yourself: Take a XLR-female plug, a mic cable and a 3.5mm 4pin plug and solder as described.
• buy it ready made: really hard to get, but you can get it from AVnet.ca
• modify a ready made: There are some cables with the correct plugs but false pin-out. But you could modifiy one of these by removing the bridge within the XLR plug.
• use adapter cables: as they are easier to get
• 3,5mm Stereo<-->6,3mm Stereo (without bridge!)
• 6,3mm Stereo<-->XLR Female (without bridge!)
• Plus a TRRS to TRS adapter cable like descripted below

Get Phantom Power

As i wrote above, there seems to be no phantom power from SX20, but it´s easy to solve. Simply get an external phantom power supply, which adds the needed voltage (usually 48Volts). It should be a box with external power supply instead of a battery. Check your mic, which voltage is needed. Many mics accept a range between 12-48V. These little helpers are cheap to get anywhere online or offline.

Rolls PB23 Phantom Power Supply

Even I do not recommend it, you could also make the phantom power supply by yourself by using the 11Volts from mic control. You should check the specs of your microphone, if the provided voltage is high enough. You should also consider, that most microphones do have a lower sensitivity and quality at lower voltages and longer cables yould also cause a lower sound level.

Prevent shortening mic control and ground

I´ve measured 11Volts between Mic Control and Ground. A 3pin 3,5mm Plug would shorten these two pins. It even works but is not optimal. If you´ve made your own cable by using a 4pin plug, simply do not connect the 3rd pin. I´ve used a little headset adapter used for PCs or Mobile Phones to prevent this:

TRRS to TRS Adapter Cable from Lindy

It has the following pin-out:

T(ip): Audio+ <---> Green Headset jack (T)ip

R(ing): Audio- <---> Green Headset jack R(ing)

R(ing): Mic <---> Pink Mic jack (Tip)

S(leeve): Ground <---> Green and Pink jack (S)leeve

Simply put the plug into the SX20 and the mic cable into the green headset jack.

Note: Adapters which are made for Apple iphone, ipad or HTC etc. do not work. They do have a different pin-out. Mic and ground are reversed. A very good overview regarding this can be found here.

The final solution with adapters

External Links

http://www.cisco.com/en/US/docs/telepresence/endpoint/quick-set-sx20/tc6/administration_guide/sx20_quickset_administrator_guide_tc61.pdf

http://www.avshop.ca/wire-amp-cable/adaptors-wire/avshop-cisco-sx20-to-external-microphone

http://www.rolls.com/product.php?pid=PB23

http://www.lindy-usa.com/pc-to-earphone-mic-headset-adapter-cable-35509.html