Setting up the Kramer Audio Matrix Part 2

Continuing from the last post (albeit the delay), the Kramer finally showed up. To my surprise, I realised looking at the connections on the back, that the Kramer wasn't just a 16x16 switcher, but it was stereo 16x16! Awesome! Looking in more detail at the connections:

Kramer Back Connections

there are 3 connections for each input/output. In(or out), Loop, and Gnd. Two of the connections are obvious but I was a bit mystified at the loop. Using a multimeter confirmed my suspicions of it being a passthrough connection - presumably to allow connection to another Kramer switcher of some kind. This did leave me slightly confused though. Why would they bother putting a loop through connection that is just directly connected to the In/Out when you could simply connect two wires into the Phoenix connectors on the back? Anyway, the loop inputs were quite useful as they are next to the ground meaning you can use them as the main In/Out connector if you're skimpy on phoenix connectors!
You may also notice that there a couple of audio transformers on the back of the switcher. This was to convert from balanced audio going to my room to unbalanced which the kramer uses (and also allieviates any ground loop problems that may arrise as well).

We used ethernet to actually pass the audio to various rooms in our house. The original plan was to use the layout Left Ground Right NC NC Right Ground Left layered ontop of the T568B RJ45 colour codes with the first group as input to the kramer and the last group as output. This had the side effect of twisting together the right of the input signal with right of the output signal causing significant crosstalk along the length of a 10m ethernet cable.

Eventually, we changed to another layout seperating the two Left/Right signal pairs and ground pairs resulting in Left Right Ground NC NC Ground Left Right with again the first group being input and last group being output from the kramer.

Another thing to note, the switcher has an "input 0" to disconnect an output, however, this can cause some buzzing and slight crosstalk from other input channels. I have found it better to use an extra input and short it to ground and use that as a disconnect.

RS232 Control

The RS232 control as detailed in the 2000-spec is bidirectional. We use a python program to simply write the 4 bytes and then read back. There is also the need to read all the time as if a connection is made locally (on the front panel), the kramer will output this over serial and we will need to update the web interface.

Code for this can be found on github: (Currently Private - should be public soon) https://github.com/JoshuaCurry/Python-Kramer-Audio
The send_kramer.py takes in on stdin "Input,Output" which will map the input to the output and on stdout will give in the same format any feedback from the kramer (including if a connection is made on the front panel)

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