So, I decided that CSR Technical Services needed a blog. So now it does! Here’s a copy of my first post there, about the recent issues we’ve had with the Station Switcher
Here at CSR, we have a number of different places which we might want to broadcast from. For example, we have two studios (Studio 1 at the University of Kent, Studio 2 at Christ Church University) a sustainer service and we often perform Outside Broadcasts which require taking a signal from another source again.
Therefore, we have an airchain which includes a station switcher (specifically two Sonifex Redbox OA3 daisychained together to give us 5 possible audio sources). In order to allow presenters to switch the station output between the sustainer service and the studio output, we provide two buttons – offer and accept. The station switcher allows the operator of a studio to “offer” to take control from other studios, and “accept” control for other studios. The sustainer, since there is no human present, cannot offer to take control – therefore a special case is given whereby the studios may force it to take control by holding the offer button then pressing accept.
The buttons (and indicator lights) are connected to the station switcher via a D-sub connector, which goes into a control interface. This is fine – however we wanted to be able to control the switcher from within Engineering where the switcher is located. Thus, many years ago, one of the Tech crew built a 2U rackmount box which allowed us to “press” the offer and accept buttons remotely. It essentially consisted of a box with a bunch of push to make buttons mounted on the front panel.
However, there was one slight issue – Studio 2 is at a remote site, and obviously we can’t run a piece of wire from the buttons half way across Canterbury to the switcher! Therefore, we made use of the remote GPIO pins on the MDO STL-IP units which we use to transport audio to and from Studio 2. This allows us to send digital signals (i.e. high or low) over IP – we connected up a switch box at Studio 2, plumbed this into the GPIO ports which were replicated up at Kent. However, the signals which came from the STL-IP unit were not the same as the ones required by the switcher. Therefore, in our custom made box we had to build some level shifting circuitry which converted the levels to ones which the switcher could accept.
This circuit was built on veroboard, and looked a lot like this:
The chip at the bottom is a octal darlington array, the one at the top a hex inverter. This successfully shifts the level, and everything worked perfectly for the time which we had Studio 2 online.
This all worked just fine up until recently – however when we took out the STL-IP boxes for maintainance we discovered a slight flaw. Since the STL-IP box was disconnected, the inputs on the CMOS inverter were floating. This caused the control signals to the switcher to fluctuate wildly – this in turn caused the switcher to constantly give Studio 2 control of the switcher! This was not good (thankfully we have backups in place to prevent this being a showstopping problem).
The solution: attack the insides of the switch box with a hacksaw, remove the circuit, and disconnect it permenantly. Now, it shall never bother us again.
Until we need to work out how to remotely put Studio 2 on air.
