Broadcast

Note: You may find it useful to consult Appendix 1: Wiring Diagrams while reading this.

The broadcast system is composed of various broadcast sources (situated in the cool room) which are connected through a computer-controlled switch to the modulator situated in the transmission cupboard. We will consider each part of the system in turn.

Sustain
'Sustain' is the name given to the signal broadcast when we have nothing else to broadcast, i.e. most of the time. It consists of a computer-generated slide show with sound from IC Radio.
Sound is brought into the cool room from IC Radio's transmission DA in the transmission cupboard on a stereo pair of wires. Here it is run through a DA in the sources rack (nearest the door) where one spur is run into the matrix (appearing on source 31) and one spur is run straight into the airswitch.
Video is generated by a computer mounted in the sources rack, known generally as 'Textbox'. This runs a slide show of graphics uploaded into it and generates a video signal via a GameVision VGA-PAL converter box plugged into the back of it. The details of the implementation are given in the computers section. The only important thing to note is that sometimes the GameVision will go into black and white or stop working; usually powering it down and up again by unplugging its power supply will get it working again.

Playout
This is an automated playout system based around a computer in the sources rack playing MPEG files (again generally known as MPEG box which is controlled from an SQL database on Crossroads. Details of the implementation are in the computers section. It outputs using a hardware MPEG playout card (a 4Reel/Pro single-channel model) which is connected directly into the airswitch. The switch is controlled by a parallel link to Crossroads.

Airswitch
The airswitch is a standard three-channel (one video, two audio) parallel switcher, controlled by Crossroads, mounted in the sources rack. It takes inputs from sustain, playout and the studio mix DA (for live transmission) and outputs directly to the modulator in the transmission cupboard. Technically it forms part of the playout system, fully described in the computers section.
Important: Behind the airswitch is permanently on-air. Be careful!

Audio Compression
In order to improve broadcast audio signal quality, the broadcast output is fed through a compressor. This effectively eliminates parts of the audio signal in order to improve the signal quality. It does mean that you can't level compensate for broadcast directly from the sound desk or broadcast source. The compressor is mounted in the audio rack towards the top and sits between the airswitch and the broadcast lines to the transmission cupboard. This means that it's permanently live so be careful when adjusting it.
????FIXME - how do you adjust it????

Where the DAs Are
As can be seen from the wiring diagrams there are many DAs in the broadcast system, not all of which are live. Sustain goes through audio and video DAs to feed it into the matrix, where it is used as an auxiliary input (aka 'Panic Button') on the JVC vision mixer. The audio DA is situated in the sources rack in the cool room (labelled 'Sustain Audio DA') and the video DA is in the card-based video DA racks in the video rack. Playout feeds directly into the airswitch. DAs in the studio system are covered in the audio and video sections.
Behind the airswitch, video runs directly to the transmission cupboard and audio runs there via the compressor in the audio rack. Once in the transmission suite the signal runs through a combined video-stereo audio DA (the Transmission DA), one output of which is connected into the modulator (we believe this is mono but connect both left and right anyway). The other outputs are left to feed into patch within the transmission cupboard for patching to tielines etc.

Modulation - To Infinity and Beyond!
We have made many references so far to the modulator. This is a small piece of kit which takes in video and audio signals and converts them to a standard UHF signal suitable for plugging into a telly. There are two modulators in the transmission rack; our one, broadcasting on UHF channel 21 and IC Radio's broadcasting on UHF channel 28. The modulators are built with a pass-through on them so they modulate their signal over an input UHF signal. So one gets an input from a roof aerial, modulating its own signal over, and the other gets the combined signal from the first. They then run to a UHF DA mounted on the wall. This has a large gain on it, probably capable of driving a signal all the way to Southside. One of the outputs connects down to a standard television in the rack for monitoring. The others connect to boxes on the wall which are direct inputs to the UHF network.

UHF Network
Our signal is broadcast across College via a network of satellite-grade UHF transmission cable. The cables are terminated in small wallboxes with professional (or 'thick') UHF connectors on them.

UHF wallbox in the transmission cupboard
IMPORTANT: when dealing with the UHF network you must remember that different buildings in College can have different earth levels. Since the cable runs without any active elements, this means that you may get shocks off the cable shield when working on it. Also it's not a good idea to feed this straight into equipment. The signal should be decoupled before connecting to equipment - essentially running it through a couple of capacitors to block the DC component. The principle on our network is that 'thick' UHF connectors are not decoupled and 'thin' (i.e. standard domestic UHF connectors) are. Decoupled connection boxes have a capacitor symbol on them:

Decoupling wallbox installed in the JCR. The large black cable connects to the 'thick' UHF wallbox.
Union Bars
At the time of writing, only dB's is connected. The cable is run up to the Union roof and down into the dB's DJ booth, converted to 'thin' UHF along the way, where it is connected to a VCR along with the Sky feed. This is run on UHF to the video projector mounted in the ceiling.

Diagram showing the cable route between the Media Centre and dBs and the aerial