This Wednesday I was honoured to be involved with the launch of Insanity Radio’s FM broadcasts as a Community Radio station. The main switch-on was planned for midnight on Wednesday, and a special space-themed party hosted at the Student’s Union was planned to celebrate, with a countdown to the switch-on and at midnight, pyrotechnics, balloons dropped from the roof, and Insanity DJs from 9PM through to 3AM.
Of course, we wanted to celebrate with a special broadcast, and we had to manage the technical side of actually switching on FM. This is a quick writeup of how we pulled it off.
We basically wanted to have the studio broadcasting regular shows till 5 minutes to midnight, then we would switch over to the outside broadcast from the SU, the countdown would be held on air, and the switch-on at midnight would go directly live from the OB.
To get audio from the Student’s Union PA system we fed a feed from an aux output of their PA system into a standard desktop PC. This PC was running Ubuntu Linux 11.10 and we had a counterpart to this machine sat in our studio. Each machine was configured with an OpenOB link, running in Linear PCM mode at 48kHz (16-bit) over RTP with a total latency of around 30 milliseconds (giving us a comfy jitter buffer for a reliable network). At the studio this machine fed into a channel on the desk, letting us treat it like any other audio source. The desk output went as normal to our broadcast chain.
To avoid unfortunate switch-overs to backup playout systems in the event of quiet periods from the broadcast (not something we potentially anticipated, but it’s happened in the past) we had the backup playout systems muted throughout the broadcast, with monitoring of modulation done via FM/AM radios so we could intervene in the case of a real fault. This required us to retrofit an alternative backup playout system to our FM transmission system to permit us to remotely switch the system on and off – a Linux box running Rivendell was installed temporarily. This and our primary backup in the studio were muted to prevent any switch-overs by overzealous silence detectors, and restored after the broadcast.
The actual switch-on went very smoothly – we’ve been running the link from the studio to the transmitter for the last week to help iron out any kinks. Like the OB link, the STL is an OpenOB Linear PCM link, providing us with a lossless link to the processor at the TX site. The computer providing the input to the processor was simply muted along with the backup system a few hours before we were due to switch on (previously, our test transmissions had been broadcasted from a compact flash device at the TX site). Thus we had nice quiet unmodulated carrier for the actual switch-on.
Last but not least we had all the clocks of computers involved in the broadcast synchronized via the Network Time Protocol to the station’s time server, ensuring that the countdown video which was scheduled to play just before midnight would go off right on schedule. A countdown timer on the OpenOB source computer gave us a very accurate timer to coordinate with.
A couple of seconds before midnight we sent the unmute command via ssh to alsamixer, and dead on schedule we heard the station manager, David Lamb, and current vice-president of communications of campaigns at SURHUL and previous assistant manager Sarah Honeycombe come on air and announce the switch-on to the crowds.
All in all everything went spectacularly well and we had very few technical glitches in the evening, the few we had related to ADC clipping on our encoder systems at the studio, which weren’t being monitored as closely as needed. This aside, everyone had a lot of fun and we had lots of great feedback on the broadcast.