CQ / Hello World…

A quick introduction: by day, I'm a DevOps Engineer at Red Gate, a software company in Cambridge, UK. Outside of work, I enjoy both amateur radio (hence the callsign, M0VFC) and community broadcast radio at Cambridge 105. This blog aims to span all those interests - so feel free to ignore the posts that aren't relevant!

Feel free to get in touch on Twitter (@rmc47).

73 / Best wishes,

Remembrance Sunday on Cambridge 105

8 November 2015 | Comments

This morning, Cambridge 105 covered the Remembrance Sunday service at the war memorial on Hills Road, Cambridge. You can listen again to the coverage here.

We are no strangers to outside broadcasts, covering everything from the Cambridge Rock Festival to the Addenbrookes Hospital Christmas Concert, but this was a little different.

The war memorial stands in a very public location, and the adjacent roads were only due to be closed for a short period. This, combined with a need for our presence to be discrete and avoid distracting from the service, meant we opted for a completely wireless microphone rig.

The readings and prayers were given from a stand-mounted handheld microphone near the memorial - the only one the public were probably aware of. This also fed a pair of PA speakers (an independently mixed output from the broadcast audio).

The band of the Salvation Army was captured using a radio mic cable tied to a nearby tree when they were in their final position, and while approaching as part of a parade, another clipped to a convenient traffic light did the job. Finally, a shotgun microphone provided ambient crowd noise and filled in as required.

Julian Clover, presenting for the broadcast, wore a further radio mic as well as a wireless in-ear monitor. This meant he was able to move within the crowd to offer commentary on the proceedings while being able to hear the broadcast mix.

The OB went very smoothly, with the only real technical challenges being wind noise on microphones, and a slightly enthusiastic singer a little near to one of the mics during one hymn!

K3 repair - deafness after HI RFI warning

7 October 2015 | Comments

While operating at GB15NH, we managed to let the magic blue smoke out of two radios: Gavin M1BXF's IC-756ProIII, and my Elecraft K3.

The cause was entirely self-inflicted: our 20m and 17m antennas were probably 1m apart, and we didn't have a 17m band-pass filter to hand. We survived a day like this, but on the second day, the beams ended up facing each other, and this proved too much. The ProIII let out some smoke. We swapped it for the K3, which showed the "HI RFI" warning, then went suspiciously deaf.

Symptoms were:

A bit of searching online pointed to D25, a double PIN diode. Iain M0PCB also reported problems with D5 on the KXV3A board.

Inspecting D25 (on the bottom of the RF board, near the LPA) showed it looking a bit supicious, and trying to remove it to test led to it disintegrating:

Original faulty D25

D25 is a BAR 64-05 PIN diode, and costs all of £0.40 from Mouser - plus a £12 delivery charge for small orders! I ordered 10, to give me plenty of spares if it happens again...

The new part installed:

New D25

Everything sprung back into life, with a -40dBm test signal from the Rigol DSA815's tracking generator lighting up the S-meter to 9+50dB, a much more reasonable value.

Engineering Homegrown music festival with Cambridge 105

31 August 2015 | Comments

This bank holiday weekend, Cambridge 105 broadcast live from the Homegrown music festival in Barrow, Suffolk. We bought sets live from the two stages, as well as interviews and live acoustics sets from a backstage green room area.

Jon Watts has published some fantastic photos of the weekend if you want to see what it's like from the point of view of an audience member, but this is a look at the behind-the-scenes engineering.

At the highest level, it's a simple problem: you need some set of sources (microphones, music sources, feeds from stages), a way of mixing them together, and some way of getting the output back to the studio in the centre of Cambridge. However, the reality looks more like this:

By using a digital mixer like the X32, a lot of routing that would previously been complex is made easy. For example:

Talkback was particularly important for this setup: we engineered the show from our outside broadcast van, but the presenters and acoustic performers were in a green room area, out of sight. We needed to be able to cue presenters when the main stages (also out of sight) were about to start, stop, go into new tracks and so on, all while the presenters were live on air.

As well as getting the engineering of this right, it's a real skill for the presenters to be able to keep going while having a voice in their ear talking at them - all credit to Neil and Julian for doing this so well!

Along with the digital mixer comes digital snakes or multi-cores. The S16 allows us to have 16 inputs and 8 outputs remote from the mixer using only a single Cat5E cable - much lighter and easier to handle than a conventional snake.

Finally, we have to get the signal back to the studio. Barrow is much further away from the studio than most of our outside broadcasts, so this demanded a new approach. We used a combination of an IP stream from a laptop in the OB van over 3G and a band 1 (~53MHz FM) link back to a remote receive site where it then hit a separate IP stream. The 3G stream was our primary feed, but automatically fell back to the band 1-fed stream in case it dropped. In the event, 3G held up remarkably well, but it's dangerous to rely on it as the only option, particularly at a festival where a much larger than normal collection of people suddenly turn up in a rural area with their mobile devices.