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Mashed Digital TV

  • TVP
  • 30 Jun 08, 11:02 AM

Simon Thompson of BBC Research and Innovation writes:

This all started what seems an age ago. A while back, you may have read about an internal BBC event, called the Festival of Technology - I was there, and I had a problem! My project until recently was looking at the signalling required for digital switchover - and I'd recently created an "Autoretune" specification which I needed to present. Unfortunately most other members of my team also needed presentation space - so I could only have one screen. To solve the problem, it was decided that the Powerpoint needed to go into the Digital TV feed I was using - so off I went, armed with an old Linux box, and a couple of commercial pieces of TV hardware to make a TV service to show the Powerpoint.

Next door to me at the Festival of Technology was the BBC Backstage team who became interested as soon as they realised that, despite all their great work on getting people to use our data, they've never got anyone to hack with our main data feed - the telly.


So, the idea was run-up the flagpole, synergies were leveraged and learnings actioned and for Mashed it was decided that we'd build a TV station in a box, with quite a tight budget (i.e. less than it costs for one laboratory modulator), rebroadcasting a BBC service as live with two additional content feeds, one of interesting hacks from around the web, and one with stuff from Mashed. A mate of mine, Matt Hammond, got involved at this point with the Linux video encoding, playout scheduling, metadata creation box, whilst I created the Windows based modulator and all the radio-frequency hardware. Another great mate of mine, Alia Sheikh, got involved at this point with the organising of the content (and the engineers involved :-) ) including all the Makezine style stuff, the fantastic idents and the Test Card compilations.

Around a month before the event, the TV Platforms Group guys came to us and asked about including their own Mashed-specific MHEG data streams, and allowing the participants to write their own interactive Apps. Luckily, the hardware system I'd designed for the multiplexor (the item of kit which takes a load of diverse audio, video and data and creates a Digital TV service) easily allowed this.

Two of our more recent staff members (Peter Schübel and Sam Davies) were interested, so we asked them to write a GUI for metadata generation. Unfortunately, due to the fact that the first opportunity we had to test it was Setup Day, we hit a bug (which has now been fixed) and couldn't use it. Since this meant hand-editing of all scheduling and metadata files, we'll be relying on that software next time!

Meanwhile, Alia, Matt Cashmore and myself had been chasing Ofcom for our transmitting licence - I'd gone for a "Non-operational" licence for the building of innovative spectrum appliances - as that was basically what we were doing - allowing people to write innovative software using our TV services, whilst Matt C arranged for our use of BBC 1.

The Techno-babble

For those interested we had:

  • a professional DVB-T receiver outputting Crystal Palace Mux 1 over DVB-ASI

  • a Ubuntu Linux PC running:

    • FFMPEG for transcoding

    • BBC ASI playout software

    • bespoke scheduling software

    • bespoke metadata creation software

  • a Windows PC running:

    • proprietary DVB multiplexor + configuration editor

    • DVB-T PCI modulator

    • playout monitoring

  • Linear RF amplifier

  • MHEG feed from TV Platforms staff


The setup day arrived and Matt and I drove up to Ally Pally from Kingswood Warren (only got lost once!) at ridiculous o'clock, met up with a drenched Alia, and set the kit up - by lunchtime we were radiating. The rest of the team were busy that night so I spent the evening talking to a person from NASA and dancing to Matt Cashmore's Vengaboys album.

The day arrived, we had presentations about Digital TV from Steve Jolly (another mate we'd roped-in who'd written our public-facing user guides for us) and Nick Gallon, we had ingesting to do, we had support to provide and hacks to help with. We provided "Dr Who" to the masses and I spent the evening exhausted in a mock-up of a plane trying to do vector calculus in my head (I was navigating as Alia flew) whilst coming up with corny lines from war movies. Lots of other Kingswood-ites also passed by and leant us a hand - Thanks all!

Sunday arrived and we were beginning to see a lot of Hacks using various Digital TV based data, using software we'd provided and knowledge from our teams - hopefully epitomising the Reithian ideals. Ariel [the BBC internal mag] came to interview us - and got pretty excited about broadcasts resuming from Ally Pally after a 52 year absence and we finally got to have a look around!

Now we've got to work out what to do with what we've learned - but I'm pretty sure we'll be back next time (with a faster Linux box)! Who knows, there may even be Open-Source TV-Station-In-A-Box to play with.


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