Fixing Freeview

Jul 11, 2011 18:52 · 990 words · 5 minute read

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I’m watching increasingly little broadcast TV - at least at the time of transmission. As a result I’ve never been particularly fussed about having the latest, greatest, widest, high definionist TV gear - it’s a cheapo widescreen LCD that (if memory serves) was picked up for next to nothing from Sainsburys.

For reasons best known to itself, recently it crashed completely and I had to set it back up again from scratch. Once that was done, a bunch of the channels more-or-less disappeared - or at least, were unwatchable because of picture breakup and audio drop outs.

There didn’t appear to be any reason for this - there was no transmission problems that I could find information about, and retuning didn’t have any effect. Had it been ITV Sport+2 HD or some other channel, I probably wouldn’t have been too bothered. But as a Guardian-reading effete middle-class member of the chattering intelligentsia class, losing BBC4 is a a right pain.

The problem, as it turned out, was relatively simple to fix - but it did mean I ended up learning a whole load of new stuff about TV transmissions. There’s also plenty of information about how to retune a TV or set top box to get new channels, but not much on what to do if the reception’s crap despite retuning. Hopefully if this post gets picked up by Google, it’ll help somebody out.

Unless you’re particularly interested in the minutia of broadcast transmission, your mileage may be limited, though.  Don’t say I didn’t warn you…

# Background

Now for the science bit. In the UK, TV is broadcast at frequencies ranging from about 470MHz to 850Mhz. You’d think that tuning into a particular station would simply be a case of hitting a specific frequency, in the way that you do with FM radio - Radio 4 is often to be found at about 94.3KHz for example.

But compared to radio signals, TV transmissions have enormously larger bandwidth - so you actually need to pick up the signal across a band of frequencies. Rather than expressing it as specific frequencies, TV signals are broadcast on channels. Depending on where you live, you might find BBC1 on Channel 31 for example - or a frequency range around 551MHz.

Back in the old analogue days, a TV channel took up a whole channel to itself. But with the advent of digital TV, you can squeeze vastly more TV channels into the same bandwidth as an analogue channel because of the way the signal is digitised and compressed.

So rather than have a one-to-one channel-to-station relationship, digital TV is broadcast as groups of stations in multiplexes. The stations are grouped together - so the BBC channels are grouped together into Mux 1, while BBC4 and Sky Sports (among others) are found on Mux A.

Although the quality of the digital image is much higher than the 625-line analogue version, ironically digital TV is actually much more sensitive to interference, because the effects are much more noticeable. A bit of snow or ghosting on an analogue picture has to be quite severe to be noticeable - whereas digital errors result in pixellated frames, frozen pictures and audio dropouts.

The signals themselves are broadcast from a network of transmitters at various locations and operating at various power levels. Around here, Emley Moor is the main transmitter - it’s the tallest freestanding structure in the UK at 382m, and it pumps out a megawatt of signal to reach pretty much the width of the country.

But signal reception is very dependent on the local topography, so the signal is boosted by “filling in” through a network of much less powerful transmitters.

Our aerial actually points at the Tapton Hill transmitter which puts out less than a thousandth of the power of Emley Moor - and it’s this difference which proved to be the root cause of the problem.

When you use the automatic tuning function of a TV or a set top box, unless it’s a reasonably sophisticated it will simply start at the lowest frequency and scan up to the highest, looking for multiplexes as it goes. Once it’s found a mux, it’ll get loaded into the slots - and once the slot is filled, the box won’t refill it with another signal from another transmitter.

My box was merrily searching away and finding enough of a Mux 1 signal being broadcast by Emley Moor - even though the aerial points in more-or-less the opposite direction. The Mux 1 slot was filled, and that was it. While the signal was strong enough to trip the tuner, it wasn’t strong enough to provide a reliable signal - hence the picture and audio dropouts.

Until I realised this was what was happening, repeated retuning wouldn’t resolve the problem.

The fix

The solution, it turns out, is embarrassingly simple. Rather than auto retuning, I needed to tune manually. The first step was to clear the current, problematic, settings. This had me stumped for a while - a factory reset just triggered the auto tuning process. Eventually I hit on a workaround - starting an auto retune and then immediately stopping it cleared the memory without reloading any channels.

Then it was a process of adding the TV channels back mux-by-mux by scanning each channel that they’re broadcast on in turn. It’s a six step process, there being six muxes - but it’s actually quicker than the automatic process as that has to scan all 47 channels one by one.

Figuring this all out took a while, at least partly because the official Freeview site is incredibly lacking in anything approaching technical detail. But with a bit of Googling, it’s possible to grab an incredible amount of detail - the ATV site, for example, has everything from frequencies and channel lists, to polar diagrams for every transmitter in the country.