When the channel is off-air the BBC transmits a continuous stream of programme clips, in what is known as the Barker. The test card appears after an hour, with the audio sync test 50 minutes after that.

So one hour after the BBC HD channel programmes come to an end you can see the test card. One hour and fifty minutes after closedown the audio sync test appears.

In brief the audio sync test shows two moving white bars, one vertical and one horizontal. The vertical one is a clapperboard while the horizontal bar shows the video timing relative to the audio. Simply listen to the audio and see where the horizontal bar has reached at the point that the clapperboard sound is heard. The numerical scale will show you how far in advance or behind the pictures are compared to the sound.

Both test signals can conveniently be recorded on a Freesat or Sk+ HD recorder by using the manual recording option BUT you may well find that the audio sync on replay is slightly different to that on live viewing!

For fuller details on the audio sync test signal see the BBC Blog here.

The latest TVs and Blu-Ray DVD players both support 1080p video, which has twice as many lines a  second and therefore a higher data rate. Consequently a category 1 cable just doesn’t cut the mustard. Although it might appear to work, it is quite likely to create intermittent problems.

The specification of Category 2 cables requires that they support signals of up to 340 MHz – over four times higher than Category 1. This allows them to support 1080p signals, even allowing for future increases in sample resolution to over 8 bits per colour and frame (refresh) rates of up to 120 frames per second.

So in other words when you hook your £700 TV to your £200 Blu-Ray player, don’t use a £5 cable!

The original HD boxes were all made by Thomson but the current generation of Sky HD boxes are made by Pace, Samsing and Amstrad. Some customers with the new generation of boxes had initial problems with lip sync which appeared to have been solved with over-the-air updates last year.

Well to be truthful it had packed up so it wasn’t much of a loss. And it was only a 1Gb drive in the first place. As you can see below once the lid is off all the major mechanical components are exposed. You may have seen elsewhere that a hair or even a particle of smoke is enough to make a hard drive head crash fatally into the disc – so don’t try this at home. Unless your drive has taken a dive too of course.

The disc platter is very rigid. It’s made of aluminium with a magnetic coating and it is spun round at 7200 or 5400 RPM by a motor underneath. The head is a tiny ferromagnetic device that is aerodynamically designed so that it flies above the disk in the air-flow generated by the disk’s rotation. The head generates a magnetic field when current is passed through it so data can be recorded onto the disk. The clever bit is that it also generates a current when a changing magnetic field is passed across it which means that the same head can also be used to read data from the disk.

The drive is controlled by electronic circuitry on a PCB (printed circuit board). The PCB is on the bottom of the drive.

The control circuit moves the actuator arm to position the head over the correct part of the disk. The electronics can tell by reading the formatting information where the invisible magnetic tracks start and stop.