FROM Land's End to John O'Groats, from Shetland to the Isles of Scilly, wherever a stereo is switched on, the echoes lead back to Swindon or many of them do at least.

For most of the year, the EMI CD pressing and packing plant at Greenbridge produces between 200,000 and 300,000 discs a day, five days a week, 24 hours a day.

Between October and December, that figure rises to 400,000 a day in a six day week.

Over the three month festive build-up, therefore, some 36 million discs are made and dispatched from the site.

Whether music lovers' tastes run to Robbie Williams or Daniel O'Donnell, Ibiza compilations or punk karaoke singalong, choral classics or Pink Floyd, the chances are that a lot of the music on their CD racks started life at Greenbridge.

Key Christmas releases include new albums by Robbie Williams, Blue and Smashing Pumpkins, as well as the Now 50 compilation album.

Other important items include albums by Kylie Minogue, Mick Jagger, Paul McCartney and Diana Ross, and a double compilation album of Pink Floyd classics.

Over the festive period, the 180 staff at the plant are supplemented by about 75 temporary workers, drafted in to help cope with the rush.

Many of those are set to work packing the various special CD box sets, which are popular gifts but too fiddly for even up-to-the-minute machinery to assemble.

Plant spokeswoman Faith Moore said: "Christmas is our busiest period, so we have extra staff.

"We are capable of producing 450,000 CDs a day."

The process of making a CD is a complex one.

Once the music is recorded by the artist, it is encoded as digital information and later burned with a laser onto a highly-polished glass master disc.

The surface of this disc is then coated with a substance which will conduct electricity.

Then, using an electrical plating process resembling a distant cousin of the process by which a silver coin can be plated with copper in a school chemistry lab, nickel copies of the glass master are produced.

These become the metal stampers which are installed into the machines on which the compact discs themselves are produced.

Each CD starts life as thousands of tiny plastic granules which are pumped from a hopper into the stamping machines.

There, they are heated until they melt, and the molten plastic is injected into the mould formed by the metal copies taken from the original glass disc.

Jack Champ, 25, whose job as a CD matrix operator includes preparing the metal masters said: "Each one is good for producing anything from 5,000 to 20,000 CDs."

There are several moulding machines, each with their own specialist operators, known as moulding technicians.

One of them, 57-year-old John Wicks, has worked for EMI for 15 years.

He said: "The stamping process takes five seconds, and 2.8 of those seconds are for cooling the disc."

The discs as they emerge from the stamper are clear plastic, and only become play-able when a thin layer of aluminium is added, followed by a coating of clear lacquer to protect it.

It is this layer that allows CD players to read the encoded digital signals that produce the sound.

After a quality check to identify faulty discs, labels are added to them, using a mechanised screen printing technique.

The discs are then packed, either by machine or by hand, ready to be sent to distribution centres and shops.