CDs and DVDs - how do they work?

Though they may seem quite different, an optical disc (CD or DVD) works much like a record.

Just like a needle on a turntable reads the bumps and grooves in a spiralling line on a vinyl record, the laser in a DVD or CD player reads the tiny 'pits' and 'lands' that are embedded in the disc, transforming the data into audio.

Well that’s the simple explanation anyway. But first, it's probably wise to look specifically at a CD and how it's made.

Digital CD data consists of thousands of 0s and 1s arranged in various combinations called binary code. This code (which is eventually converted into an electronic signal) is read via the patterns created on the disc from the pits and lands we talked about earlier (lands are the spaces in between the pits). This data is then translated into audio - the sounds you hear from your speakers.

These pits and lands are pressed into the plastic polycarbonate of the CD during replication, creating a microscopic pattern across the disc. However, a CD player wouldn't be able to read this data as it is, as the player's laser would simply shine right through the clear disc. So a reflective layer of aluminium is applied to the bumpy, patterned polycarbonate through a process called 'sputtering'. A layer of lacquer is applied to the disc as protection, and once complete, the CD is ready to be printed. (Fig. 1)

So what happens when you pop a CD into your player?

The CD player's drive motor begins to spin the disc. As the CD turns, the laser focuses in on the disc, starting from the centre and guided by a tracking mechanism. The laser passes through the protective lacquer and through the polycarbonate, reflecting off the aluminium-coated pits and lands. The reflected beam is then transmitted back to a detection device within the player that picks up changes in light.

As the laser light reflects off the varying sized pits and lands, a mechanism within the player reads the information on the disc based on the different light reflections caused by the pits and lands. The pulses generated by the indentations are then converted into digital data which is then transformed by a Digital to Analogue (D/A) converter into an analogue signal. This signal then passes to your amplifier and speakers. The result? High quality, crystal clear audio. (Fig. 2)

The CD player mechanism that reads a CD is extremely precise. It has to be - like a needle reading a record, the player laser reads the information on the disc, but unlikea needle and record, it doesn't touch the medium nor is it guided by grooves imprinted on the medium's surface. To assist, error correction information is included on the CD. In the 1990s shock-absorbing technology improved portable CD players which were prone to constant skipping and jumping, and sales took off.

DVDs

A DVD works much like a CD, but instead consists of two layers and, in some cases, two sides. The pits and lands are smaller and compacted more closely together, and less error correction information on the disc is required. This means that a DVD has the advantage of holding much more data – up to 12 times that of a CD for DVD 9. There are three main types of DVD, each with different data capacities:

• DVD 5: Single sided with a single layer of data. Holds 4.7GB of data.
• DVD 9: Single sided with two layers of data. Holds 8.7GB of data (Fig. 3).
• DVD 10: Double sided with a single layer of data accessible from each side of the disc. Holds 9.4GB data.
• DVD 18: Double sided with two layers accessible from each side of the disc making a total of 4 layers. Holds 17.4GB of data. The disc needs to be flipped in the player to access all data (this format is not very common as it is time-consuming and expensive to manufacture).

Like a CD, a DVD has an initial layer of pits and lands coated with a reflective layer either silver or aluminium. However, DVDs with two layers have an outer semi-reflective layer which is coated in silver, allowing the laser to reflect and read off both layers. In the case of a dual-layered DVD 9, the first layer (Layer 0) is a semi-reflective layer coated in silver that allows the laser to pass through and read the second layer as well (Layer 1). Interestingly, you may have noticed that your player sometimes pauses slightly halfway through a DVD. This is caused by the laser focusing onto the next DVD data layer. In the case of a double-sided DVD (DVD 10 or DVD 18), you will need to flip the DVD over to continue watching when prompted.