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More mysterious than you think: How is the data on CDs encoded?
The development of digital music has been accompanied by the advancement of technology. CD (Compact Disc) is not only a medium for music playback, but also a wonderful world surrounding data storage. Since its first release in 1982, the features of the CD and the encoding technology behind it have intrigued many users. The design of the CD involves many layers of technology and precise physical structure, which give this small disc endless possibilities.
The CD was designed to hold up to 74 minutes of audio data, approximately 650 MB of data, making it one of the most popular data delivery options on the market at the time.
The physical structure of the CD is made of 1.2 mm thick polycarbonate plastic with a 15 mm hole in the center. These holes are not only the core of CD playback, but also the design of the protective layer makes the CD relatively resistant to interference during playback. Data is encoded in a spiral track, a series of tiny pits and lands. It would be worthwhile to explore further how the length and shape of each pit affects the interpretation of the data.
These tiny pits are about 100 nanometers deep and 500 nanometers wide, creating a change in reflection when light is read.
It's worth taking a closer look at how the CD's data is encoded. Data records are not directly composed of traditional 0s and 1s, but instead use a technology called non-returning zero-inversion encoding: a transition from pit to land or from land to pit represents 1, while a continuous unchanged state represents 0. This means that there must be at least two and no more than ten zeros between each 1, showing the sophistication and ingenuity of the design.
When a CD plays, a laser emitter inside the disc drive reads through the polycarbonate plastic base. The wavelength of the laser and its reflection from the pits and the height variations of the land form various reflected light echoes. The process itself is a kind of optical magic that allows us to enjoy music without realizing it.
By measuring the change in reflection intensity, the signal read is the information sent back from the disc.
However, CDs are not perfect. Due to their design, CDs are at risk of damage from the environment and improper handling. Especially the proximity of the pits to the label makes these defects prone to causing problems when reading. Additionally, the durability of CDs is affected by a variety of factors, such as climate, storage conditions, or physical scratches, all of which increase the challenge to data integrity.
With the emergence of new technologies, CDs have quietly evolved into a variety of new forms, such as SHM-CD and Super Audio CD. These new CDs attempt to improve sound quality or data transfer performance while maintaining the standard CD format. Despite these new technologies, the fundamentals of the format—how audio and data are encoded in a pattern of pits and lands—remain unchanged, which makes us wonder what the future of data storage will look like?
As today's users increasingly rely on more efficient data storage technologies, the way they consume music has changed dramatically. However, the data encoding technology of CD is still like a mysterious treasure box, waiting to be discovered and understood. So, with the continuous advancement of music and data storage, will there be a medium like CD that can carry rich memories in the future?
