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 how do CD's work

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PostSubject: how do CD's work   Thu Dec 04, 2014 11:47 am

CDs  - 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.

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PostSubject: Re: how do CD's work   Thu Dec 04, 2014 12:26 pm

The Vanderbilt studies.

My teaching on laser came mostly about through listening and experiments but when I was comissioned to work on the Vanderbilt "star wars" laser lab I came loaded with questions.

History
The videodisc player of the 1970s was an ancestor of the modern CD player. It also used a laser to pick up information encoded on the surface of a spinning optical disc. In a laserdisc player, a tube containing helium and neon gases produced laser light. When the first small, practical laser diodes were developed a few years later, they replaced the more expensive, bulky gas laser tubes. These less expensive lasers made new applications, like music players, possible.


Light
Unlike the light from a light bulb or light-emitting diode (LED), a laser's light consists of a single, pure color. All the light's waves are in sync, allowing it to travel in precise beams that don't spread out. These properties are important for a compact disc because the music data is grouped into tracks about one-thousandth of a millimeter wide. A lens focuses the laser light to a tiny spot that can find these tracks.


Pits
In the CD's tracks, music has been recorded as digital data as a series of tiny pits 125 nanometers (nm) deep and 500 nm wide. The laser light reflects off these pits as the CD spins, sending a stream of binary, on-off flashes to a detector. This sends data from the disc at a rate of over four million bits per second. As the laser light's frequency is billions of times faster than this, it can easily carry this much information.


Detector
An electronic detector called a photodiode receives the rapid light pulses being reflected from the surface of the CD. The photodiode is sensitive to the laser light, converting it to electrical signals. It feeds these signals to electronic circuits in the player, which decode the information into stereo music. These signals also contain the album and song titles, track location, and other data.


Recording
Some CD players are able to record music. To do this, you need a recordable CD, called a CD-R or CD-RW. To play music, the player can use a laser diode of modest power. To record music, the diode must be more powerful, capable of heating tiny spots in the CD's material and writing the music data permanently into the CD.

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PostSubject: Re: how do CD's work   Thu Dec 04, 2014 12:32 pm

review

CD Players
CD players operate according to the same principles that DVDs and laser discs do. They're somewhat simpler than those devices because they only carry audio information, not video information, but otherwise they're pretty much the same. To understand how the laser of the CD player reads the music, it helps to understand how the CDs themselves are constructed.


Construction
CDs consist of a thin layer of aluminum sandwiched between a clear piece of plastic on the bottom and a piece of acrylic on top. The aluminum contains a series of bumps and divots, which extend outward in a long, continuous spiral. The bumps are so tiny as to be invisible to the naked eye. They form an elaborate binary code: a series of 1s and 0s, much the same as the information stored in a computer. That code contains the music of the CD. It then becomes a question of translating it as such.


Laser
The laser in the CD player moves along the string of code when it plays the CD. It starts on the innermost part of the CD and slowly spirals outward--much like an old record player, only in reverse. As it reflects off the aluminum, it bounces back into an optical reader. However, when it strikes one of the bumps, it changes directions slightly and the receiver doesn't pick it up. Those two states form the basis of the binary code--on/off or 1/0--which creates the digital signal. The hardware in the CD player then translates that into sound and amplifies it to play out of your speakers.


Tracking
The laser has to move at a steady pace across the CD into order to pick up the signals properly. The bumps on the outside of the CD move past it more quickly than the inside of the CD, which can disrupt the signal. So a tracking system within the CD player makes sure that the laser stays pointed where it's supposed to and a spindle motor varies the speed of the disc so that the bumps pass over the laser at the same rate throughout the playing time.


Subcode Data
In addition to the bumps themselves, most CDs contain subcode data, which help divide the information into specific sections. That allows you to skip ahead on the CD to a new track automatically. It can also tell more sophisticated CD players the title of the song, the title of the album and the artist.

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PostSubject: Re: how do CD's work   Thu Dec 04, 2014 12:51 pm

CD tweaks

There are a few interesting CD Disc tweaks, but in my own listening I found them to be hit and miss. For example there are people who color parts of the cd. You've seen the green treatments for the edge. Through experimenting I found that each CD can actually be tuned for a particular player. Problem is what if you switch players, or desire the sound to be different from the treated sound? This is where I separate myself from those who say "better sound", because I know that all sound is variable and you can look at the recorded code from different points of view.

So I took this question to my friends and here's what they showed and told me.

A CD player in years to come, if they ever make it to the next stage will need to be treated more like a turntable tone arm only instead of adjusting the needle and arm you would adjust the laser intensity and filtering not on the CD disc itself but the laser. I sat back while this was demo-ed with my jaw on the floor. Of course, why are we treating the CD Disc when we should be keeping them clean and ocasionally de-staticed and tuning them from the laser end of things.

Nice to have smart friends Wink

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