How Do CD Players Work?

Most of us understand what a CD player is; a box we insert CDs and play them.

But, what’s going on inside? How do they work? What are the basic elements of a CD player and why do Audiophiles not use them very often, preferring instead CD transports and DACs?

Back to the beginning

On October 1, 1982, Sony placed the first nail in the LP’s coffin with its release of the world’s first commercial compact disc player, the CDP-101, in Japan. The launch signaled a new audio medium that promised to deliver a crystal-clear music experience for a generation of consumers accustomed to vinyl records.

It was touted as “PerfectSound Forever”. It was anything but perfect.

The CDP-101 retailed for about $674 (roughly $1609 in today’s dollars), and was launched alongside a group of 50 classical and pop CDs published by CBS Records. Names like Mozart, Tchaikovsky, and Schubert shared the bill with more modern artists such as Billy Joel, Pink Floyd, and Journey. Each disc cost $14 apiece (about $33 in today’s dollars).

CDP101a

From vinyl to optical

It was Sony and Philips that together invented the CD player, though they hadn’t come up with the technology itself. Instead, American engineer James T. Russell invented the first system to record digital information on an optical transparent foil. He applied for a patent in 1966, and it was granted in 1970. Sony and Philips hijacked the technology, at first for laser discs, later for the audio CD, but Russell (and the company he had sold the patent to) took them to court and won, forcing them to license his invention.

That first player

So, what was inside that first player? The same three basic elements found in every CD and DVD player today: a turntable, an optical reader, and a DAC.

Let’s first take a look at the turntable and optical reader.

cd-laser

The turntable is exactly what it sounds like, a spinning platter that rotates the CD itself. Speeds are quite a bit faster than an album, averaging 600 rpm as opposed to 33 1/3 rpm for vinyl. When you place a CD atop the turntable, the label faces you, the optical surface with all the bits faces down.

Below the CD optical disc waits a laser mounted to a moving sled or carriage. The carriage can move back and forth along the CD as it spins. In close proximity to the laser beam is a photo receptor that converts the reflected laser light into an electric current.

Once converted to electrical signals, the ones and zeros we’re so familiar with are sent to the CD player’s internal DAC.

The DAC inside

DAC stands for Digital To Audio Converter. As its name implies, this circuitry takes the ones and zeros we call digital audio, and converts their message into audio—sound we can then use to play through speakers or headphones.

The bits of digital audio form a type of machine language with words, just like in any language. In digital audio, words represent numbers, and each number represents a higher or lower part of the music’s loudness.

The maximum number of bits within a word determine the number of possible words. CD words have 16 bits, which means CD systems are limited to 65,536 words (compared to 16,777,216 possible words in a 24 bit system). Within those restrictions, digital words form louder and softer levels that the DAC turns into sound.

The difference between a CD player and transport

Most Audiophiles no longer use CD players, preferring instead to separate the duties of the CD player into two separate pieces of equipment, the Transport and DAC.

An aftermarket transport contains the turntable, laser mechanism, photo receptor, and associated electronics to convert what is on the CD to electrical ones and zeros.

One of the more interesting aspects of the optical extraction process is the format the ones and zeros are prepared in, called I2S.

I2S (pronounced, “I squared S”) has multiple elements of digital audio: several clocks and the digital music data itself. Clocks are needed to keep all the bits coming at the proper time and organizes the order in which they appear.

Some transports output this I2S format, but most do not. Instead, the four separate digital audio clocks and data of I2S, are mixed together and output through a single RCA connector, optical cable, or XLR. This new format, which must later be untangled back into I2S inside the DAC, is known as S/PDIF, an acronym for Sony Philips Digital Interface.

Why CD players have fallen out of favor

The vast majority of high-end audio systems separate the duties of the all-in-one CD player into two units, the transport and DAC. Separating these necessary components in a digital audio system results in better sound, or at least that’s the theory.

There’s no reason whatsoever that separating CD players into two chassis is superior other than the fact it permits manufacturers to lavish more attention on each, as well as allows consumers to mix and match to best advantage.

The venerable all-in-one CD player might just have a comeback someday, if enough high-end manufacturers decide it’s something their customers want. Otherwise, CD players—the very equipment that forever changed the audio landscape—will likely be relegated to the basement and audio museums.

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