CD Player

Overview

The CD player, introduced by Sony and Philips in 1982, reads digital audio from a 120 mm polycarbonate disc carrying up to about 80 minutes of stereo PCM at 16 bits and 44.1 kHz. The data is physically a spiral of pits, each about 0.5 µm wide on a 1.6 µm track pitch, a track over five kilometers long read without contact by a focused laser. The player's job divides cleanly: the Disc Transport spins the disc and positions the Optical Pickup, the Servo and Decoder Board keeps the laser on track and decodes the data, and the DAC Board converts the recovered samples to an analog signal.

Reading the disc

The Laser Diode emits at 780 nm. The beam passes a Diffraction Grating that splits off two side spots, then a Beam Splitter, and the Objective Lens focuses it through the disc's transparent substrate to a 1.7 µm spot on the reflective data layer. Pits are one quarter-wavelength deep: light reflected from a pit travels half a wavelength further than light from the surrounding land, interfering destructively, so the pit edges read as intensity transitions at the Photodiode Array.

Nothing about the disc is precise enough to read passively. A disc may wobble half a millimeter vertically while focus depth is about ±2 µm, and its eccentricity dwarfs the 1.6 µm track pitch. The Actuator Coils therefore move the objective lens continuously, commanded by the Servo DSP. Focus error comes from an astigmatic lens that distorts the spot shape on the quadrant detector as focus drifts; tracking error comes from comparing the two side spots straddling the track. The Sled Motor follows with coarse radial motion as the spiral progresses, through a Helical Gear Pair reduction, while the Spindle Motor varies between roughly 500 and 200 rpm to hold constant linear velocity, the data rate, not the rotation rate, is what the format fixes.

From pits to PCM

The RF Amplifier shapes the photodiode signals into the RF eye pattern. Bits on disc are EFM-coded, each 8-bit symbol stored as 14 channel bits chosen so transitions are never too close together for the optics nor so far apart that the clock recovery loses lock. The EFM Decoder demodulates this and applies the Cross-Interleaved Reed-Solomon Code: data is interleaved across a span of the spiral, so a scratch destroying thousands of consecutive channel bits becomes many small, correctable errors after de-interleaving. CIRC fully corrects burst errors up to about 4,000 bits (a 2.5 mm scratch); beyond that the player interpolates, and only gross damage produces audible artifacts. An Microcontroller supervises the whole mechanism, runs the table of contents, and handles track seeks.

Conversion to analog

The recovered 44.1 kHz samples pass through the Oversampling Filter, which interpolates to typically eight times the sample rate. This moves the spectral images that conversion produces from just above 22 kHz to far above the audio band, so the analog reconstruction filter in the Analog Output Stage can be gentle, with benign phase behavior. The DAC Chip performs the conversion with more than 96 dB of dynamic range, clocked from the Crystal Oscillator, 16.9344 MHz divides exactly to 44.1 kHz. The output stage buffers the result to the standard 2 V RMS at the rear Connectors, and a Mute Relay silences the output during seeks and power transitions. Most players also pass the raw bitstream out as S/PDIF for an external DAC.

Mechanics and power

The Loading Tray, driven by a Drive Belt from a small motor, carries the disc in and lowers it onto the Turntable Hub; the magnetic Disc Clamp presses it flat. The whole transport sits on Isolation Mounts inside the Chassis so footfalls and cabinet vibration do not disturb tracking. The Power Supply keeps the analog rails separate: a Regulator Bank derives quiet supplies for the DAC Board away from the motor and logic rails, since spindle current pulses would otherwise appear in the output as noise. The Vacuum Fluorescent Display on the front panel shows track and time, with commands arriving from the Front Panel Buttons or through the IR Receiver.