MOTM

Replicated Audio CD = Philips/Sony Redbook Standard CD is 
basically a few layers of digital transformation on top of  a 
smaller version of an analog LaserDisc (remember those, I'll bet 
King Paul does). A spiral "pit track" (starts in the center on a CD - 
opposite an LP) at an optimal spacing of 1.6 um is read by a 
laser at CLV (constant linear velocity) which is about 500 RPM to 
200 RPM (at the outer edge). The CLV ranges from 1.2 to 1.4 
meters /sec depending on the mastered scanning velocity of the 
disc. The readout  speed is regulated to maintain a constant bit 
rate of 4.3218 MHz. Pits and lands (non-pits) are detected from 
the bottom of the polycarbonate substrate to the top or 
information side of the substrate (which is metalized) by a laser 
pickup. Lands provide about 60 to 90% reflectivity depending on 
the metal, aluminum is the most common these days and 
provides about 75%. The pit depth is(was) selected to cause 
phase cancellation for very little direct reflectivity (about 1/4 
wavelength, remember the light travels through the substrate 
twice). So we have a total of 1/2 the wavelength and fair amount 
of destructive interference which disperses the light. The 
information is stored as NRZI (non-return-to-zero-inverted) 
signal by 8 different lengths of pits and lands - 3T (720 kHz) 
through 11T (196 kHz) [T=Period]. NRZI represents the data with 
(statistically) the fewest state transitions. The pickup 
(transducer) produces a fairly sine-ish HF waveform of changing 
frequency. Digital?

Barry