Chip's CD Media Resource Center:
CD-DA (Digital Audio) page 6

Bit Density & Encoding

The next thing I want to figure out is how many "bits per micron" can be stored on a CD. The answer to this question required a bit of research. Let's look first of all at how the bits are encoded in the pits and lands on the surface of the CD.

The transition between a pit and a land denotes a binary 1. The absence of any transition denotes a binary 0. In other words, it is the edge of a pit, not the pit itself, which represents a 1 bit.

Pits are nominally 500 or 600nm wide, as we already discussed in the section on Physical Structure. But how long are the pits? We know from the discussion in that section that the pits, and the space between pits ("lands") must never be smaller than than the spot size of the reading laser beam. We calculated that spot size as 682nm, so we expect this space to be 700nm or more. We will calculate the exact size below. The minimum size at the lowest standard speed will be shown to be 833nm.

CDs use an encoding scheme called Eight to Fourteen Modulation (EFM) which avoids really short pits or having pits too close together by ensuring that there are always at least two 0 bits between any 1 bit. It also avoids tracking problems by ensuring that there are no more than ten 0 bits between any 1 bit. The size of a single bit is designated "T", and so pits or lands are never any smaller than 3T ("001") nor larger than 11T ("00000000001").

The raw bits being read from the CD are called channel bits to help distinguish them from the data bits. As we shall see, it an audio CD uses about 3 channel bits to reliably represent a single data bit.

How Large is a Bit?

I explained that CDs are CLV (constant linear velocity) devices, and now it's time to talk a bit more about velocity. ECMA-130 says, "The scanning velocity during recording shall be between 1.20 m/s and 1.40 m/s with a Channel bit rate of 4.3218 Mbit/s. The velocity variation for a disk when recorded shall be within 0.01 m/s" (section 11.4). In other words

• Discs are indeed recorded at a constant velocity within 0.01/1.3 = 0.8% tolerance.
• The actual recording velocity can be between 1.2 and 1.4 m/s, so long as it's constant.
• Since the channel bit rate is held constant (4.3218 Mbit/sec = 75 blocks/sec * 98 frames/block * 588 channel bits/frame. More on this later), then the density of the bits must vary with recording velocity. In other words, those 4,321,800 channel bits that encode 1 second of audio could be stored in as little as 1.2 linear meters or as much as 1.4 linear meters.

The table below shows the size of a channel bit at several different recording velocities, along with the corrsponding sizes of a 3T and 11T pit or land.

The bit size is computed just by dividing the speed by the bit rate. For example

    Channel bit size = 1.2 m/sec / 4,321,800 bit/sec
                     = 278nm (0.278 microns)

The raw bit capacity of a CD, including Lead-in and Lead-out areas, is thus

    5680 meters / 0.278 microns
    = 2.1E+10, about 20 gigabits or 2.5 gigabytes.

But don't expect to use one audio CD to store 3 hours of material. Read on for what happens to most of those bits...