Someone wrote to me with questions about commercial CDs (technically, Compact Disc - Digital Audio or CD-DA). Responding to her made it obvious that I have again taken for granted information which is hard to gather from the variuos Internet sources, so I will summarize it here.
The format for CD-DA was defined in a formal specification commonly called the red book (for the color of its cover). It is maintained by Philips and is available at a substantial price as are other specifications known as the orange and yellow books for other formats. The red book specifies that an audio CD may carry the CD logo if and only if it complies with the specification. In particular, the contents must be as follows. Let me note that there are some conventions in acronyms: k means a thousand, K means 1024; b means bits, B means eight-bit bytes.
What about higher sample rates?
44.1 ksps provides audio response beyond 22 kHz, therefore beyond the
capability of ordinary playback equipment to reproduce or most users to hear as
pure tones. In fact, there are audible losses due to that sampling when real
music is offered, but that was the compromise selected in the 1970s when the
technology was developing. Higher sampling rates would have meant much more
expensive equipment for relatively little return. In addition, that technology
would have meant shorter recording time.When DVD-Video came along, 48 ksps was
selected; the practical difference between them is negligible. However,
mastering and some audio sources justified still higher rates and has led to
standards beyond that of CD-DA such as 96 and 192 ksps.
What about more bits per sample
In one sense, that is provided in the spec: it allows for four-channel (quad)
recording and playback. That halved the recording time and never caught on. But
16 bits corresponds to more than 90 db of signal-to-noise ratio (SNR). Since
good tape sources provided only about 60 db SNR, more bits per channel would
mean only improved encoding of the noise, which was already encoded in the last
several bits. Among the many options for DVD-Audio, more bits per sample are
supported.
What length can a CD-DA offer?
The specification for the timing on a CD-DA and other CD formats is based on a
spiral along which the read laser tracks. That spiral had a nominal length for
writing the full disc which corresponds to 74 minutes. Tolerance was
established so that the actual pitch of the spiral would fill the disc at no
less than 63 nor more than 80 minutes (actually, 79 minutes, 59 74/75 seconds -
written 79:59:74). While it has always been clear that a steeper pitch,
providing longer recording time, meant more errors than a shallower one, the
payoff in playing time was deemed more important by the mass market so
gradually shorter blanks for CD recording (CD-R) lost market. At this writing
(2006), general-use CD-R blanks are available only at "80" minutes
with 74-minute media sold only for archival use. Still longer discs have been
marketed - up to 99 minutes - but they have high error rates and are seldom
employed. It is worth noting that the largest 'length' number which can be
written to the disc is 79:59:74; longer discs cannot tell the truth about their
length and later track starts are not specifiable in the TOC.
What's the difference between CD-R, CD-RW and pressed (commercial)
discs?
There are three technologies involved: CD-R uses a dye to record information;
it can be written only once. CD-RW uses the change of state of an alloy and can
be erased and rewritten. These are discussed elsewhere in this primer. A
pressed disc is, in fact, pressed. The information of the audio stream and the
TOC is prepared by a special program to create a glass master. That master is
used to make one or more metal stampers. A stamper is pressed into a fluid
plastic layer of the blank disc to impress the information. Once set, that disc
is given an aluminum coat from which the read laser is reflected. At any point
in that process, a speck of dust means a flaw in reproduction, so pressing
requires a clean room and special operations to avoid contamination.
Fortunately, CD-R and CD-RW are more tolerant and small amounts of
contamination are seldom detected.
Errors in digital recording? Is that for real?
Unfortunately, it is. CDs of all sorts are read by analogue mechanisms and
digitized by gating. The raw error rate is surprisingly high and both encoding
and error correction are used on all CDs and DVDs to bring it to a manageable
level. A data disc has an extra layer of error correcting code (ECC) which uses
about 13% of the space; that is not used for audio so the player provides error
concealment to reduce the effect of misreading. The subject rapidly becomes
complicated; it is discussed throughout this primer and in the CD-R FAQ.
E-mail me at cdrecording@mrichter.com
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