Optical Disc Formats for Audio Reproduction and Recording 1145DVD-Audio). Similarly, many D/A converters use over-
sampling to increase the sampling rate of the output sig-
nal and thus move the image spectra from the audio
band. DSD coding uses a high sampling frequency, but
does not require decimation filtering and multibit PCM
quantization; instead, the original sampling frequency is
retained. One-bit data is recorded directly on the disc.
Moreover, DSD does not employ interpolation (over-
sampling) filtering during playback.
DSD uses sigma-delta modulation (SDM) and noise
shaping. In a simple SDM encoder, the 1-bit output
signal is used as a compensation signal. It is delayed by
one sample and subtracted from the input analog signal
using a negative feedback loop. If the input waveform
rises above the value accumulated in the negative feed-
back loop during the previous sample, the converter
outputs a logical 1. Similarly, if the waveform falls rela-
tive to the accumulated value, a logical 0 is output. The
output pulses represent the magnitude of the input
signal; pulse density modulation can be used. Because
the integrator in the SDM encoder acts as low-pass filter,
the low-frequency error content is reduced while the
high-frequency content is increased. Higher-order noise
shaping feedback filters can further decrease error in the
audible range of frequencies. In principle, a low-pass
filter can decode SDM signals, and also remove
high-frequency noise resulting from noise shaping.
On SACD recordings, the DSD modulation employs
a sampling frequency of 2.8224 MHz and each sample
is quantized as a 2-bit word. Overall, the bit rate is thus
four times higher than on a CD. In principle, the
Nyquist frequency of DSD is 1.4112 MHz. However, to
remove high-frequency noise introduced by noise
shaping, some SACD players incorporate a 50 kHz
low-pass filter (e.g., 3 dB at 50 kHz) for use with
conventional power amplifiers and speakers. A 20 kHz
low-pass filter is recommended when making SACD
audio measurements. The 1-bit DSD signal can be
converted to standard multibit PCM sampling rates.
30.6 DVD Disc Format
In its early development, DVD was envisioned as a con-
sumer video disc playback system. Subsequent develop-
ment expanded the scope of the standard. The resulting
family of DVD optical disc formats encompasses video,
audio, and computer applications, with both play-
back-only and recordable technologies. Because the
scope of these applications far exceeded digital video,
the original name of digital video disc was changed to
digital versatile disc, but that name was never accepted.
Instead, the format is simply called DVD.
Whereas the CD was originally designed exclu-
sively for audio storage, and subsequently adapted to
other applications, the DVD family was designed as a
universal storage platform. The CD was designed to
work with or without microcontrollers in the player. In
contrast, DVD employs sophisticated microcontroller
functions to read its file structure and interact with the
disc and its contents. The CD was designed to play back
a continuous stream of data. Thus, addressing was not
provided; addressing capability was only subsequently
developed for CD-ROM. In DVD, all data is address-
able and randomly accessible; all DVD contents are
essentially viewed as software data. Although its outer
physical dimensions are identical, one DVD data layer
provides about seven times the storage capacity of a
CD. This increase is due to the shorter wavelength laser,
higher numerical aperture, smaller track pitch, and other
aspects.
The DVD family contains six DVD books: Book A
is DVD-ROM (read only), Book B is DVD-Video,
Book C is DVD-Audio, Book D is DVD-R
(write-once), Book E is DVD-RAM (random access
memory), and Book F is DVD-RW (rewritable). In each
book, Part 1 defines the physical specifications, Part 2
defines the file system specifications, and subsequent
parts define specific applications and extensions. For
example, Part 3 defines the video application, Part 4
defines the audio application, and Part 5 defines the
VAN extension. DVD-ROM video and audio discs use
the same disc specifications and physical format as well
as file system. DVD-R, DVD-RAM, and DVD-RW
discs are more unique. The DVD format employs other
specifications. For example, the DVD file system uses
elements of the UDF, ISO 9660 and ISO 13346 specifi-
cations, and DVD-Video uses MPEG video coding and
Dolby Digital (AC-3) audio coding.
The physical specifications for the DVD-ROM,
DVD-Video, and DVD-Audio discs are identical and
these read-only formats share disc construction, modula-
tion code, error correction, etc. Discs are 120 mm or
80 mm in diameter and 1.2 mm in thickness, and have
two bonded substrates with single or dual data layer per
substrate. DVD discs use a pit/land structure to store
data. The DVD track pitch is 0.74μm. The track
constant linear velocity (CLV) is 3.49 m/s on a single
layer and 3.84 m/s on a dual layer. Minimum/maximum
pit length is 0.40/1.87μm (single layer) and
0.44/2.05μm (dual layer). The laser beam used to read
DVDs uses a wavelength of 635 nm or 650 nm. The
objective lens has a numerical aperture of 0.6. A DVD
layer can store 4.37 Gbytes (measured in 8-bit bytes) of
data and multiple data layers provide greater capacity.