Handbook for Sound Engineers

1144 Chapter 30

computer peripherals. Software supports track-at-once,
disc-at-once, and multisession recording. When CD-RW
discs are appropriately formatted, the CD-Universal
Device Format (CD-UDF) specification permits easy
file-by-file rewriting; for example, users can write to
CD-RW discs with dragging and dropping.

30.5 Super Audio CD

The super audio CD (SACD) standard provides
high-density storage to support two-channel CD and
two-channel and multichannel SACD audio recordings.
SACD recordings use 1-bit direct stream digital (DSD)
coding with a high sampling frequency to achieve a fre-
quency response to 100 kHz and a dynamic range of
120 dB in the 0 to 20 kHz band. Hybrid SACD discs
can hold both a high-density DSD data layer (containing
both a 5.1-channel mix and a stereo mix) as well as a
Red Book compatible (44.1kHz/16-bit) data layer.
SACD players play back both SACD and CD discs. To
achieve this, dual laser pickups operate at both the
SACD 650 nm wavelength and the CD 780 nm wave-
length. The SACD format also specifies a lossless cod-
ing algorithm known as direct stream transfer (DST); it
uses an adaptive prediction filter and arithmetic coding
to effectively double disc capacity. The SACD standard
is described in the Scarlet Book.

30.5.1 SACD Specifications

SACD discs have a 12 cm diameter and 1.2 mm thick-
ness, the same as a CD. Other specifications allow
greater density; the laser wavelength is 650 nm, the lens
numerical aperture (NA) is 0.60, the minimum pit/land
length is 0.40μm, and the track pitch is 0.74μm. A sin-
gle-layer SACD disc holds 4.7 Gbytes of data; this pro-
vides about 110 minutes of playing time for a
two-channel stereo DSD recording. Several disc types
are specified in the SACD format including sin-
gle-layer, dual-layer, and hybrid disc constructions. The

single-layer disc contains one layer of DSD content

(4.7 Gbytes); the dual layer contains one or two layers

of DSD content (8.5 Gbytes with two layers); and the

hybrid disc is a dual layer disc that contains one inner

layer of DSD content (4.7 Gbytes) and one outer layer

of Red Book CD content (780 Mbytes) that can be

played in ordinary CD players. In dual-layer discs, two

0.6 mm substrates are bonded together. There is only

one data side in all implementations. A semireflective

layer (20–40% reflective) covers the embedded inner

data layer, and a fully reflective top metal layer (at least

70% reflective) covers the outer data surface. The outer

data surface is protected by an acrylic layer and a

printed label. Fig. 30-13 shows a hybrid disc and a dual

pickup (650 and 780 nm) reading SACD and CD layers.

SACD players can play both SACD and CD discs

(and hybrid SACD discs). CD data is passed to the

digital filter and SACD data is applied to the DSD

decoder. DSD data is output as a 1-bit signal and

applied to a pulse density modulation processor. The

data signal is converted to a complementary signal; each

logical 1 creates a wide pulse and each logical 0 creates

a narrow pulse. A current pulse D/A converter converts

the voltage pulse output into a current pulse. This signal

is applied to an analog low-pass filter to create the

analog audio waveform. SACD recordings with DSD

coding are not compatible with the DVD-Audio stan-

dard and its PCM coding. Some players may include

decoders to accommodate both disc formats.

30.5.2 Direct Stream Digital Coding

SACD recordings employ direct stream digital (DSD)

coding which uses 1-bit pulse density representation

and sigma-delta modulation to code audio signals.

Many A/D converters use sigma-delta techniques to

sample the input signal at a high sampling frequency.

The signal is applied to a decimation filter and quan-

tized for output as a PCM signal at a nominal sampling

frequency of 44.1 kHz (for CD) and up to 192 kHz (for

Figure 30-13. A hybrid SACD disc contains two data layers holding CD and SACD data.

CD layer: Completely reflective

SACD layer: Reflects 650 nm wavelength

and is penetrated by 780 nm laser

CD pickup wavelength: 780 nm, Aperture: 0.45

Focused only on the CD layer

SACD pickup wavelength: 650 nm, Aperture: 0.6

Focused only on the SACD layer

0.6 mm

0.6 mm