988 Chapter 25
digital consoles. It carried up to 56 audio channels (now
64) originally down inexpensive TV-style 75:coax
(plenty enough for a 48 track recorder) and owed a lot
to FDDI, an older communications network backbone
format. Being unidirectional meant that a MADI link in
each direction to a recorder was necessary. Latency was
quite low, and the availability of chipsets made imple-
mentation fairly straightforward. An oldie-but-goodie, it
is still used to an extent in the pro audio world by some
manufacturers for overall system interconnection and
intra console knitting.
25.25.2.2 ADAT
ADAT is a simple (both in hardware and signal format)
unidirectional fiber-optic interconnection, originally, to
get 8 audio signals into or out of the once highly
popular Alesis ADAT VCR-based 8-track recorders
(which can be thanked as being the likely tipping point
of recording from uptown to basement). It is still an
interconnect of choice in semi-pro recording equipment,
where “pieces of eight” is adequate or sensible, as when
additional functionality is marketed in such a modular
fashion.
Being a strictly hardware interface it is wholly deter-
ministic (audio arrives exactly when expected) and with
very low latency. Although inexpensive chipsets are
available, the format lends itself to low-impact imple-
mentation in (possibly already existing) FPGAs
(Field-Programmable Gate Arrays) within a product
design, so incurring near-zero add-in cost.
An ADAT frame, which can carry up to eight 24 bit
audio words, is 256 bits long at a clock rate of
12.288 MHz for 48 kHz. There is a 16 bit preamble
containing a 10 bit frame-sync period and four user bits
for control/messaging. (The arithmetically astute will
wonder where the other 46 bits went; they are used
throughout the frame after every 4 bit nibble—except in
the frame-sync period—as synchronization zero-value
bits). The bits are scrambled (Manchester-encoded) to
non-return-to-zero to remove any tendency to have a dc
component. Some of this—in particular, the syncing and
NRZ—had a lot to do with coping with the vagaries of
VCR tape transports, but as a long-standing standard
with millions of installed instances it holds up very well
and doesn’t warrant the potential confusion revisitation
and redesign would incur. It is hard to envisage a
simpler robust multichannel self-clocking interface, and
its designers deserve full credit!
25.25.2.3 USBA somewhat surprising development has been the adop-
tion of the humble USB connectivity of PCs to move
moderate amounts of audio about. This reflects the
massive shift over recent years from the large-scale
studio-as-shrine approach of the recording business to
small-scale home or demo studio recording becoming
the new mainstream.
Conceived as a replacement and expansion for
RS-232 serial connections (and related mouse/keyboard
interfaces) for PCs, the early USB implementation (e.g.,
v1.1) was hard pressed to reliably move a stereo pair of
44.1 kHz about, but the upgraded USB2 with its
nominal 480 MHz data rate changed all that. As an
example, Fig. 25-154 shows a 1U rack-unit box by
Tascam (beneath the laptop, above the mega mic
preamps) that readily simultaneously transports 16
audio paths to, and 4 back from, a PC running DAW
software, all via USB2. USB2 seems to have eclipsed
Fire Wire (IEEE 1394), a similar-speed (if
network-capable) interconnection that hitherto briefly
reigned in the sphere of small-scale PC audio transport
to external A/D and D/A boxes and such.It is not at all uncommon for small mixers— analog
or digital—to present their outputs and accept a
returning pair of inputs via USB; small hand-held
recorders likewise; microphone preamplifiers; evenFigure 25-154. A modest-sized DAW running on a USB link
between the audio interface unit (center) and the laptop.
The (almost free) DAW software allows for 48 simultaneous
recorded tracks, with significant audio processing. Com-
plete with shown external mic-amps and computer, the
cost of this outfit is about that of a decent microphone; its
1990 equivalent in facilities and performance would have
cost the same as a decent car, while the 1970 version
would have equated a decent house, which would have
been needed to fit it all in, too.