Mackie Digital 8 Bus Owner's Manual page 203

Clipping
of how an audio signal is represented in the
digital domain, it's time to talk about clipping.
This is important because clipping takes on a
different characteristic in the digital domain,
and is not at all as subtle or excusable as it can
be (in small doses) in the analog domain. When
recording with analog tape recorders, if the sig-
nal level gets too hot it can start to saturate the
tape and cause distortion. In most cases, occa-
sional excesses of this nature will go unnoticed.
But if you exceed the maximum recording level
with a digital tape recorder, there's no ignoring
it. Clipping in the digital domain sounds very
different than in the analog domain, with a very
harsh and unpleasant character to the sound. If
you're tracking, you'll have to record the track
over again with the level turned down a bit. If
you're mixing down, you should first make a
dry run of the transfer (in Record-Pause) to
make sure the levels are set correctly and you
don't exceed 0 dB FS.
Digital Interface Standards
developed over the years for transmitting digital
audio from one piece of equipment to another.
We'll introduce a few of the most common ones,
and explain the differences between them.
AES/EBU
audio interface in the professional audio
industry. It was developed jointly by the Audio
Engineering Society and the European Broad-
casting Union, and is officially named AES3. It
uses balanced cables with XLR connectors for
transmitting data. Two channels of digital audio
are multiplexed on a single pair of wires. This
means that a digital word from the first channel
is transmitted, followed by a digital word from
the second channel, followed by the next digital
word from the first channel, and so on. It can
handle digital words up to 24 bits, and transmits
data at 64 times the sampling frequency.
S/PDIF
standard for transmitting digital audio between
consumer devices, so the Sony/Philips Digital
Interface Format, or S/PDIF, was developed.
Many of the engineers who worked on the
AES3 standard were from Sony and Philips, so
S/PDIF is very similar to AES3. The main dif-
ference is that it uses unbalanced cables with
RCA connectors for transmitting data. Like
AES3, two channels are multiplexed together
on a single conductor, and it can handle up to
24-bit words.
B-4
Digital 101
Now that we have a general understanding
A number of interface standards have been
This is probably the most accepted digital
Sony and Philips both wanted to develop a
ADAT Optical
Alesis introduced the ADAT modular digital
multitrack recorder (MDM) in 1992, capable of
recording eight tracks of digital audio on an
S-VHS videocassette. They developed a
proprietary optical digital audio interface called
the ADAT Multichannel Optical Interface for
transmitting all eight channels of digital audio
(sometimes referred to as the ADI interface, or
Lightpipe). It uses a single fiber-optic cable to
transmit eight multiplexed channels, and can
handle up to 24-bit words, even though the
original ADAT records in 16 bits. The data-
transmission rate is 256 times the sample rate,
which is four times faster than AES/EBU and
S/PDIF. This makes sense, however, because it
is transmitting four times the number of
channels as AES/EBU and S/PDIF.
TDIF
Soon after the ADAT was introduced,
TASCAM came out with the DA-88, its version
of an eight-track MDM. Their proprietary inter-
face is called TEAC Digital Interface Format,
or TDIF, which uses 25-pin D-sub connectors
with 25-conductor cables to transmit and re-
ceive eight channels of digital audio. Unlike
any of the previously described interface stan-
dards, which require separate cables to
transmit and receive, TDIF transmits and re-
ceives on a single cable. Each wire in the cable
carries two multiplexed channels in a format
similar to AES/EBU, with a maximum resolu-
tion of 24 bits.
Jitter
Jitter is the result of instability in the timing
of the digital data. This can be caused by the
clock in the A/D or D/A converter circuits,
called sampled jitter, or by errors introduced by
the transmitting or receiving circuits, or even by
the transmitting cable, called data jitter. Jitter
can result in data errors in the transmission of
digital audio, causing increased noise and distor-
tion in the analog audio signal after conversion.
We could go on about other aspects of
digital audio, like oversampling, the different
types of analog-to-digital and digital-to-analog
convertors, error correction, sample rate
conversion, and digital signal processing, but
there's just not enough room for it in this
owner's manual. If you still want more, refer to
Appendix K, "Recommended Books."