Appendix Ii: About Digital Audio Interconnects - Apogee AD-1000 Operating Manual

AD-1000 Operating Manual
Appendix II – Digital Audio Interconnects
Digital Audio Without Making Your Eyes Glaze Over
You've probably read, or at least started to read many articles on digital audio. Like many people, you may be
guilty of skipping the technical diagrams and jumping to the last page for the conclusions. Understanding how
digital audio works is akin to getting into the details of how MIDI controls musical instruments... it's handy infor-
mation, but not necessary for making music. Most digital audio users' eyes glaze over when discussing the tech-
nical aspects of the subject. On the other hand, discussing why one digital audio box won't talk to another can
make the same eyes bug out and face turn red! Many of us have experienced the frustration of trying to make
one piece of digital audio gear connect to another without success. Digital audio is not so new anymore, so it's
reasonable to assume that interconnects should be "no brainers". Because they aren't, requires some under-
standing of what makes them tick so we can get the most out of them. This section will give you insight to deal
with the peculiarities of digital interconnects without the usual technical smoke screen.
The Difference Between Good Old Analog And Digital Audio
Sound is transmitted through air as movement of individual air molecules. A microphone turns this movement
of air into a changing voltage which represents the air movement. This changing voltage is called an analog of
the air movement. Sound analogs can also be mechanical, such as a phonograph groove, electrical current, mag-
netic field, optical energy, or any continuously varying representation.
Digital audio uses numbers to represent sound. These numbers have to be big enough to capture the smallest
and biggest details in sounds – accurately. The same numbers also need to be changed fast enough so our ear
is not aware of them stepping by. You are probably aware that cartoons consist of a sequence of individual
drawings changing fast enough to give the illusion of motion. If we slow the sequence of drawings down, the
image starts to flicker like the old movies and motion becomes jerky.
To fool our eyes into seeing fluid motion, the images need to change from one to the next at least 25 times per
second. There are some motion picture systems – such as the one from Showscan in Culver City, CA – that
increase the rate to 60 per second, resulting in an amazingly grain-less and fluid motion.
The frozen visual images of individual movie frames are analogous to the individual numbers of digital audio.
Our ear doesn't get fooled into thinking that these numbers sound real until they change at around 32,000 times
a second. The individual numbers are called samples and represent audio in narrow slivers of time. The rate
these frozen slices of audio change per second is called the sample rate.
You will often see sample rates represented as kHz or kiloHertz (k = one thousand; Hz = cycles/times per sec-
ond). A sample rate of 32 kHz (32 thousand samples per second) is used in digital broadcasting applications.
Compact Discs use a 44.1 kHz sample rate (44,100 samples per second). These individual samples are different
to the musical instrument or vocal samples used in assembling music tracks. Sound samples are made up from
strings of the individual "slices of time" samples much as a video clip is a sequence of individual video frames.
You can see it takes a lot of numbers in the digital world to represent an analog version of the same sound. An
analog signal path may need a frequency response of 100 kHz to faithfully reproduce 20 kHz audio. A digital
signal path for the same 20 kHz audio requires a frequency response of several million Hertz (Megahertz or
MHz). Bandwidth is a measure of the lowest to the highest frequency a path can handle. The wide bandwidth
required for digital audio is due to the way the individual numbers are transmitted across an interconnect. There
are a number of different methods of making digital audio connections inside equipment and externally to other
devices.
Digital Audio Interconnects
In the early days of digital audio there was no accepted standard for interconnecting different devices, so the
manufacturers invented their own schemes. A interconnect needs to pass the individual numbers of each sam-
ple along with timing information and any useful control information such as if pre-emphasis was applied or not.
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