Apogee AD-1000 Operating Manual page 51

AD-1000 Operating Manual
The numbers of digital audio are transmitted in binary form. Instead of using our familiar ten-finger-oriented
decimal numbers, we substitute one-finger binary numbers. Any decimal number can be represented as a bina-
ry number and vice versa. The big advantage of using binary coding to represent digital audio samples is that
each individual digit of a complete binary number takes only one of two values instead of the ten when we use
our familiar decimal method of counting. Binary digits are called bits and, because they have only two values or
states, can be easily represented by electronic circuits as either on or off, high or low voltage etc. The most
common digital audio numbers in use today are 16 bits long, with a small (but growing) percentage of recorders
and workstations capable of handling or storing more.
All At Once or A Bit At A Time
When manufacturers had to come up with schemes to interconnect their products, before they could agree on
a standard (pre AES/EBU), the main requirement was to minimize the number of interconnections. When mak-
ing interconnections within a digital device, it is usually most efficient to move the numbers around as complete
chunks of the individual bits. Sixteen bit systems can use 16 separate lines to transfer entire samples in single
steps. This is known as parallel operation. A parallel interconnect between different audio devices is cumber-
some, requiring over 32 connections for a stereo 16-bit system – plus additional lines for grounds, timing and
control information. A more efficient method is to send the 16-bit numbers across one wire, one bit at a time.
This is called a serial interconnect and can be visualized as sending individual bits down a hose and reassem-
bling them into complete numbers at the other end. It's important to know when the 16 bit numbers start and
finish to correctly unravel them at the other end, so timing information is also included – as either a separate
connection or included with the 16-bit audio and identified with an additional unique pattern of bits. You can
think of the timing as the pulse of a digital audio system; every time it beats, it signals a sequence of events
such as the beginning of a transfer of a sample, one bit at a time. The main pulse is at the sample rate, beating
at 44,100 times a second for a CD player. In addition to the sample rate beat, there are additional higher fre-
quency pulses used to co-ordinate all the activity going on between the slower sample rate timing. You could
visualize this relationship in musical terms as a one-measure loop with the main pulse on one and the other as
⁄ note pulses. The high frequency pulses are often called the bit clock, which is passed across interconnects in
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one form or another.
It's All In The Timing
A drummer's timing can make the difference between good music and a memorable hit. Digital audio, likewise,
needs good timing to make it from one place to another with uncompromised sound quality. The timing in the
interconnect is used to unscramble all the bits for accurate recovery of the exact samples transmitted. The tim-
ing also needs to be very regular.
Timing jitter is any irregularity in the timing passed across an interconnect. If the samples become messed up
in the interconnect, the effects are usually very audible, varying from occasional clicks to a loud, harsh fuzz.
Timing jitter can cause more subtle effects. In digital to analog converters for example, the location of instru-
ments across the audio sound stage can become less focused. Note: A "sound stage" is the mental picture you
form when you listen to a piece of music and localize the various instruments and vocals as if they were on stage
in front of you (closing your eyes can help form the image). A well defined sound stage has width, depth, and
focused locations all defined by subtle reflections, reverb tails and tonal quality in a stereo mix.
These Interconnects Sound Different!
You may have heard critical digital audio listeners complain "if digital audio is so perfect, then how come it
sounds different when I use different interconnects?" Some experts will tell them it must be their imagination
because if the numbers are sent correctly on each interconnect they both must sound the same. That makes
sense, but it's only part of the story...
When a digital to analog converter receives the samples from an interconnect, it must also extract the timing
information and regenerate its own timing "clock". A good analogy is a drummer playing to a click track. If the
drummer is good, he can nail the basic tempo of the click and add in faster patterns of his own, such as a six-
teenth-note high hat. When digital devices receive the clock from an interconnect, they lock up to the sample
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