Apogee AD-1000 Operating Manual page 54

AD-1000 Operating Manual
on a twisted pair of wires enclosed in an outer metal shield. The shield is usually a continuous, flexible braided
wire jacket or in applications where flex is unnecessary, a metal foil wrap is often used (inside patch bays and
consoles for example). The shield provides a ground connection and reduces the influence of outside electrical
interference on the two wires carrying the audio. Two wires are used instead of one to further reduce the effects
of outside interference. Because the two wires are twisted together, they follow almost exactly the same path.
Any interference managing to make it through the tubular shaped shield tends to affect both wires equally. An
example would be running the microphone cable alongside a power transformer. The magnetic energy radiat-
ed from the transformer causes the two wires to develop the same AC mains related hum voltage. If the two
wires were driven into a transformer, this hum voltage would not come out the other side of the transformer
because both wires have the same voltage at any moment due to the hum. For the transformer to give any out-
put, there must be a voltage difference between the two wires. The transformer input is called differential
because the analog audio is carried as the voltage difference between the two wires. The noise signals picked
up along the way are called common mode inputs and the ability of the transformer to ignore them is rated as
common mode rejection. In professional audio we call differential inputs and outputs balanced and because
transformers are bulky and expensive, they are outnumbered in modern equipment by their more economical
electronic equivalent: electronically balanced inputs and outputs.
As compared to other digital formats which rely on multiple interconnects for clock, left and right data,
AES/EBU simplifies the cable connections and uses readily available wire interconnects that are already in use
at most professional and semi-professional facilities.
A single line connection of stereo digital audio must transfer a string of data packages containing left and right
audio samples repeated at the sample rate. One package is referred to as a frame. The single line AES/EBU
interconnect divides each package into 64 little pieces of binary bits with 32 for the left sample and 32 for the
right. Each chunk of 32 bits is called a subframe. To make it easy to recover the data on the receiving end, each
bit is further divided in two. Patterns of full bits and half bits are coded to indicate whether the bits represent
one binary state or another, often referred to as zeros and ones. In some older multi-line interfaces, the loca-
tion of the beginning of samples is marked with a separate word clock line. To find the beginning of the left and
right samples in the AES/EBU format, each 32-bit subframe includes a unique pattern of half bits and at least
one delay equal to one full and one half bit joined together. Receiver circuits can recognize the longer one and
a half sync bit and use it to extract the left/right synchronizing information for sample decoding and word clock
separation.
The audio samples can be up to 24 bits long and the sync pattern uses four more bits. With 32 bits available,
there are four extra bits left to send more information. Digital audio samples must change very quickly where-
as other information can be updated at a slower rate. For example, emphasis is usually selected at the begin-
ning of a session and remains on or off, so updating the emphasis status 44,100 times a second would be redun-
dant. The AES/EBU interconnect takes two bits of each subframe and calls them user data and channel status
bits. To pack more information into the one channel status bit location, 192 bits are sent sequentially, one bit at
a time. These 192 bits can represent vast amounts of data at a slower rate than the one bit alone. The begin-
ning of one of these sequences is marked with a special sync pattern in place of the normal sync pattern for a
left sample. At the receiving end, the status bit is picked off at every frame and assembled one at a time into
a string 192 bits long. The collection of 192 bits repeats every 230 time a second for a 44.1 kHz sampling rate.
The status bits can represent controls for a variety of important data. Sample Rate, Emphasis and Copy pro-
tection are represented. Even control of redundancy checking is implemented. Bits for 'indexing' are support-
ed. Identification of professional or consumer format is also indicated.
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