Audio Engineering

570 Chapter 18

inversion since it is only a frequency (and not its phase) that needs to be detected. The
resulting signal has much in common with an analogue FM signal and since the two
frequencies are harmonically related (an octave apart), it is a simple matter to extract the
bit clock from the composite incoming data stream.

In data format terms the digital audio signal is divided into frames. Each digital audio
frame contains a complete digital audio sample for both left and right channels. If 48-kHz
sampling is used, it is obvious that the 48 thousand frames pass over the link in every
second, leading to a fi nal baud rate of 3.072 Mbit/s. If 44.1-kHz sampling is employed,
44 thousand one-hundred frames are transmitted every second, leading to a fi nal baud rate
of 2.8224 Mbit/s. The lowest allowable transfer rate is 2.084 Mbit/s when 32 kHz is used.
Just as each complete frame contains a left and right channel sample, so each frame may
be further divided into individual audio samples known as subframes. A diagram of a
complete frame consisting of two subframes is given in Figure 18.3.

It is extremely important that any piece of equipment receiving the digital audio signal,
as shown in Figure 18.3 , must know where the boundaries between frames and subframes
lie. That is the purpose of the “ sync preamble ” section of each frame and subframe. The
sync preamble section of the digital audio signal differs from all other data sent over the
digital interface in that it violates the rules of a biphase mark encoded signal. In terms
of the FM analogy given earlier you can think of the sync preamble as containing a third
nonharmonically related frequency, which, when detected, establishes the start of each
subframe. There exists a family of three slightly different sync preambles, one to mark
the beginning of a left sample subframe and another to mark the start of the right channel
subframe. The third sync preamble pattern is used only once every 192 frames (or once
every 4 ms in the case of 48-kHz sampling) and is used to establish a 192 bit repeating
pattern to the channel-status bit labeled C in Figure 18.3.

The 192 bit repeat pattern of the C bit builds up into a table of 24 bytes of channel-status
information for the transmitted signal. It is in this one bit of data every subframe that the

20 20

64

VUCP VUCP

44 4 44 4

Left audio

channel data

Left audio

Sync Aux Sync Aux channel data

Figure 18.3 : Digital audio data format.