Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
Figure 5.19 - AES/EBU connections using XLR connectors
If you need to transmit more than two channels of digital audio between devices, there
are several options available. The most common is the ADAT Optical Interface (Alesis Digital
Audio Tape). Alesis developed the system to allow signal transfer between their eight-track
digital audio tape recorders, but the system has since been widely adopted for multi-channel
digital signal transmission between devices at short distances. ADAT can transmit eight channels
of audio at sampling rates up to 48 kHz or four channels at sampling rates up to 96 kHz. ADAT
uses the same optical TOSLINK cable used for S/PDIF. This makes it relatively inexpensive for
the consumer. However, the protocol must be licensed from Alesis if a manufacturer wants to
implement it in their equipment.
There are several other emerging standards for multi-channel digital audio transmission,
more than we can cover in the scope of this chapter. What is important to know is that most
protocols allow digital transmission of 64 channels or more of digital audio over long distances
using fiber optic, or CAT-5e cable. Examples of this kind of transmission include MADI, AVB,
CobraNet, A-Net, and mLAN. If you need this level of functionality, you will likely be able to
purchase interface cards that use these protocols for most computers and digital mixing consoles.
5.1.4 Signal Path in an Audio Recording System
In Section 5.1.1, we illustrated a typical setup for a computer-based digital audio recording and
editing system. Let’s look at this more closely now, with special attention to the signal path and
conversions of the audio stream between analog and digital forms.
Figure 5.20 illustrates a recording session where a singer is singing into a microphone
and monitoring the recording session with headphones. As the audio stream makes its way along
the signal path, it passes through a variety of hardware and software, including the microphone,
audio interface, audio driver, CPU, input and output buffers (which could be hardware or
software), and hard drive. The CPU (central processing unit) is the main hardware workhorse
of a computer, doing the actual computation that is required for tasks like accepting audio
streams, running application programs, sending data to the hard drive, sending files to the printer,
and so forth. The CPU works hand-in-hand with the operating system, which is the software
program that manages which task is currently being worked on, like a conductor conducting an
orchestra. Under the direction of the operating system, the CPU can give little slots of times to
various processes, swapping among these processes very quickly so that it looks like each one is
advancing normally. This way, multiple processes can appear to be running simultaneously.