Handbook for Sound Engineers

Digital Audio Interfacing and Networking 1461

39.1 Background

In most cases it is preferred to interface digital audio
devices in the digital domain, instead of using analog
interconnections. This is because every time audio is
transformed from analog to digital, or digital to analog,
there are inevitable quality losses. While analog inter-
facing is simple and well understood, there are few
cases in which it would be desirable to interface two
digital audio devices in the analog domain. If the digital
audio devices are not provided with digital audio inter-
faces, for example, analog interfacing will be required.
Such an analog interface, however, will result in subtle
changes in the digital audio from one side of the inter-
face to the other. The exact sequence of numbers that
make up the digital audio will not be reproduced at the
far side of an analog interface.
The numbering system commonly used in digital
audio is called binary. Each of the digits (called bits) in
the binary numbering system can be either a 1 or a 0. If
two binary numbers are identical, then all their bits will
match.

Digital audio interfaces have the potential to allow
bit accurate transfer of the digital audio from one digital
audio device to another, thus insuring no changes in the
sequence of numbers that makes up the digital audio,
and therefore potentially perfect accuracy. In order for
this potential to be realized both digital audio devices
must be synchronized.
Digital audio consists of a series of consecutive
numeric samples of the audio, each of which must be
received in sequence. If the samples are not received in
proper sequence, or if samples are lost or repeated, then
the audio will be distorted.
In order for digital audio devices to interconnect
digitally, both ends of each connection must run at the
same sampling rate. If the source is running at even a
(very slightly faster) rate than the receiver, sooner or
later the source will output a sample that the receiver is
not ready to receive yet. This will result in the sample
being lost. Similarly, if the source is running at even a
(very slightly slower rate) than the receiver, eventually
the receiver will be looking for a sample before the
source is ready to send it. This will result in a new false
sample being inserted into the data stream.

39.1.1 Synchronous Connections

The most straightforward way to carry digital audio is
over a synchronous connection. In such a scheme, the
data is transmitted at the exact same rate it is created, in
other words, at the sample rate. When additional data is

sent along with the audio in a synchronous system, it is

added to the audio data and the whole package of infor-

mation is transmitted at the audio sampling rate. Such

systems send information in fixed-size groups of data,

and introduce very little signal delay of latency. In a

synchronous system the audio data words are sent and

received at the audio sampling rate and both ends of the

system must be locked to the same master sampling rate

clock, Fig. 39-1. AES3 and IEC 90958 are examples of

synchronous digital audio interconnection schemes.

39.1.2 Asynchronous Connections

An asynchronous system is in many ways the exact

opposite of a synchronous system. Information is not

sent at any particular time. The size of a given packet of

information may vary. The time that it takes to get a

given piece of information across an asynchronous

connection may well be indeterminate. There is no

common master clock that both ends of the connection

refer to.

Examples of asynchronous transmission abound.

When you mail a letter, it may contain just a short note

on a single page, or it might contain the manuscript for

a book. You put your letter in the mailbox (the outbound

buffer) in the expectation that it will be picked up some-

time later that day. The letter will pass through many

different stages of transmission and storage along the

way to its destination. You might know that the average

delivery time is 3 days, however, in some cases the

delivery might happen in 2 days, and in others it might

be 6 days. You can be (almost) certain the letter will

reach its destination eventually, but the exact delivery

time can’t be known.

Other examples of asynchronous transmission

include the Internet, and most common computer inter-

faces and networks including RS-232 serial, and

Ethernet networking.

RealAudio and Windows Media Audio (WMA) are

two common schemes for providing a synchronous

Figure 39-1. Synchronous connection between input and

output.

Input

Output

Slack

Synchronous

transport