Handbook for Sound Engineers

1462 Chapter 39

audio connection over the asynchronous Internet. While
there can be a large receive buffer of as much as 5 to 10
seconds, you will still experience dropouts in the audio
due to network conditions on the Internet inserting
delay on some audio packets that exceed the receive
buffer delay. Generally the audio gets through OK, but
every once in a while it drops out.

39.1.3 Isochronous Connections

Isochronous connections share properties of both the
synchronous and asynchronous systems, and bridge the
gap between the two. Information is not sent at a
constant rate locked to a master clock. It provides a
maximum delivery time for information that is only
rarely if ever exceeded. By using buffers at each end, it
can carry information such as audio where the delivery
of audio words in proper sequence, and with a known
and constant delay, is essential. In a properly designed
isochronous system latency can be very low providing
near real time operation, and reliability can be very
high, Fig. 39-2.

Examples of isochronous systems include ATM that
is commonly used for transmitting telephone calls and
computer data, and CobraNet® audio networking.
FireWire (IEEE-1394) is a networking scheme that
combines both isochronous and asynchronous elements.

39.1.4 AES5

AES5 standardizes on a primary sampling frequency for

professional audio use of 48 kHz r10 parts per million

(ppm). It also allows 44.1 kHz to be used when compat-

ibility with consumer equipment is required. For broad-

cast and transmission-related applications where a 15

kHz bandwidth is acceptable it allows a sampling

frequency of 32 kHz to be used. For applications where

a wider than 20 kHz bandwidth is desired, or a relaxed

slope of the antialiasing filter is preferred, a sampling

rate of 96 kHz r10 ppm may be used.

Higher and in some cases much higher sampling

rates are in use internally in digital audio equipment.

When such a higher rate appears on an external digital

audio interface, the AES recommends the rate be a

multiple of a factor of two of one of the approved

sampling rates above.

AES5 discourages the use of other sampling rates,

although others are in use.

The above information is based on AES5-2003. It is

always advisable to obtain the latest revision of the

standard.

39.1.5 Digital Audio Interconnections

In a simple chain of interconnected digital audio

devices, it is possible for each device to look at the

sampling rate of the incoming digital audio, and lock

itself to that incoming rate. One problem with this

system is that the sampling rate as recovered from the

incoming digital audio is less than a perfect steady rate.

It will have slight variations in its rate known as jitter.

While there are techniques available to reduce this jitter,

they add cost, and are never perfect. Each consecutive

device in the chain will tend to increase this jitter. If the

jitter gets too high, the receiving device may not

correctly interpret the digital audio signals, and bit

accuracy will be lost. Worse, the performance of analog

to digital and digital to analog convertors is very depen-

dent on a precise and steady clock. Even very small

amounts of jitter can significantly degrade the perfor-

mance of convertors. As a result, it is not recommended

to cascade very many digital audio devices in this

manner.

If a single digital audio device such as a mixer will

be receiving digital audio from more than one source,

then this simple scheme for synchronizing to the

incoming digital audio signal breaks down, since it is

only possible to synchronize to a single source at a time.

There are two ways to solve this problem.

Figure 39-2. An isochronous system.

Synchronous input

Packetizer

Transport

Isochronous

data packets

Transport

Synchronous output

Serializer

Isochronous

data packets