Handbook for Sound Engineers

1486 Chapter 39

Actual data rates used will vary. fifty six channels at
48 kHz + 12.5% or 28 channels at 96 kHz + 12.5%
results in a data rate of 96.768 megabits per second,
while 56 channels at 32 kHz – 12.5% results in a data
rate of 50.176 megabits per second.

39.7.3 Synchronization

Unlike AES3, MADI does not carry synchronization
information. Therefore a separate AES3 signal must be
provided to both the transmitter and receiver for
synchronization purposes.
A MADI transmitter must start each frame within
5% of the sample period timing of the external reference
signal. A MADI receiver must accept frames that start
within 25% of the sample period timing of the external
reference signal.

39.7.4 Electrical Characteristics

Either 75: coax or optical fiber is allowed for the
transmission media. Optical interfacing is described
below.
The line driver has an impedance of 75:r 2 :
average output level when terminated into 75: is
0 V ±0.1 V. The peak-to-peak output voltage is between
0.3 V and 0.6 V into 75:. Rise and fall times between
the 20% and 80% amplitude points must be no longer
than 3 ns, and no shorter than 1 ns with a relative timing
difference to the average of the amplitude points of no
more than r0.5 ns.
Interestingly there is no input impedance specified
for the receiver, although the example schematic shows
a 75: termination.
When a signal meeting the limits shown in Fig.
39-20 is applied to the input of a MADI receiver, it must
correctly interpret it.
Cabling to interconnecting MADI devices must be
75 :r 2 : and have a loss of less than 0.1 dB/m over
the range from 1–100 MHz. Cables are equipped with
75 : BNC-type male connectors and have a 50 m
maximum length. Chassis connectors are female.
At the receive end of the cable the eye pattern must
be no worse than what is shown in Fig. 39-20. Equaliza-
tion is not allowed.
The cable shield must be grounded to the chassis at
the transmitter. If the shield is not grounded directly to
the chassis at the receiver, it must be grounded above
30 MHz. This can be achieved by capacitively coupling
the shield to the chassis through a suitable low induc-
tance capacitor of around 1.0 nF.

39.7.5 Optical Interface

Graded-index fiber with a core diameter of 62.5 nm,

nominal cladding diameter of 125 nm, and a numerical

aperture of 0.275 is to be used with ST1 connectors.

This will allow links of up to 2000 m.

39.8 Soundweb

All of the interconnect schemes we have looked at so

far have been point to point, and not networked.

Soundweb is a good example of a simple yet useful

networking scheme that is part of a family of signal-

processing devices from BSS Audio. In the following

discussion we will only consider the digital audio

networking aspect of Soundweb. The chapter on virtual

systems describes the sound-processing aspects of this

family of products.

Unlike the standards-based digital audio interconnect

methods discussed so far, the protocol for Soundweb is

not published and is available only in products from a

single manufacturer, BSS Audio. Nontheless it is in

wide use, and needs to be examined from an applica-

tions viewpoint.

Each Soundweb component has network in and out

connectors for interconnecting the devices. Most

devices have just 1 in and 1 out, but their active hub has

three in and three out connectors. Each of the six

network connectors on a hub may be used to terminate

one end of a chain. Virtual wiring inside the hub is then

used to interconnect the 6 networks as desired.

An output is connected to an input with a Category 5

(Cat 5) data cable of up to 300 m in length. By using

special fiber converters that distance can be extended to

Figure 39-20. AES10 eye pattern for minimum and

maximum input signals where tnom=8ns; tmin=6 s; Vmax=

0.6 V; Vmin= 0.15 V. The MADI receiver must correctly

interpret signals within this eye diagram as applied to its

input.

Tmin

Tnom

Vmin

Vmax