Lab.gruppen PLM Series Operation Manual - Application Guide 12
PLM Series Operation Manual 67
APPLICATION GUIDE 12
For transmission distances up to 100 m, use a screened
twisted-pair cable, ideally with an impedance of 110
ohms at all frequencies up to 12 MHz. Cable meeting
these requirements will generally be marketed as
being suitable for digital audio. Some cable types
with lower HF attenuation characteristics may permit
transmission distances of up to 200 m. These figures
apply to an AES/EBU signal at 96 kHz sample rate;
practical distances may be double at 48 kHz. Users
are strongly recommended to experiment with cable
types and lengths before setting up a system.
It should be noted that significant differences
in performance exist between the various
cables sold as “suitable for digital audio“.
Users are recommended to experiment
“off-site”.
12.5.5 Accessories
As will be seen from the preceding paragraphs, much
greater care needs to be exercised in distributing
AES/EBU digital audio than analog audio. In some
cases, some items of additional hardware may be
required. Note that none of these items alter the
audio data itself in any way.
12.5.5.1 Passive splitters
A passive splitter is a device using specially-designed
HF transformers to split a digital audio signal to
(usually) 2 outputs. They do not compensate for cable
losses and will generally exhibit signal attenuation.
However they will often be suitable for situations
when only a 2-way split is required, the input signal
is of good quality, and only short cable distances
are involved.
12.5.5.2 Distribution amplifiers
Distribution amplifiers are a more reliable method of
splitting a digital audio signal, as they electronically
“refresh” the input signal, thus correcting waveform
degradation resulting from cable loss. Units with 4,
6, 8 or more outputs are available.
There are two methods in use for “refreshing” the
digital audio signal: repeating and reclocking.
Repeaters are HF switching devices which restore
the waveform edges of the input signal and reset the
output signal amplitude to the standard level. They
do not correct for any inherent jitter (timing errors)
in the input signal, and thus should only be used in
situations where the received AES/EBU signal is
known to be of good quality.
In a situation where the digital audio signal has suffered
deterioration resulting in jitter, it may be necessary to
use a distribution amplifier which reclocks the signal.
Rather than reshaping the waveform, a reclocker
effectively regenerates the signal as new, using a
cleaned-up version of the signal’s embedded clock to
time the new data. With this method, the DA outputs
are virtually identical to the original source signal.
Depending on the design, reclocking may introduce
a small additional amount of latency, which may
need to be taken into account when calculating time
delays.
12.5.6 Application hints
With analog audio distribution, audio quality can be
said to decrease linearly with distance – the longer
the cable, the more the degradation.
Deterioration of digital audio is different. Signal
recovery error correction techniques can recover a
near-perfect signal from one that has suffered cable
loss and has data errors. However, these techniques
will only be effective up to a point; once the received
digital audio signal has degraded beyond a certain
level, audible artifacts will start to occur, usually as
pops and clicks or brief dropouts. Once this point is
reached, should the signal degrade further, it is likely
that the receiver will be unable to decode a usable
signal at all, and all transmission will cease.
The point that should be appreciated is that the
margin between the onset of audible artifacts and
complete loss of data is usually very small. For this
reason, a “margin of safety” should always be built
in, particularly in the matter of cable lengths. In the
case of portable systems, it may well be found that
a cable length that provides error-free operation at
one venue may be unusable at another - with the
same equipment, due to noise from lighting rigs or
similar interference.
Bench test techniques are available for accurately
evaluating the degradation of digital audio signals by
observing the “eye pattern” on an oscilloscope, but
such tests are generally impractical with a portable
system. Thus it is recommended that where possible,
users should experiment with cable lengths in