Cable Types And Distance Limitations; Signal Degradation And Loss; External Signal Distribution Hardware; Distribution Amplifiers - Lab.gruppen PLM series Operation Manual

12. Application Guide
AES/EBU input on the first PLM. Connect the adjacent Link output on the same unit to the Input on the next
PLM, and continue connecting in this manner. As many as 20 PLM units may be daisy-chained in this man-
ner, though the total may be dependent on environmental conditions and the quality of connecting cables.
AES/EBU distribution amplifier – Connect the input of the distribution amplifier to the signal source, and
connect the outputs individually to the PLM units. The number of available outputs must be equal to or
greater than the number of PLM units to be driven. Note that the distribution amplifier must be specifically
designed for AES/EBU signals; a device made for analog signals will not function in this capacity. (See 12.5.4
following.)
Consider you application requirements carefully before choosing an interconnection configuration. Note that
the daisy-chain option presents a potential "single point of failure" scenario: failure of one cable or connec-
tion will affect subsequent units in the chain. Use of distribution amplifiers avoids this scenario, though extra
expense is involved and potential failure of the distribution amplifier itself is introduced.

12.5.3.3 Cable types and distance limitations

All connections should be made with 100 ohm balanced cables wired according to the AES/EBU standard.
Although standard analog microphone cabling may function in limited circumstances, the potential for
problems is greatly increased. AES/EBU "audio" is actually a high-speed data stream, and requires an
effective bandwidth of up to 12 MHz, far beyond the 20 kHz required for analog audio.
The distance allowed between a signal source and a PLM unit is dependent on both cable quality and the
sampling rate used. At a 96 kHz sampling rate, any good quality AES/EBU cable should allow a cable run
of 100 m with no data losses beyond the capability of internal error correction. The best cables may allow
longer cable runs, though careful trials are recommended before use in the field. Sampling rate also governs
allowed cable length: a 100 m length at 96 kHz might extend to 200 m at 48 kHz, but be cut to 50 m at 192
kHz.

12.5.3.4 Signal degradation and loss

A weak or degraded AES/EBU signal will exhibit no audible loss of quality as long as the robustness of the
data stream remains above the threshold required for internal error correction. However, as the degradation
approaches the threshold, audible artifacts may be heard, including pops, clicks and momentary dropouts.
Any such indications require immediate attention, as often the "window" of acceptable data loss between
artifacts and complete audio loss can be very narrow.
As a precautionary measure four touring applications, it is advisable to configure all digital audio wiring "in
the shop" prior to use. It is recommended that all cables be tested for error-free performance at lengths
20% to 25% greater than lengths to be used in the field in order to provide a comfortable margin of safety.

12.5.4. External signal distribution hardware

12.5.4.1 Distribution amplifiers

Dedicated distribution amplifiers for AES/EBU signals are available from several manufacturers. The most
common format is one input and six outputs. Digital distribution amplifiers are designed to "refresh" or
"reconstruct" the signal as well making up for line losses. One type of distribution amplifier is a simple
repeater, which restores the waveform shape and brings the signal amplitude back up the required level.
Some distribution amplifiers also offer a reclocking feature, which also re-times the signal to prevent signal
degradation from clocking errors ("jitter"). Units that offer re-clocking often make the feature switchable;
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PLM Series Operation Manual rev 1.1.3