Speaker Configurations; Connecting Digital Audio Devices; Aes/Ebu Digital Audio; Sampling Rates And Latency - Lab.gruppen PLM Series Operation Manual

12.4 speaker Configurations

Connecting two speakers in parallel to a PLM power
output presents a load to the amplifier which is half
the impedance of that presented by one speaker.
Thus the current that two speakers will attempt to
draw from the output stage is double that for one
speaker, and this higher current may be sufficient to
cause the Current Peak Limiter to become active. The
more speakers connected to an output in parallel,
the lower the impedance and the higher the current
draw.
Multiple loudspeakers may be driven by a PLM power
output more satisfactorily if a "series-parallel" wiring
configuration is adopted. Care must be taken with
polarities.
In this case, the nominal impedance is the same as
with one speaker; however, the principle of power
sharing still applies, and it is not possible to get
the amplifier section to deliver more than its rated
power.
Nominal loads as low as 2 ohms are
supported within the PLM Series; however,
it is important to understand that a 2 ohm
nominal load will have impedance dips at
its resonances that can go substantially below 2
ohms. In such cases it is likely that the resulting
higher current will cause the Current Peak Limiter to
become active.
12.5 Connecting Digital Audio
Devices
When connecting inherently digital audio equipment
fitted with both analog and digital inputs and/or
outputs, the digital option is generally preferable.
Connecting digitally avoids unnecessary conversions
between the analog and digital domains, and as these
conversions are the main source of distortion and
errors in a digital system, maximum audio fidelity
is maintained.
Digital audio is data, not audio. It therefore requires
different handling and interconnection techniques
from line level analog audio. This section of the
APPLICATION GUIDE 12
manual gives some guidance for those unfamiliar
with these techniques.

12.5.1 AEs/EbU digital audio

The PLM is able to directly accept an AES/EBU digital
audio signal as an input. It is important to remember
that a single AES/EBU connection can carry two
channels of audio. These will generally be the L & R
of a stereo pair, but can just as easily be two unrelated
mono signals.
Electrically, the AES/EBU interface uses a 3-wire
balanced connection in accordance with the RS422
standard for data transmission. See figure 12.5.1.
Note the "hot" and "cold" wires are a twisted pair.
Pin 1: Ground/shield
Pin 2: Hot (+)
Pin 3: Cold (-)
Figure 12.5.1: AES connector wiring and pinout
Although the PLM (in common with most professional
audio equipment) uses industry standard XLRs for
connection of both digital and analog audio signals,
it is important not to confuse the two types.
NEVER CONNECT A DIGITAL AUDIO SOURCE TO
AN Analog INPUT, OR VICE-VERSA.
The AES/EBU standard allows for two channels
of audio data at up to 24 bits resolution. Some
digital audio sources may only have a resolution of
16 or 20 bits; it should be possible to interconnect
professional equipment with AES/EBU inputs and
outputs correctly without the user being aware of
any resolution differences. In addition to the audio
data, the AES/EBU standard also carries control data,
which, for example, can provide a receiving device
with information about the signal.

12.5.2 sampling rates and latency

Any piece of equipment processing audio digitally
will impose a small time delay on the original analog
input. Most of this delay is generated internally by the
conversions between analog and digital (ADC), and
then back to analog (DAC) again. This delay is often
referred to as the latency of a piece of equipment.
Ultimately, the delays may become large enough to be
audible and lead to phasing problems. For this reason,
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