Monitoring On Loudspeakers And Headphones - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION ORBAN MODEL 6300
put, this can cause the peak levels of individual samples to increase above the nomi-
nal threshold of limiting. Because the 6300's peak limiter is oversampled at 192 kHz,
this increase will be less than 0.5dB.
Second is additional processing, such as equalization. Equalization that applies
boosts at certain frequencies is very likely to add peak level and thus cause clipping.
However, equalization that attenuates certain frequencies can also cause overshoots
because of added phase shifts. So be wary of any equalization and allow headroom
to accommodate it.
Third is headroom in lossy data compression systems. A well-designed perceptual en-
coder will accept samples up to 0dBFS and will have enough internal headroom to
avoid clipping. However, there is no guarantee that receiver manufacturers or de-
coder providers will implement perceptual decoders with sufficient headroom to
avoid clipping overshoots. Such overshoots are the inevitable side effect of increas-
ing the quantization noise in the channel, and can be as large as 3-4dB. Most per-
ceptual encoder algorithms are designed to have unity gain from input to output.
So if peak levels at the input frequently come up to 0dBFS, peak levels at the output
will frequently exceed 0dBFS (and will be clipped) unless the decoder algorithm is
adjusted to have less than unity gain.
Canny engineers familiarize themselves with the performance of real-world receiv-
ers and reduce the peak modulation of the transmissions if it turns out that most re-
ceivers are clipping due to perceptual encoding overshoots. Our experience to date
suggests that allowing 3dB headroom will prevent audible overshoot-induced clip-
ping in low bite-rate systems (e.g., 32 kbps streams), while 2dB is adequate for
128kbps and above. While some clipping may still occur, it will have a very low duty
cycle and will almost certainly be inaudible.

Monitoring on Loudspeakers and Headphones

In live operations, highly processed audio often causes a problem with the DJ or
presenter's headphones. The delay through OPTIMOD 6300 can be as much as
25ms (or more, if the installer purposely adds frame-makeup delay). This delay, al-
though not usually audible as a distinct echo, can cause bone conduction comb fil-
tering of the DJ/presenter's voice in his/her ears. This is almost always very uncom-
fortable to them.
OPTIMOD 6300's processing can emit the output of either the multiband compressor
or AGC before their associated look-ahead peak limiters, which is where the major-
ity of the delay occurs. The AGC and multiband pre-limiter signals can be routed to
any output. The input/output delay of the pre-limiter signal is approximately 3 ms
for the AGC or 5 ms at the multiband compressor. This delay can still be uncomfort-
able to some, but most DJ/presenters find it acceptable and almost anyone can get
used to it.
See page 6-43 for a diagram of the signal routing.