Built-In Calibrated Bypass Test Mode; Setting Output/Modulation Levels - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION ORBAN MODEL 6300
You can adjust the frequency and modulation level of the built-in line-up tone via
the front panel or PC Remote software. You can use the front panel, the PC Remote
software, or the opto-isolated remote control interface ports to activate the Test
Tone.

Built-in Calibrated Bypass Test Mode

A BYPASS Test Mode is available to transparently pass line-up tones generated ear-
lier in the system. It will also pass program material, with no gain reduction or pro-
tection against overmodulation. It can transparently pass any line-up tone applied
to its input up to about 130% output modulation, at which point clipping may oc-
cur.

Setting Output/Modulation Levels

In a perfect world, one could set the peak level at OPTIMOD 6300's output to 0 dBfs.
However, there are at several potential problems that may make it desirable to set
the modulation level slightly lower.
First is asynchronous re-sampling, which we have discussed at length earlier in this
chapter. (See page 1-13, for example.) If any digital processing that causes its output
samples to be asynchronous to its input samples is used after OPTIMOD 6300's out-
put, this can cause the peak levels of individual samples to increase above the nomi-
nal threshold of limiting. This increase is typically 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 ad-
justed 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.