Orban OPTIMOD 6200, Руководство по эксплуатации

Страница: 32 / 195

1-14
INTRODUCTION ORBAN Model 6200
First is asynchronous re-sampling, which we have discussed at length earlier in this
chapter. (See page 1-8, for example.) If any digital processing that causes its output
samples to be asynchronous to its input samples is used after the 6200’s output, this can
cause the peak levels of individual samples to increase above the nominal threshold of
limiting. This increase is typically less than 0.5dB.
Second is headroom in lossy data compression systems. A well-designed perceptual en-
coder will accept samples up to 0dBfs and will have internal headroom sufficient to
avoid clipping. However, there is no guarantee that receiver manufacturers will imple-
ment perceptual decoders with sufficient headroom to avoid clipping overshoots. Such
overshoots are the inevitable side effect of increasing the quantization noise in the chan-
nel, and can be as large as 3-4dB. Most perceptual 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 be less than unity gain.
The canny station engineer will therefore familiarize him/herself with the performance
of real-world receivers and will reduce the peak modulation of the transmissions if it
turns out that most receivers are clipping due to perceptual encoding overshoots.
Monitoring on Loudspeakers and Headphones
In live operations, highly processed audio often causes a problem with the DJ or pre-
senter’s headphones. The delay through the 6200 can be as much as 14ms (or more, if
the installer purposely adds frame-makeup delay). This delay, although unlikely to be
audible as a distinct echo, can cause bone conduction comb filtering of the
DJ/presenter’s voice in his/her ears. This is almost always very uncomfortable to them.
OPTIMOD-DAB’s Monitor Output can be switched so that it is driven after the multi-
band compressor but before the look-ahead peak limiter, which is where the majority of
the delay occurs. When driven by the multi-band compressor alone, the input/output de-
lay is approximately 3-4ms (depending on whether the analog or digital input is used and
whether sample rate conversion is used). This delay can still be uncomfortable to some,
but many DJ/presenters find it acceptable.
Such problems can be completely avoided if the DJ/presenter’s headphones are driven
directly from the program line or, better, by an inexpensive compressor connected to the
program line. If the DJ/presenter relies principally on headphones to determine whether
the station is on the air, simple loss-of-carrier and loss-of-audio alarms should be added
to the system. Such alarms could be configured to cut off audio to the DJ/presenter’s
phones when an audio or carrier failure occurs.