Orban OPTIMOD-AM 9300 Operating Manual Download

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INTRODUCTION

ORBAN MODEL 9300

Digital Links

Digital links may pass audio as straightforward PCM encoding or they may apply los-
sy data reduction processing to the signal to reduce the number of bits per second
required for transmission through the digital link. Such processing will almost in-
variably distort peak levels; such links must therefore be carefully qualified before
you use them to carry the peak-controlled output of the 9300 to the transmitter. For
any lossy compression system the higher the data rate, the less the peak levels will
be corrupted by added noise, so use the highest data rate practical in your system.

As stated above, links using lossy data reduction cannot pass an OPTIMOD-AM–
processed signal without distorting it. However, it is practical (though not ideal) to
use lossy data reduction to pass

unprocessed

audio to the 9300’s input. The data rate

should be at least of “contribution quality” — the higher, the better. If any part of
the studio chain is analog, we recommend using at least 20-bit A/D conversion be-
fore encoding. Because the 9300 uses multiband limiting, it can dynamically change
the frequency response of the channel. This can violate the psychoacoustic masking
assumptions made in designing the lossy data reduction algorithm. Therefore, you
need to leave “headroom” in the algorithm so that the 9300’s multiband processing
will not unmask quantization noise. This is also true of any lossy data reduction ap-
plied in the studio (such as hard disk digital delivery systems).

For MPEG Layer 2 encoding, we recommend 384 kB/second or higher.

Some links may use straightforward PCM (pulse-code modulation) without lossy da-
ta reduction. If you connect to these through an AES3 digital interface, these can be
very transparent if they do not truncate the digital words produced by the devices
driving their inputs. Because the 9300’s output is tightly band-limited to 9.5 kHz or
lower (depending on the 9300’s lowpass filter setting), any link with 32 kHz or high-
er sample frequency can pass either output without additional overshoot.

Currently available sample rate converters use phase-linear filters. These have con-
stant group delay at all frequencies. Sample rate conversion, whether upward or
downward, will not add overshoot to the signal if it does not remove spectral en-
ergy from the original signal.

If the link does not have an AES3 input, you must drive its analog input from the
9300’s analog output. This is less desirable because the link’s analog input circuitry
may not meet all requirements for passing processed audio without overshoot.

NICAM

is a sort of hybrid between PCM and lossy data reduction systems. It uses a

block-companded floating-point representation of the signal with J.17 preemphasis.

Older technology converters (including some older NICAM encoders) may exhibit
quantization distortion unless they have been correctly dithered. Additionally, they
can exhibit rapid changes in group delay around cutoff because their analog filters
are ordinarily not group-delay equalized. The installing engineer should be aware of
all of these potential problems when designing a transmission system.

Any problems can be minimized by always driving a digital STL with an AES3 digital
output, which will provide the most accurate interface to the STL. The 9300’s digital