Using Lossy Data Reduction In The Audio Chain Before The 6300; Interfacing To The Transmitter; Sync Input - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION
Using Lossy Data Reduction in the Audio Chain
Before the 6300

Interfacing to the Transmitter

Sync Input

istics and phase shifts, even the best landlines tend to veil audio quality slightly.
They will certainly be the weakest link in a DAB broadcast chain.
Slight frequency response irregularities and non-constant group delay characteristics
will alter the peak-to-average ratio, and will thus reduce the effectiveness of any
peak limiting performed prior to their inputs.
Many broadcasters are now using lossy data reduction algorithms like MPEG-1 Layer
2, Layer 3, or Dolby AC2 to increase the storage time of digital playback media. In
addition, source material is often supplied through a lossy data reduction algorithm,
whether from satellite or over landlines. Sometimes, several encode/decode cycles
will be cascaded before the material is finally presented to OPTIMOD 6300's input.
All such algorithms operate by increasing the quantization noise in discrete fre-
quency bands. If not psychoacoustically masked by the program material, this noise
may be perceived as distortion, "gurgling," swishing, or other interference. Psycho-
acoustic calculations are used to ensure that the added noise is masked by the de-
sired program material and not heard. Cascading several stages of such processing
can raise the added quantization noise above the threshold of masking, making it
audible.
In addition, at least one other mechanism can cause the noise to become audible at
the radio. OPTIMOD 6300's multiband limiter performs an "automatic equalization"
function that can radically change the frequency balance of the program (some-
times by more than 10 dB). This can cause noise that would otherwise have been
masked to become unmasked because the psychoacoustic masking conditions under
which the masking thresholds were originally computed have changed. Accordingly,
if you use lossy data reduction in the studio, you should use the highest data rate
possible. This maximizes the headroom between the added noise and the threshold
where it will be heard. In addition, you should minimize the number of encode and
decode cycles because each cycle moves the added noise closer to the threshold
where the added noise is heard.
In the Eureka-147 system, several programs are combined into one "ensemble mul-
tiplex." This requires synchronization of the sample rates applied to the transmitter.
DTV and HD Radio also require synchronization.
The 6300 provides a BNC connector that accepts "house sync" in AES11id (75 un-
balanced) or wordclock (square wave at the sample frequency) format. A setup
menu selection determines whether the 6300's output will be synchronized to word-
clock, AES11, the 6300's internal clock, or the signal applied to its AES3 audio input.
ORBAN MODEL 6300