Orban OPTIMOD 6200, Руководство по эксплуатации

Страница: 30 / 195

1-12
INTRODUCTION ORBAN Model 6200
Sometimes, several encode/decode cycles will be cascaded before the material is finally
presented to OPTIMOD-DAB’s input.
All such algorithms operate by increasing the quantization noise in discrete frequency
bands. If not psychoacoustically masked by the program material, this noise may be per-
ceived as distortion, “gurgling,” or other interference. Psychoacoustic calculations are
used to ensure that the added noise is masked by the desired program material and not
heard. Cascading several stages of such processing can raise the added quantization
noise above the threshold of masking, such that it is heard. In addition, there is at least
one other mechanism that can cause the noise to become audible at the radio.
OPTIMOD-DAB’s multi-band limiter performs an “automatic equalization” function
that can radically change the frequency balance of the program. 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. Also, 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.
Interfacing to the Transmitter
Sync Input
In the Eureka-147 system several programs are combined into one “ensemble multi-
plex.” This requires synchronization of the sample rates applied to the transmitter. DTV
also requires synchronization. OPTIMOD-DAB provides a second AES/EBU input to
accept “house sync,” which allows OPTIMOD-DAB’s output to be synchronized to a
master sync generator. Regardless of whether its analog or digital inputs are used, its
AES/EBU output will be synchronized to the AES/EBU signal at its
SYNC INPUT
. Be-
cause OPTIMOD-DAB’s digital input is equipped with a sample rate converter, the
SYNC INPUT
allows an asynchronous digital input to be applied to OPTIMOD-DAB
while ensuring that OPTIMOD-DAB’s output is in sync with the master sync generator.
If there is no signal present at the SYNC input, the 6200 can still sync its output to the
signal present at the AES/EBU input.
Sample Rate and Audio Bandwidth
Most DAB audio is at a 48kHz sample rate. However, several of the proposed AM IBOC
systems operate at 32kHz, requiring 15kHz audio bandwidth. The Eureka-147 system
offers a 24kHz sample rate option, requiring 10kHz audio bandwidth. The proposed
shortwave systems require audio bandwidths as low as 4.5kHz for speech-grade services.