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1-28
INTRODUCTION
Using Lossy Data Reduction in the Audio Chain
before OPTIMOD-PC
Interfacing to the Transmitter
Sync Input
Sample Rate and Audio Bandwidth
Many broadcasters and netcasters are now using lossy data reduction algorithms like
MPEG-1 Layer 2 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 cas-
caded before the material is finally presented to OPTIMOD-PC'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," phasiness, 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, 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-PC's multiband limiter performs an "auto-
matic 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. 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 DAB system, several programs are combined into one "ensemble multiplex."
This requires synchronization of the sample rates applied to the transmitter. DTV
and HD Radio® also require synchronization. OPTIMOD-PC allows you to use one of
the AES3 inputs to accept "house sync," which permits OPTIMOD-PC's output to be
synchronized to a master sync generator. Regardless of whether its analog or digital
inputs are used, its AES3 output will be synchronized to the AES3 signal at this input.
Because OPTIMOD-PC's digital inputs are equipped with sample rate converters,
locking the output sample rate to the house sync reference allows an asynchronous
digital input to be applied to OPTIMOD-PC's remaining AES3 input while ensuring
that OPTIMOD-PC's output is in sync with the master sync generator. See also page
1-22: Genlocking OPTIMOD-PC Cards to a Reference Sample Frequency
Most DAB audio is at 48 kHz sample rate. However, AM HD Radio operates at 32
kHz, requiring 15 kHz audio bandwidth. The DAB system offers a 24 kHz sample rate
option, requiring 10 kHz audio bandwidth.
ORBAN MODEL 1101
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