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OPTIMOD
INTRODUCTION
Where maximum quality is desired, it is wise to request that all equipment in the signal
path after the studio be carefully measured and aligned and qualified to meet the appro-
priate standards for bandwidth, distortion group delay and gain stability. Such equipment
should be measured at reasonable intervals.
OPTIMOD-DAB at the Transmitter:
Gain Control before the STL
The audio received at OPTIMOD-DAB's input should have the highest possible quality.
To achieve the full audible benefit of OPTIMOD-DAB processing, use a studio-
transmitter link (STL) that is as flat as the bandwidth of OPTIMOD-DAB as used in
your plant (usually 20kHz). Ideally, you should use a 20-bit (or better) uncompressed
digital link with at least 44.1kHz sample frequency.
Because the audio processor controls peaks, it is not important that the audio link feed-
ing OPTIMOD-DAB's input terminals be phase-linear. However, the link should have
low noise, the flattest possible frequency response from 20-20,000Hz, and low non-
linear distortion.
If the audio link between the studio and the transmitter is noisy (or, if digital, is limited
to 16 bits or less), performing the AGC function at the studio site can minimize the audi-
bility of this noise. AGC applied before the audio link improves the signal-to-noise ratio
because the average level on the link will be greater. Further, many STLs require level
control to prevent the STL from being overloaded.
To apply such level control and compression, we recommend the Orban Model 8200ST
Compressor/Limiter/HF Limiter/Clipper before the STL transmitter. The 8200ST per-
forms the function of OPTIMOD-DAB's internal broadband automatic gain control
(AGC), while simultaneously protecting the STL. If this is done, defeat OPTIMOD-
DAB's broadband AGC by accessing the
ST CHASSIS
function within the Setup menu
and setting it to
.
yes
In DTV applications, the AGC function in Two-Band presets is normally off. We do not
recommend using additional AGC ahead of the STL in this application. Instead, either
align STL operating levels to allow sufficient headroom to pass unprocessed audio, or
use an Orban Transmission Limiter (which is ordinarily operated below threshold) to
protect the link from operator error.
Using Lossy Data Reduction in the Studio
Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 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 satel-
lite or over landlines.
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