OPTIMOD-FM INSTALLATION
2-9
dB. The reference level is adjustable using the Digital Reference Level
control.
The 8400 is a “multi-rate” system and its internal sample rate is 32 kHz and
multiples thereof (up to 512 kHz). The output is strictly band-limited to 16
kHz. Therefore, the output can be passed through a 32 kHz uncompressed
link with bit-for-bit transparency. Because sample rate conversion is a
phase-linear process that does not add bandwidth, the 8400’s output signal
will continue to be compatible with 32 kHz links even if it undergoes inter-
mediate sample rate conversions (for example, 32 kHz to 48 kHz to 32 kHz).
The 8400HD option adds:
• A second AES/EBU digital output, which carries the HD-processed signal.
• A second AES/EBU sync input. The sample rate of the HD-processed signal can be
genlocked to this input, independent of the sample rate of the main (FM-processed)
AES/EBU output.
Composite Output and Subcarrier Input
There are two composite outputs. These carry the encoded stereo signal, the stereo pilot
tone, and any subcarriers that may have been applied to the 8400’s subcarrier inputs.
The output level of each output is independently adjustable in the In-
put/Output: Composite screen from –12.0 dBu to +12.0 dBu. Output imped-
ance may be configured for 0 or 75Ω by internal jumpers (refer to Mainte-
nance section). Each output can drive up to 75Ω in parallel with 0.047μF be-
fore performance deteriorates significantly (see footnote on page 1-12). A
Ground Lift switch is available on the rear panel. This is useful to prevent
ground loops between the 8400 and the transmitter.
The second composite output can be reconfigured in software to provide the stereo pilot
tone only, which can provide a pilot reference for RDS or RBDS subcarrier generators.
Connect the 8400’s composite output to the exciter input with up to 100 feet (30.5m) of
RG-58/U or RG-59/U coaxial cable terminated in BNC connectors.
Longer runs of coax may increase problems with noise, hum, and RF pickup
at the exciter. In general, the least troublesome installations place the 8400
close to the exciter and limit the length of the composite cable to less than 6
feet (1.8m).
We do not recommend that the exciter input be terminated by 50Ω or 75Ω
unless this is unavoidable. The frequencies in the stereo baseband are low by
comparison to RF and video, and the characteristic impedance of coaxial ca-
ble is not constant at very low frequencies. Therefore, the transmission sys-
tem will tend to have more accurate amplitude and phase response (and thus,
better stereo separation) if the coax is driven by a very low impedance
source and is terminated by greater than 1kΩ at the exciter end. This also
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