D*AP8
78
The second step of calibration is to set the values for
Pilot Tone, RDS, SCA(DARC) Deviation
.
The necessary values depend on the setup of the respective encoders. Please refer to the respective
manuals.
After calibration the
FM Conditioner
will now display the available audio headroom.
Here is an example with an assumed deviation of ~ 12 % of 75kHz for the extra services:
20*log (75kHz – 8.8kHz) / 40kHz = 4,4dB
or -4.6dBFS
This calculation is performed internally and updates automatically when any of the involved parameters
change. The resulting value is called the Ceiling. It is important to know that the
Resulting Ceiling
is
calculated with the
Pre-Emphasis
filtering of the FM transmitter included. Thus the wideband true peak
level of the audio signal before
Pre-Emphasis
needs to be lower. A look at the level relation diagram
may help understanding this:
Pre-Emphasis
is a filter system where the higher frequencies are raised by a shelving filter at
transmission stage and equivalently reduced at the receiver end. The
Pre-Emphasis
filter utilizes a
time constant of 50 µs (or 75 µs in the USA) which results in a gain of 10dB at 10kHz.
This procedure creates a significantly improved signal-to-noise ratio.
