Pag. 46
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
Electronic Broadcast Equipment
The signal goes to the stereo-encoder circuit’s input sections if present. In the mean time, the signal on
the mono right channel path is sent to a low-pass filter, consisting of the section built around IC10 and IC11,
which attenuates the frequencies above 15 kHz for operation in mono.
The switch IC14 selects the signal issuing from the non-pre-emphasised input section through R124 or
from the pre-emphasis-and-filter section through R128 or from the stereo-encoder through R131. IC12b
buffers the chosen signal and mix it with that issuing from the auxiliary channel. When required, the diode
D17 further limits the resulting total signal. The latter is then sent to the FM modulating/exciting circuit via
IC12c buffer circuit and adjusted in level by RT6 as required. A separate section of IC12 separately buffers
the modulation signal for monitoring purpose, and sends it to the modulation output connector. IC15
deserialise the digital signal sent by the CPU to control the transmission channels with IC14 and preemphasis
action with IC9. Two output lines from IC15 are used to latch the remote output lines "Failure" and "On the
air".
THE STEREO-ENCODER MODULE
The encoding circuit uses an 8-step switching technique, which ensures excellent performance with a
relatively simple circuit. With this technique the first harmonics that are associated with the resulting stereo
multiplex signal are the 7th and 9th (266 and 342 kHz); this simplifies the design of the low-pass filter on
this signal.
The audio signal is filtered beyond 15 kHz by the two precision active low-pass filters built around IC1
÷ IC4. It is then buffered by IC3d and IC4c and applied to the encoding circuit present in IC8. To remove
higher order harmonic products, another higher frequency low-pass filter follows. Also this filter is made with
highly precise active circuitry built around IC5 and IC6a, b. The latter section (c) of IC6 performs phase
equalization. The four analog switches contained in IC7 allow to select the mono or the stereo-encoded
signal and to slightly vary the encoder gain to adjust for the 90% audio modulation in stereo vs. 100% in
mono. Two jumpers on BD1 are used to select either Left or Right or Left+Right channel for mono operation,
with no output level change. As factory configured, both jumpers are installed, to mix Left and Right channel
for "MONO L+R" operation.
Circuits IC10, IC11 supply the encoder’s time base; IC9 synthesizes the 19kHz pilot frequency which
is filtered and buffered by IC5a. A separate 1Vpp output is provided on J2 to drive carrier synchronization
on a possible external RDS generator.
