Q1 (Q2) is a recover/expansion gate with a threshold about 18 dB below the normal program
level. The amount of short-term expansion and time for gain recovery is controlled by the
PROCESSING control, located on the front panel display board. (See section 3.5.)
Audio components above 15,200 Hz are greatly attenuated by eighth-order switched-capacitor
elliptical filter, U5 (U11). The filter cut-off frequency is determined by a 1.52-MHz clock (100 x
15,200 Hz) signal from the stereo generator section of the board. The broadband signal level
at the output of U5 (U11) is about 5 dB below that required for full modulation. (With normal
program material, the 5 dB of headroom will be filled with pre-emphasized audio.)
Pre-emphasis in microseconds is the product of the capacitance of C7 (C17), multiplied by the
current-gain of U6 (U12), times the value of R22 (R62). (For description of the device used for
U6 (12), see explanation for U3 (U9) above.) For a 75 micro-seconds pre-emphasis, the gain
of U6 (U12) will be about 1.11.
Selection of the pre-emphasis curve (75 μS, 50 μS, or Flat) is made by moving the jumper on
HD1 to the pins designated on the board. Fine adjustment of the pre-emphasis is made with
R23 (R63). (See section 5.1.)
For high-band processing, the peak output of U7A (U13A) and U7B (U13B) is detected and
gain-reduction bias is generated, as with the broadband processor. The high-band processing,
however, shifts the pre-emphasis curve rather than affecting overall gain. Peak audio voltages
are compared to plus and minus 5-volt reference voltages at the outputs of U19A and U19B.
This same reference voltage is used in the stereo generator section.
A stereo phasing error occurs when left and right inputs are of equal amplitude but opposite
polarity. The most common cause is incorrect wiring of a left or right balanced audio line
somewhere in the program chain-sometimes at the source of a recording. When this happens,
all the audio is in the left-minus-right stereo subcarrier-none in the left-plus-right baseband.
The error can go unnoticed by one listening on a stereo receiver, but the audio may disappear
on a mono receiver. In normal programming there may be short-term polarity reversals of left
versus right, either incidental or-for effect-intentional. A phase error of several seconds dura-
tion is processed by U14A and U14B and interpreted as a real error. During a phasing error
the right-channel level is gradually reduced by 6 dB. For a listener to a stereo radio, the right-
channel volume will be lower, while on a mono receiver there will be a reduction of volume.
NORMAL/TEST switch. In the TEST position, the stage gains are set to a fixed level. See sec-
tion 6.2
4.2.2 Stereo Generator Section
Composite stereo signal is generated from left and right-channel audio inputs. This section
also has the amplifier (U201) for an optional external composite input and provision for inser-
tion of SCA signal(s).
4-4 FM600 User’s Manual
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