73
The output of this module is a very precise controlled amplitude signal. U603 and U604, along with
associated components form a phase linear 7th order low pass filter. With a cut-off frequency of 9.2
kHz and an elliptical response, the NRSC 10 kHz bandwidth is achieved. Due to the phase linearity of
the filter, and prior overshoot control of the GIBBS MODULE, overshoot of the final filter is kept to
a minimum. Safety clippers D501 and D502 eliminate an residual overshoot from the system. The
output of U605A/B will be a pre-emphasized, amplitude and frequency limited signal that will now be
used to feed the discrete left/right output sections located on the motherboard.
Transmitter Tilt is corrected through the use of the RC equalizer network “around” U-606A/B.
Adjustment of VR-1 A/B will set the amount of Tilt correction, while adjustment of VR-2 A/B will
set the optimum frequency for correction. AM transmitter Tilt occurs because of the negligence of LF
response of the modulator stage. This is very common in older plate modulated transmitters. The Tilt
correction circuit actually creates a “pre-distortion” signal exactly opposite that of the transmitter Tilt.
When the “pre-distorted” signal is modulated by the transmitter, the two “opposing” tilt signals cancel
each other out, and optimum low frequency modulation is achieved. Refer to the discussion on page
54 of this manual for a detailed description.
MULTIBAND LEVELER AND LIMITER ALGORITHMS
The processing algorithms for leveling and limiting are performed on circuit boards # 4 ,#5, and #7.
Boards #4 and #5 handle the multiband levelers, #4 for left, #5 for right. Board #7 takes care of the
multiband limiting. For both leveling and limiting only one band of one channel will be detailed.
Since this system is a four band stereo processor, there are a total of eight levelers and eight limiters
overall.
MULTIBAND LEVELERS
Circuit Boards #4 & #5
C o n t r o l f o r t h e M U L T I B A N D L E V E L E R S i s d e v e l o p e d u s i n g a f e e d - f o r w a r d
a p p r o a c h . T h e i n p u t a u d i o s i g n a l a p p e a r s a t p i n s 1 - 4 o f J 1 . T h i s i s b a n d l i m i t e d
a u d i o a s s e t b y t h e c r o s s - o v e r n e t w o r k . P i n 1 i s f o r t h e l o w b a n d , p i n 2 the mid
b a n d , p i n 3 t h e p r e s e n c e b a n d , a n d p i n 4 i s t h e h i g h b a n d s i g n a l . U 3 0 1 B , D 3 0 2 ,
D 3 0 3 , R 3 1 2 , a n d R 3 1 3 f o r m t h e f u l l - w a v e r e c t i f i e r f o r t h e f o l l o w i n g i n t e g r a t o r s .
U 3 0 1 A , U 3 0 2 A / B a r e c o n f i g u r e d a s i n t e g r a t o r s t h a t w i l l d e v e l o p c o n t r o l v o l t a g e s
f o r v a r i o u s a s p e c t s o f t h e s i g n a l w a v e f o r m .
U 3 0 2 A f o r m s t h e i n t e g r a t o r t h a t p r o v i d e s c o n t r o l s i g n a l f o r t h e G A T E d e t e c t o r .
A s w a s p r e v i o u s l y d i s c u s s e d i n t h e
W B - A G C
s e c t i o n , t h e o p e r a t i o n o f t h e G A T E
d e t e c t i o n i s e x a c t l y t h e s a m e . W h e n t h e “ a v e r a g e d ” l e v e l f e l l s b e l o w t h e G A T E
r e f e r e n c e , U 3 0 3 B c h a n g e s t o a h i g h s t a t e , a n d t h e f o l l o w i n g t w o o c c u r r e n c e s
r e s u l t . F i r s t , b i l a t e r a l s w i t c h U 3 0 7 D w i l l s w i t c h o n t o b e g i n r e s e t t i n g t h e g a i n t o
