C lR C U I TO P E R A T I O N( C o n ' t . )
The side chain output is amplified further by D, and then
r e c t i f i e da n d s m o o t h e db y a n o n - l i n e a ri n t e g r a t o r( b l o c k G ) .
T h e r e s u l t i n gd c c o n t r o l s i g n a li s f e d b a c k t o v a r y r e s i s t a n c e
F . W h e n t h i s c o n t r o l v o l t a g ee x c e e d sa t h r e s h o l dv a l u e ,t h e
r e s i s t a n c eo f F f a l l s , c a u s i n gt h e t u r n o v e r f r e q u e n c y o f t h e
secondpart of the filter (R302, C303, and F) to increase,
t h e r e b y a t t e n u a t i n g l o w a n d m e d i u m f r e q u e n c y s i g n a l si n
t h e s i d e c h a i n . W i t h i n c r e a s i n gi n p u t l e v e l s ,t h e s i d e c h a i n
s i g n a l b e c o m e sa d e c r e a s i n gp r o p o r t i o n o f t h e m a i n s i g n a l ,
p r o d u c i n gt h e t r a n s f e rc h a r a c t e r i s t i c ps l o t t e d i n F i g u r e 1 7 .
At OdB the side chain signalis very smallcomparedwith
t h e m a i n s i g n a l ,s o t h a t t h e s i g n a li s s u b s t a n t i a l l yu n c h a n g e d
by the addedcomponent.
T h e c o n t r o l c i r c u i t u s e sa n o n - l i n e a rs m o o t h i n g s c h e m et o
avoid the generationof modulation products while pro'
v i d i n g f a s t r e s p o n s eu n d e r t r a n s i e n t c o n d i t i o n s . F o r s m a l l
changesin signal level, the charge and d ischargetime-
constants are of the order of 'l00ms, but for sudden
i n c r e a s e si n s i g n a ll e v e l ,t h e a t t a c h t i m e i s r e d u c e dt o a b o u t
2 m s . T o a v o i d o v e r s h o o t si n t h e o u t p u t d u r i n g t h i s i n t e r v a l ,
the side chain includes overshoot suppressorH, which
operatesonly under extreme transient conditions during
w h i c h t h e s i d e c h a i n i s r e - e s t a b l i s h i n gi t s s t e a d y - s t a t e
operatingpoint. Thus. for a very short period,the encoded
o u t p u t c o n s i s t so f a l a r g ep u r e s i g n a l( v i a t h e d i r e c t p a t h )
and a smallclippedcomponent (viathe sidechain).Under
s t e a d ys t a t ec o n d i t i o n st h e c l i p p e r H i s i n o p e r a t i v e .
F o r d e c o d i n g ,e s s e n t i a l l yt h e s a m ec i r c u i t r y i s r , r s e db, u t i n
t h i s c a s et h e s i d e c h a i n f o r m s p a r t o f a n e g a t i v ef e e d b a c k
l o o p i n w h i c h t h e n o i s e r e d u c t i o n s i g n a li s s u b t r a c t e df r o m
( i n s t e a do f a d d e dt o ) t h e m a i n p a t h s i g n a l .
T h e d e c o d e rc h a r a c t e r i s t i casr e t h e r e f o r ec o m p l e m e n t a r yt o
t h e e n c o d e r c h a r a c t e r i s t i c sR. e f e r r i n g t o F i g u r e 2 5 , w h i c h
shows the decoder,the inverter K inverts the phaseof the
s i g n a lf e e d i n gt h e s i d e c h a i n r e l a t i v et o t h a t i n t h e e n c o d e r .
S T A N D A R DZIA T I O NO F L E V E L S
F o r c l o s e m a t c h i n g ,t h e t w o p r o c e s s o r os f a c o n r p l e t ec h a i n
m u s t o D e r a t eu n d e r t h e s a m e c o n d i t i o n s .T h i s m e a n st h a t
the audio channel (tape recorder,broadcasttransmitter-
receiverpath, etc.) between encoder and decoder must have
a f l a t f r e q u e n c y r e s p o n s ea n d a f i x e d g a i n , s u c h t h a t t h e
s i g n a lv o l t a g e i n t h e d e c o d e p r o c e s s o ri s t h e s a m e ( w i t h i n
2 d B ) a s t h a t i n t h e e n c o d e p r o c e s s o rT. h e r e q u i r e m e n tf o r
i n t e r c h a n g e a b i l i t yo f r e c o r d i n g sa n d e q u i p m e n t i m p o s e sa
further condition. The signal levelsin the noise reduction
system must be related to levelsin the interveningchannel
i n a f i x e d a n d i n t e r n a t i o n a l l ys t a n d a r d i z e dm a n n e r .
I n o r d e r t o r e l a t et h e v a r i o u sv o l t a g e l e v e l sa n d f l u x l e v e l s
u s e d i n t h e c o m p l e t e r e c o r d i n go r t r a n s m i s s i o nc h a i n , t h e
c o n c e p t o f D o l b y L e v e l i s e m p l o y e d . D o l b y L e v e l b e a r sa
f i x e d a m p l i t u d e r e l a t i o n s h i pf i r s t l y t o t h e n o i s e r e d u c t i o n
compression and expansion parameters, and secondly to
conditions in the chain between processors.lt corresponds
with a flux level in open reel tapesof 185nWb/m, a flux
S I G N E T I CDS O L B Y . BN O I S EP R O C E S S OrR5 4 5
l e v e l o n c a s s e t t e so f 2 0 0 n W b / m , a m o d u l a t i o n d e v i a t i o n
o n F M t r a n s m i s s i o n so f + 3 7 . 5 k H 2 , a n d a v o l t a g e l e v e l a t
t h e o u t p u t o f t h e D o l b y B - t y p e p r o c e s s o r so f 5 8 0 m V r m s .
M A T C H I N GA N D M I S M A T C H I N G
Two processor units in a correctly manufactured and
a l i g n e d c h a i n c a n m a t c h a t a l l f r e q u e n c i e so f i n t e r e s ta n d
a l l l e v e l st o w i t h i n a b o u t t 1 d B .
M i s m a t c h i n g c a n o c c u r f r o m a n u m b e r o f c a u s e s .M o s t
c o m m o n l y ,t h e s i g n a lc h a n n e lb e t w e e ne n c o d e ra n d d e c o d e r
m a y n o t b e c o r r e c t l y a d j r - r s t e de, i t h e r o n a w i d e b a n d
b a s i s( i . e . g a i n e r r o r ) o r a t h i g h f r e q u e n c i e so n l y ( f r e q u e n c y
responseerror); the results of gain errors are shown in
F i g u r e1 8 .
Other oossibleerrors arise from use of out of tolerance
componenls.
S Y S T E MR E O U I R E M E N T S
F i g u r e 3 2 i l l u s t r a t e st h e b a s i c b l o c k d i a g r a r no f a D o l b y
type noisereduction systemas appliedto a typical record-
p l a y b a c k c h a i n . T h e c o n f i g u r a t i o n se m p l o y e d f o r e n c o d i n g
a n d d e c o d i n g a r e v e r y s i m i l a r , a n d c a n b e r e g a r d e da s t h e
s a m ec i r c u i t s w i t c h e dt o o p e r a t ei n e i t h e r m o d e .
Figure 25 shows the detailedblock diagramof a practical
circuit for incorporationin, for example,a consumertape
r e c o r d e r .S o m e o f t h e i n p u t , o u t p u t a n d o t h e r i n t e r f a c i n g
c h a r a c t e r i s t i c so f a p r a c t i c a l c i r c u i t c a n b e a r b i t r a r i l y
d e c i d e d ,b u t i t i s s e n s i b l et o d e s i g ns o t h a t t h e c i r c u i t m a y
m o s t e c o n o m i c a l l yb e i n c o r p o r a t e di n c o n s u m e re q u i p m e n t .
H e n c e ,f o r e x a m p l e ,i n p u t a n d o u t p L r tl e v e l sa n d i m p e d a n c e s
s h o u l d p e r m i t t h e d e s i g n e ro f a r e c o r d e rt o d i s p e n s ew i t h
additional amplifiers. However, it is important that the
i n p u t c i r c ui t r y s h o ul d n o t d e g r a d e t h e p o t e n t i a l n o i s e
performanceof the rest of the equipment. For this reason
a c o m p r o m i s e l i n e i n p u t s e n s i t i v i t yo f 3 0 m V i s e m p l o y e d ;
t h i s w i l l a c c o m m o d a t ea l l i n t e r n a t i o n a lr e q u i r e m e n t se x c e p t
thoseof the Dl N input stage.
The features of an overall svstem mav then be listed:
Input
sufficient sensitivity to be used directly as
input of consurner equipment, i.e.30mV
a p p r o x i m a t e l yi n t o a m i n i m u m o f 5 0 k o h m s .
Output
s u f fi c i e n t l e v e lt o b e u s e d d i r e c t l y a s o u t p u t o f
consumer equipment, including meeting DIN
s p e c i fi c a t i o n s , i . e . 0 . 5 - 1 . 0 v o l t s f r o m a l o w
impedance.
D i s t o r t i o n l e s st h a n t h a t i n t r o d u c e d b y a s s o c i a t e dt a p e o r
FM equipment, e.g. O.2% total harmonic dis-
t o r t i o n o v e r l o w a n d m i d d l e f r e q u e n c i e su p t o
p e r h a p s1 0 d B a b o v e r e f e r e n c el e v e l ,w i t h s o m e
r i s e p e r m i s s i b l ea t h i g h f r e q u e n c i e sa n d l e v e l s .
Noise
l e s st h a n t h a t i n t r o d u c e d b y a s s o c i a t e dt a p e o r
FM equipment, e.g. signalto noise ratio of at
l e a s t6 0 d B .