Technical Choices for a Video Recorder
John P. Watney · SMPTE Journal · 1984
o n of Video Tape Recorders there has been a s t e a d y improvement i n technology which first went i n t o improving p i c t u r e q u a l i t y as black and white gave way t o color and low band t o high band. When t h e 2" t r a n s v e r s e recorder gave way t o t h e1" h e l i c a l recorder t h e advance was n o t so much i n p i c t u r e q u a l i t y , e x c e p t f o r t h e e l i m i n a t i o n of segmented f i e l d s , b u t i n economy and o p e r a t i o n a l f e a t u r e s .Variable speed r e p r o d u c t i o n and p i c t u r e i n s h u t t l e are now so well e s t a b l i s h e d t h a t u s e r s wonder how t h e y e v e r managed w i t h o u t these f e a t u r e s .The a d v e n t of a f f o r d a b l e d i g i t a l p r o c e s s i n g h a s added a b e w i l d e r i n g a r r a y of c a p a b i l i t i e s t o the p r o d u c e r ' s bag of t r i c k s .It is also c o n t r i b u t i n g t o a n awareness t h a t the number of t a p e g e n e r a t i o n s , which p r e s e n t VTR t e c h n o l o g y allows w i t h o u t s i g n i f i c a n t d e g r a d a t i o n i n p i c t u r e q u a l i t y , i s n o t a d e q u a t e f o r the effects b e i n g sought.I n a d d i t i o n there is a n awareness t h a t a d e g r a d a t i o n i n p i c t u r e q u a l i t y does n o t n e c e s s a r i l y accompany storage and r e t r i e v a l .The time is r i p e f o r t h e d e f i n i t i o n of a new g e n e r a t i o n o f Video Tape Recorders which o f f e r more t h a n j u s t a m a r g i n a l improvement i n p i c t u r e q u a l i t y .D i g i t a l technology promises a q u a l i t y which is l i m i t e d o n l y by t h e data r a t e one is p r e p a r e d t o pay f o r , and computer systems have g i v e n d i g i t a l r e c o r d i n g a r e p u t a t i o n of b e i n g free from errors and free from maintenance.It is n a t u r a l , therefore, t o look t o d i g i t a l r e c o r d i n g f o r t h e new g e n e r a t i o n o f v i d e o tape r e c o r d e r s .Digital r e c o r d i n g has, however, had t o wait f o r improvements i n r e c o r d i n g technology before it was able t o offer a s u f f i c i e n t l y h i g h data r a t e a t a n economical cost.These improvements i n technology have on t h e whole n o t been i n c o r p o r a t e d i n t o a n a l o g VTR's.It is n a t u r a l , t h e n , t o a s k whether a n a n a l o g VTR format c o u l d n o t b e d e s i g n e d t o d a y which would o f f e r t h e desired improvement i n p i c t u r e q u a l i t y w i t h o u t i n c u r r i n g t h e c o s t and complexity commonly associated w i t h an e q u i v a l e n t d i g i t a l VTR.T h i s paper w i l l compare Composite and Component d i g i t a l r e c o r d i n g w i t h a c o n c e p t u a l s p e c i f i c a t i o n f o r improved a n a l o g r e c o r d i n g .Comparisons w i l l b e based on p i c t u r e q u a l i t y , c o s t and u t i l i t y .The D i g i t a l Video Tape Recorder Given a p p r o p r i a t e error p r o t e c t i o n t h e p i c t u r e q u a l i t y i n a d i g i t a l system i s f u l l y determined by t h e i n i t i a l d i g i t i z a t i o n of t h e o r i g i n a l a n a l o g s i g n a l .For a component system t h e s t a n d a r d has been s e t a t 8 b i t samples t a k e n a t a r a t e of 13.5 M samples p e r second f o r t h e luminance c h a n n e l and 6.75 M samples p e r second f o r each of two chrominance c h a n n e l s .T h i s s t a n d a r d allows v i r t u a l l y t r a n s p a r e n t coding and decoding e x c e p t f o r t h e most c r i t i c a l p i c t u r e material.DVTR s t a n d a r d i z a t i o n e f f o r t s on both s i d e s of t h e A t l a n t i c are c o n v e r g i n g on a strawman format, t h e e s s e n t i a l d e t a i l s of which are compared w i t h c o r r e s p o n d i n g parameters from t h e rrC1l format i n Table I.The p r o d u c t of head/tape s p e e d and number of c h a n n e l s is 100m/sec.While n o t r e q u i r e d by t h e f o r m a t , p r a c t i c a l c o n s i d e r a t i o n s r e q u i r e 4 head c h a n n e l s .Both s y s t e m s u s e 1" tape.