TELEVISION BY PULSE CODE MODULATION 47 



test signal, it was found that the sharpness of the contour edges was destroyed 

 by random noise when the ratio of the peak-to-peak signal to rms noise was 

 60 db. For the five-digit picture the smearing of the edges was about one- 

 tenth of the distance between the contours. Other tests which will be de- 

 scribed later suggest that the contours for the five-digit thirty-two level 

 system would be masked with an input peak-to-peak signal to rms noise 

 ratio of 40 db. 



The writer is not aware of a television system that is capable of generating 

 a signal with a peak-to-peak signal to rms noise ratio of 60 db. However, if 

 such a system were available, these results indicate that an eight- or nine- 

 digit PCM system would be needed to avoid appreciable degradation of the 

 60 db signal. 



Figure 7 shows the results for an RMA test chart with five-digit PCM 

 transmission. The resolution is limited by the input filter, the film scanner 

 having a resolution corresponding to about 10 megacycles. Using the test 

 pattern for a signal, careful comparison of the band limited transmission 

 with and without the PCM system showed only small defects in the PCM 

 transmission. 



When the PCM transmission is seen on the television screen, the contour 

 effects which are strikingly apparent for one, two, and three digits are hardly 

 noticeable for five digits. Figure 8 illustrates this performance as well as is 

 practical with photographic reproduction. About one digit is lost, and the 

 three-digit printed reproduction shows the contours with about the same 

 distinctness as four digits when viewed on the television screen. This state- 

 ment applies in general to all of the printed reproductions. 



Figure 9 shows the same results for a different subject. The contour effects 

 for a transmission system using a small number of digits are particularly 

 apparent in the sky. 



Another method of presenting the results is shown in Fig. 10. In the pre- 

 vious pictures the results have been presented for complete systems using 

 one, two, etc., up to five digits. In Fig. 10, however, the transmission of each 

 of the five digits is separately illustrated. Except for the 5th digit, for which 

 this was not possible, an attempt was made to reproduce the pictures with 

 equal contrast between black and white. The large amount of detail present 

 in the fourth and fifth digits is particularly striking. The sum picture was 

 obtained with proper weighting of the all five digits as discussed earlier in 

 the paper. 



The remaining figure (11) illustrates the effect of adding noise to the input 

 to reduce the contour effects. From these photographs it appears that add- 

 ing noise has been definitely helpful in this respect. However; a penalty is 

 paid for this result. The photographic process reduces the effect of noise by 



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