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PER CENT OF VOTES
LESS THAN INDICATED
COMMENT NUMBER
54 50 46 42 38
COUPLING LOSS IN DECIBELS
Fig. 5. Distribution contours of comments from 10 observers (flat, lumped crosstalk).
represents the reactions of the "composite" critical observer, it is seen that the crosstalk is just perceptible to him when the coupling loss is about 61 db and that he still does not object to it (votes no more than comment No. 4) when the crosstalk is made 4 db stronger. The median observer finds the crosstalk just perceptible at 58 db loss; and the uncritical observer does not notice the crosstalk until the coupling loss is reduced to 52 db.
One of the main objectives of this study of the reactions of observers to crosstalk was to arrive at a tolerable limit for the crosstalk coupling loss. The contours of observer reaction, as just discussed, will serve as a useful guide in this connection. As a criterion for tolerable limit, it is proposed that the crosstalk not exceed a value which is either objectionable to the most critical observers, or
more than just perceptible to half of the observers. Referring to the contours of Fig. 5, it is seen that a coupling loss of 58 db evoked comment No. 2 from 50% of the observers and less than comment No. 4 from the most critical observers. The limiting coupling loss is, accordingly, 58 db for flat, lumped crosstalk.
The test results for the sloping types of coupling and for both synchronous and nonsynchronous operation of the video systems are very similar to those discussed in detail for flat coupling. They differed mainly in the values of coupling loss at which the comments occurred and, to some extent, in the steepness of rise of the contours. Rather than a display of separate matrices and contours for each of the tests, a summary of the responses of the critical and median composite observers is presented in Table I which follows.
422
November 1951 Journal of the SMPTE Vol. 57