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COUPLING LOSS IN DECIBELS
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SLOPE OF DISTORTING NETWORK IN DB PER OCTAVE
Fig. 6. Four-megacycle coupling loss at which 50% of observers vote "just perceptible."
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Table I
Coupling network distortion
Coupling loss, in decibels, at 4 me
A
B
C
Flat*(l) Flatf (2) 6 db/octave* 12 db/octave* 12 db/octavef 26 db/octave*
61 60
27 17.5 19 20
57 55 22 10.5 14 12
58t 57.5 23t 11.5 15.51 14t
Column A: Critical observers rate "just perceptible" (comment No. 2)
Column B: Critical observers rate "impairment to picture, but not objectionable" (comment No. 4)
Column C: Median observers rate "just perceptible" (comment No. 2)
* Nonsynchronous: crosstalk image moves
left to right at rate of 2 frames/sec.
t Synchronous: crosstalk image moves
erratically back and forth across the main
picture.
t Selected as the limit for the indicated
type of crosstalk coupling.
Discussion of Results
One of the most striking aspects of the results tabulated above is the large
change in required coupling loss in going from flat to sloping distortion in the coupling path. Since the sloping network emphasizes the higher frequencies at the expense of the lower, the former must contribute very little to the interfering effect of the crosstalk image. This seems a reasonable conclusion when it is remembered that the high frequencies contribute to fine detail and sharp edges, neither of which is very apparent in the faint and moving crosstalk image.
To show the change in requirement with changes in slope of the coupling characteristic, the values marked { in Table I have been plotted in Fig. 6. Here the abscissas represent the slope of the coupling characteristic in decibels per octave of frequency; the ordinates indicate the required coupling loss in decibels at 4 me. It will be noted that the major portion of the reduction in requirement takes place within the first 6 db per octave; and that the requirement appears to approach an asymptotic value.
Despite the easing of requirements by as much as 35 to 44 db in going from flat to sloping couplings, it is more often the
A. D. Fowler: Reaction to Video Crosstalk
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