Dolby Laboratories CP65 - Manual (Page 60)

f. Adjustment of Surround Delay
5-42
B-Chain Alignment Procedures
The CP65 incorporates a delay line in the surround channel to ensure that sound from the
rear of the theatre arrives at the listeners’ ears approximately 20 milliseconds after the
arrival of sound from the front speakers.The delay is set into the Cat. No. 150 card via a
thumbwheel switch by using the following formula:
1. The distance between a rear seat and the nearest surround loudspeaker, in feet, is
estimated. If the metric system is used, convert the distance from meters to feet by
multiplying by three (3).
2. The distance from this seat to the front loudspeakers is estimated, in feet. If the metric
system is used, multiply the distance by three (3) to convert distance from meters to
feet.
3. Subtract the distance in 1 above from the distance in 2 above.
4. Add 20 to the difference in 3. above to obtain the delay time, in milliseconds.
Determine the Cat. No. 150 card thumbwheel switch setting (delay line setting) from the
table below.
Delay Time Thumbwheel Switch
(msec) Setting
30 1
40 2
50 3
60 4
70 5
80 6
90 7
100 8
110 9
120 10
130 11
140 12
150 13
For example:
The selected seat is 10 feet (3.3 meters) from the surround speakers.
The selected seat is 80 feet (26.7 meters) from the front speakers.
The delay is set for (80-10) + 20 = 90 milliseconds — switch position 7.
f. Adjustment of Surround Delay
5-43
B-Chain Alignment Procedures
You can verify that the delay setting is acceptable by listening to a familiar Dolby Stereo
film which ideally contains both center channel dialogue and some discernable surround
sound. The Dolby Cat. No. 251 “Jiffy” film serves excellently for this purpose. If a stereo
film is not available, the delay time can be checked with a mono film if the film is played
in format 04 Dolby stereo optical with surround. Before you set the delay time, set the
individual level adjustments for each channel because the subjective effects of channel
level and delay time interact somewhat. Make certain all speakers are ON for this test.
While the film is playing, walk around the theatre and carefully listen to the surround
speakers when there is center channel dialogue. The dialogue should appear to be
coming from the screen with no significant dialogue coming from the surround speakers.
If you hear discernable dialogue from the surround speakers, the delay
time was probably set too long.
If you hear an objectionable amount of dialogue from the surround
speakers, which persists regardless of the delay time setting, there is
probably severe gain or azimuth error in the system. Recheck both the
Dolby level and the A-chain alignment of the optical system.
In many films, the surround information is intended for subtle effects and may provide only
a low-level ambience. If the surround level and delay time have been adjusted as
previously described, the surround information will be at the level desired by the film
director. Do not be tempted to increase the surround level because the effect desired by
the film production team may be impaired or destroyed.