TELEPHONING ACROSS COOK STRAIT

Wanganui Chronicle, Volume 80, Issue 266, 9 November 1937, Page 9

TELEPHONING ACROSS COOK STRAIT

I * ' NEW APPARATUS AT WORK ’ COMPLICATED ELECTRICAL 1 OPERATION The cable recently laid across Cook • Strait from Lyal Bay, Wellington, to Blind River, -Marlborough, is now in ‘ use, and by supplementing the chan- ‘ inels provided in the old cables is giv- ■ ing seven direct circuits to ChristI church, and three to the Blenheim and ■ Nelson districts. Cross-strait telephone communication is an important fea- | ture of the Post Oilice telephone bus- ' | iness, the average number of conver- • I sations each month being 17,500. I Installing and adjusting the com- > plicated terminal equipment is almost L as heavy an undertaking as the laying . of the cable itself. Electrical currents of frequencies above the audible range :iare utilised for transmitting speech I over the cable, the range of frequen- ■ cies being from six to 60 kilocycles per '.second. To transmit speech over the » ( new cable involves a complicated elec- ’ trical operation, which, however, is | done automatically by the British- : | made equipment now being installed > i on both sides of Cook Strait. A new > repeater station has been built at I Lyall Bay, where the normal speech , currents are converted into high fre- , quency impulses for transmission to ? the South Island, while speech from ; the south has to be demodulated or t . converted into the range of audible ; | frequencies used on subscribers’ telephone lines. I If it were possible to listen in to the . traffic on the submarine cable, all that > could be heard would be a pulsating II series of high-pitched musical sounds, .Ito which the nearest parallel in nat- , Jure is the twittering of small birds. ■|On the longest waves used in this traffic these sounds could be heard | by many people, though speech would Jbe unintelligible, but as the uppei . limit of the frequency range is approached, there is no sound audible to the human ear. In this form speech between Wellington and Christchurch [ passes over cable and land lines, to be I converted at the terminal points into ’ | audible frequencies for transmission ' J to subscribers. Another important feature of longdistance telephoning is that attenuation of the original volume of speech has to be overcome by amplification, and this is done four times between the Wellington and Christchurch exchanges. Lyall Bay repeater station, replacing the modest wooden cable hut which did duty for many years, is a well designed concrete building 66ft. long, with a spacious room to accommodate the delicate electrical apparatus for the high frequency telephone channels. This apparatus is mounted on panels 10 feet high, and there is space for considerable extension as more and more of this equipment comes into use to meet the growing I needs of long-distance telephone bus- ■ iness. Power is taken from the ordin- | ary source, but to ensure smooth working and freedom from voltage fluctuations, the station operates from 'battery current, the batteries having [sufficient storage capacity to run the [system for several days if there is a breakdown in the ordinary supply oi ! current. ‘ Commercial operation of the new I cable was preceded by a long series of electrical measurements, an essential preliminary to accurate adjustment of the terminal equipment. An interesting electrical feature of the new cable is that the transmission i losses over its length of 40 nautical [miles are equivalent to those exjperienced over 200 miles of the [standard type of open aerial line. The ! temperature is another factor which j has to be taken into account by the- ' engineers, who have found that the [average temperature of the bottom lof Cook Strait is 50 degrees Fahrenjheit, and that it is almost uniform throughout lhe year. •mean temperature over the year ia 54.8 degrees.