| 释义 |
preseˈlector
[pre- A. 2.]
a. Teleph. A switch which when a subscriber lifts his receiver automatically connects the calling line to an idle trunk by a hunting action, independently of impulses produced by dialling; formerly also = line finder s.v. line n.2 32.
1912J. Poole Pract. Telephone Handbk. (ed. 5) xxxii. 535 The line-switch used by Siemens is a specially neat arrangement... It is called a ‘pre-selector’ by Messrs Siemens, and each switch is complete in itself. Ibid. 536 Secondary line-switches or pre-selectors are used in both systems to facilitate and economise the connections. 1921W. Aitken Autom. Telephone Syst. I. 3 A preselector is a switch that automatically selects an idle line of a group when the receiver is lifted. 1924[see hunt v. 9]. 1950J. Atkinson Herbert & Procter's Telephony (new ed.) II. xviii. 572/2 The 1st preselectors are arranged in groups, so that each group carries..an equal volume of traffic. The preselectors of one group are trunked via 2nd preselectors to a maximum of 100 1st selectors. 1976T. H. Flowers Introd. Exchange Syst. iv. 89 The choice between preselection and line-finding is mostly a question of economics. The first needs one exchange switch, or exchange line, and the second one switch per cord circuit. The quantity of switches needed as line finders is thus much less than the quantity as pre-selectors, but whereas line finders must be full-sized exchange switches to achieve satisfactory traffic loading, pre-selector switches may have as few as ten contacts in the banks.
b. Telecommunication. A tuned circuit preceding the first mixer in a superheterodyne receiver; an analogous filter in a microwave receiver.
1930Electronics Sept. 279/1 (heading) An improved preselector circuit for radio receivers. Ibid. 308/1 The essential features of this preselector are shown in Fig. 6. 1951A. Sheingold Fund. Radio Communications xv. 307 The preselector, when present, helps to maintain a favorable signal-to-noise ratio and minimizes interference effects. 1971M. G. Scroggie Found. Wireless & Electronics xxii. 396 Because the i.f. amplifier is relied upon for most of the selectivity, the preselector tuning circuits do not have to be very sharp, so slight errors in gauging are not serious. 1975D. G. Fink Electronics Engineers' Handbk. xxv. 72 Narrow-band filters in the receive path, often called preselectors, are built using mechanically tuned cavity resonators or electrically tuned YIG resonators. Preselectors can provide up to 80 dB suppression of signals from other radar transmitters in the same rf band but at a different operating frequency.
c. A gearbox that enables a driver to select the next gear at any time before the change is actually made (by means of a separate pedal). Usu. attrib.
1930Engineering 17 Oct. 498/2 There is one cam for each gear, all mounted on a common shaft coupled to the pre-selector lever. 1935Economist 7 Dec. 1144/1 It is a natural step from the power unit to the transmission, where most important developments have centred round such features as the fluid flywheel and the pre-selector gear. 1969Driving (Ministry of Transport) xvi. 198 ‘Pre-selector’ transmissions, mostly found on buses and coaches, have a lever by which the driver can select gears in advance, ready for later changes... No gear change takes place until a gear-change pedals is pressed and released. 1979J. Leasor Love & Land Beyond i. 13 An electrical gear change which could be used as an ordinary box or as a preselector. |