Editing Pulse dialing (section)

==Pulse rate and coding==
[[File:Bakelittelefon 1947a.jpg|thumb|Swedish phone with one pulse for 0]]
The mechanical nature of the stepping switches or relays employed at the switching system generally limited the speed of operation, the pulsing rate, to ten pulses per second.

The specifications of the [[Bell System]] in the US required service personnel to adjust dials in customer stations to a precision of 9.5 to 10.5 pulses per second (PPS), but the tolerance of the switching equipment was generally between 8 and 11 PPS.<ref>AT&T Specification No. 4566, February 1926, p.113</ref> The British (GPO, later [[Post Office Telecommunications]]) standard for  [[Strowger switch]] exchanges has been ten impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%).<ref>J. Atkinson, ''Telephony'' Volume 1, p. 142 (1948, Pitman, London)</ref><ref>Current UK standard  [http://www.sinet.bt.com/sinet/SINs/pdf/351v4p6.pdf BT SIN 351] {{Webarchive|url=https://web.archive.org/web/20160804180952/http://www.sinet.bt.com/sinet/SINs/pdf/351v4p6.pdf |date=2016-08-04 }}</ref>

In most switching systems one pulse is used for the digit 1, two pulses for 2, and so on, with ten pulses for the digit 0; this makes the code [[unary numeral system|unary]], excepting the digit 0. Exceptions to this are Sweden, with one pulse for 0, two pulses for 1, and so on, and New Zealand, with ten pulses for 0, nine pulses for 1, etc. Oslo, the capital city of Norway, used the New Zealand system, but the rest of the country did not. Systems that used this encoding of the ten digits in a sequence of up to ten pulses are known as ''decadic dialing'' systems.

Some switching systems used digit registers that doubled the allowable pulse rate up to twenty pulses per second, and the inter-digital pause could be reduced as the switch selection did not have to be completed during the pause. These included access lines to the [[panel switch]] in the 1920s, crossbar systems, the later version (7A2) of the [[rotary system]], and the earlier 1970s [[stored program control]] exchanges.

In some telephones, the pulses may be heard in the receiver as clicking sounds. However, in general, such effects were undesirable and telephone designers suppressed them with off-normal switches on the dial to exclude the receiver from the circuit, or greatly attenuated them by electrical means with a [[varistor]] connected across the receiver.

[[Image:GPO 232 Telephone without bell set 26.JPG|thumb|right|British (GPO) Type 232 telephone of 1932]]