D.c. characteristics

Most telephones which are directly connected to the exchange via a pair of wires derive their power from a central battery in the exchange, which feeds current down the line normally from a -48V or -50V supply. Some earlier telephone systems operated on local batteries, next to the telephone on the customer's premises, but this method of operation is now only reserved for special applications e.g. radio telephones. The exchange battery feed circuit is usually a Stone or a Hayes bridge. The battery feed has balanced windings with respect to earth and presents a relatively low d.c. resistance, but a high impedance to voice frequencies. The d.c. resistance is usually around 200 ohms in the battery feed and the same in the ground return with an inductance of 2 Henrys.

The d.c. characteristics of a telephone are important as they determine the operating parameters for the telephone. It is normal to have a full bridge rectifier at the front end of any telephone circuit, to guard against line reversals which may damage speech and dialling circuits. When the telephone is idle (on-hook) the telephone will extract current from the line. This is normally only insulation resistance leakage and is of the order of a few micro amperes. However, some telephone network providers permit more current to be taken from the line when the telephone is on-hook, to power electronic memory devices in the telephone or LCD displays.

When the telephone is off-hook current is drawn from the network to power the telephone. The telephone network providers set limits to the amount of current that may be drawn and the telephone must operate within the parameters set. The d.c. characteristics are nor­mally specified by a relationship between current and voltage, as in figure 58.10.

The upper limits are set to ensure that the telephone will be able to seize and hold the exchange equipment. The off-hook impedance of a telephone should be equal to or greater than the 400 ohms battery feed, to obtain optimum voice signal power transfer in the 200Hz to 3400HZ voice frequency range. Many telecommunication administrations set a lower limit on the voltage that a telephone should operate on, because it is not desirable to subject the feed bridge circuit to a short circuit. However, this is not normally the practical reason since the cable from the exchange must be present. The more practical reason is the need to ensure that sufficient current is available for more than one telephone to operate in parallel.

1 Learn the words & word combinations:

Mouthpiece	Микрофон
Reliability	Надежность
Complain	Жаловаться
Sufficient	Достаточный
Impair	Ухудшать
essential	Важный
partially	Частично
speech power levels	Уровни речевой мощности (уровни чувствительности микрофона)
ambient	Окружающий
protect	Защищать
seize	Схватить (занимать временной интервал)
hookswitch	Рычажный переключатель
cradleswitch	Рычажный переключатель
abuse	Плохое обращение
withstand	Противостоять
assess	Оценить
compare	Сравнивать
loudness rating	Уровень громкости
value	Значение
relate	Относиться
perceive	Принимать
a sound absorbing box	Звуконепроницаемая коробка (звуко-поглощающий блок)
clamped jig	Зажимное приспособление
severe	Суровый
meet requirements	Соответствовать требованиям
telephone handset	Микротелефонная трубка
Ergonomic	Эргономика
Earpiece	Головной телефон, наушники
Seal	Уплотнение устройства. Уплотнение
Sidetone	1) местный эффект (в телефонии) 2) боковая составляющая (спектра сигнала, например при АМ модуляции)
Acoustic feedback	Акустическая обратная связь
Ambient noise	Шум окружающей среды
Acoustic shock	Акустический удар
Telephone circuit	Телефонная линия
Transient	Переходной процесс, переходное или преходящее явление, переходное состояние
Power surge	(большой) выброс мощности
Discharge	Разряд
To seize	Занимать временной интервал
Sensitivity	Чувствительность
Line impedance	Входное полное сопротивление (входной импеданс) линии передачи
Frequency range	Диапазон частот, полоса частот
Line current	Линейный ток
Transmission path	Тракт передачи, канал передачи
Rating	Номинальное значение
Speech path	1) речевой сигнал 2) переговорный сигнал
Termination	1) оконечная заделка 2) конец линии
Anechoic chamber	Безэховая камера
Terminating impedance	Входное (или выходное) полное сопротивление
PCM circuit (pulse-code modulation)	Схема ИКМ (импульсно-кодовая модуляция)
Hum pickup	Фоновая наводка
Main	Магистральная линия
To sample	Производить отсчёт, выборку
Sampling frequency	Частота дискретизации, частота выборки
Digital exchange	Цифровая АТС
Bidirectional	Двунаправленный
Wire circuit	Проводная линия
Wheatstone bridge	Мост для измерения сопротивления, мост Уитстона
decoupling	1) развязка связи 2) нарушение связи

2 Find Russian equivalents:

ü A fixed mouthpiece; ü A telephone of high performance and reliability of operation; ü To form an acoustic seal with the ear; ü Does not leak away; ü Sufficient vibration absorbing material;

ü Sidetone; ü High ambient noise; ü Acoustic shock; ü A sudden surge of sound pressure; ü Comparing the performance of various telephones; ü An anechoic chamber; ü Loudness rating.

3 Find English equivalents:

ü Благодаря; ü Улавливать внешний шум; ü Гарантировать; ü Акустическая обратная связь; ü Входной речевой сигнал; ü Телефонная цепь;

ü Различные методы; ü Конкретный; ü Диапазон частот; ü Величина (значение); ü Измеряются; ü Искусственный.

1....... Answer the questions:

1 The telephone handset is designed to provide the optimum ear-mouth geometry for the average human head, isn’t it?

2 How does the earpiece need to be designed and why?

3 What is the positioning of the microphone?

4 Why is sidetone a unique feature of telephone handset design?

5 What parameters have been established to assess the transmission performance of an individual telephone?

6 What are the 4 key parameters in determining telephone performance?

7 What is a loudness rating?

5 Translate in written form points 58.6.1 – 58.6.4:

PART 3 (58.7 – 58.8)

Дата добавления: 2015-03-31