e testing Automatic Gain Control (AGC) on the microphone circuit cuts the at the point at which failure occurred. gain when the user is not talking. A manual gain control on the The test is designed to ensure that: headset allows adjustment to suit line conditions and the speech level of individual callers. A thumbwheel control is usually 1. The headset does not electrically damage the public telephone preferred to a switchable control. Preferably this should provide for system. -3dB to + 12dB gain adjustment. 2. That it meets electrical safety standards. Modular design eases logistics for maintenance purposes. 3. That no damage to the user can occur through acoustic shock. After sales service is vitally important. Headsets are fragile and an efficient and speedy after-sales back-up is essential to most users. This page intentionally left blank Telex communications David M Davidson Northern Telecom Europe Contents 59.1 Introduction 59/3 Characteristics of telex 59/3 Telex signalling 59/3 59.4 Answerback 59/5 Telex protocol 59/5 Telex terminals 59/6 59.7 References 59/6 This page intentionally left blank Introduction 59/3 Introduction design of the telex network, which in turn led to a wide variety of signalling systems to communicate between telex machines and the network. Telex has its roots in telegraphy and the first practical telegraph was installed by Wheatstone and Cooke in 1857. The development of telegraphy has been marked by continuous mechanical improve- ments. Hand sending of Morse code was laborious and so an electric Telex signalling typewriter was developed to a point where it could be connected to a telephone line and operated by a typist (the teleprinter). To send the written word, a character set had to be produced and The ability to send/receive typed messages from one part of the standardised on by all users of the telex system. As the teleprinter world to the other became of enormous importance to business and grew out of telegraphy and Morse code, then SO did the telex as the machine was able