Developments in baseband and terminal equipment also improved the quality of HF communications.
For telegraphy, the quality and throughput improved with the change over from high speed Morse to teleprinter systems. A major advance in the automation of telegraphy was the introduction of error detection and correction systems (ARQ) in 1951.
For telephony, speech compression was recognized as a method of improving transmitter loading and speech intelligibility at the receiver. A progression of these principles resulted in the development of a major advance in radio telephone speech quality. The 'LINCOMPEX' terminal equipment was system of LINked COMPression and EXansion, compressing the amplitude of speech before transmission and expanding it after reception. The compression and expansion was linked by a frequency modulated control tone transmitted over the radio link opening and closing the radio channel at the syllabic rate of the speech being transmitted. This made the channel less susceptible to noise and interference and the more constant speech amplitude enabled the transmitter to operate at a higher average power output.
It was first tried in 1965 on the London to New Delhi service and soon became established on the majority of HF radio telephone channels.