Quality in Data Transmission

09 Aug 2021 - Jim Evans, G3VDB

[Still a ‘Work in Progress’ at Feb 2022 ] Today’s Amateur Radio Data modes share one thing in common; an 'ideal' audio signal of relatively narrow bandwidth is translated by a Single Sideband Transmitter to a Radio Frequency signal. That RF signal should be an undistorted copy of the audio. If you have spent some time using these modes, you will be aware that in many instances this ideal is not met in practice. I have some suggestions as to 'why'.

With the application of the Personal Computer to Amateur Radio transmission and reception, data modes are now common place, in many cases superseding or surpassing the traditional modes of speech (phone) and Morse (CW). These 'data' modes include traditional RadioTeleprinting (RTTY), Phase Shift Keying (PSK), and various multi-tone modes (Olivia, ROS, FT8, FT4, WSPR). Together with older analogue slow-scan television (SSTV), these modes are generated by the same technique

• the computer generates a data signal in the audio bandwidth between 100Hz and 2700Hz using the sound card
• a Single Sideband Transmitter, usually set to Upper Sideband, translates that audio signal to the desired Radio Frequency.

For peaceful co-existence with other spectrum users, It is important that the initial audio signal is of high quality and that the RF translation process maintains that quality.

Here I examine what we mean by high quality and how we ensure that the RF signal maintains that quality.