Balanced audio cables were invented many years ago for professional applications such as radio, television, film and recording studios. No consumer products had them. They were for the pros.
And the pros had noisy environments of many cables, inputs, devices, transmitters, and so forth to deal with. Their inputs were almost always transformer coupled. Meaning, an audio transformer was at the input and, for the most part, the output of their devices. Using transformers kept ground loops at a minimum. Transformers also reduced noises from all the interactions of the equipment leaking into the connecting cables. Imagine a long run of audio cable between a microphone and a control room console. It could easily be upwards of fifty feet and along its path run close to power transformers, electrical outlets, perhaps even a radio transmitter if at a radio station. Lots of noise, lots of cause for concern. But the transformers isolating the equipment reduced much of this noise.
How did these transformers help with noise? As they were full range audio transformers they didn't act as filters, reducing certain frequencies. No, instead they used one of the features of all transformers to reduce the noise. In fact, they rejected noise and accepted only musical signals. How did they do that? How did they separate noise from signal? What form of wizardry could account for this? The very same technology we've been discussing as of late; balanced audio.
Tomorrow let's start to learn about something called common mode rejection.
