In other words, they can only be used for tape recording and playback rather than for a "loop" containing an equalizer (for tone control) or other circuit such as a surround sound processor.
The decoder circuit is also easy to construct. You will have to physically wire (using alligator clips for instance) the TONE out pinout from the generator to the TONE in pinout of the decoder.
The principle of the tone detection circuit is also studied and a method of designing the filter is given. At last, the coefficient corresponding to the filter is calculated.