FL1

Frequency-Agile Audio Filter

Datong's first audio filter. I bought mine for the grand sum of £30 new when they were selling them off in 1986 and I still have it.

Although superseeded by Datong's FL2 and FL3, and by present day DSP devices, it is still capable of giving a good account of itself. It has an AFC facility, missing from the later FL2 design.