Note
In the mid-1970s, Donald Hildreth, W6NRW, wrote articles on IC filters and Binaural CW reception. His Sunnyvale, California company, Hildreth Engineering, later produced a number of CW-related accessories, including a model 400 binaural synthesizer filter,the Model 1100 with Tone-Tag, and the Model 10 CW filter shown here.