Circuit developed by NASA engineers to improve intelligibility of voice
communication during Apollo moon shots passes only the three portions of
the speech spectrum required for clear speech:300-400 Hz and 2500-3000
Hz for both sexes and 900-1700Hz for males or 1100-1900 Hz for females.
Pot ad lusts null to about 600 Hz. Circuit improves readabilhy of weak
DX voice signals in noise.
-J. Fisk, Circuits and Techniques, Ham Radio,
June 1976, p 48-52
