Link: Ask the Engineer: Fix Your Phone Tape

“To eliminate this unnecessary “junk,” you will use two of my favorite tools: a high-pass and a low-pass filter. The high-pass filter will remove all of the junk below 300Hz (it allows the highs to pass) and the low-pass filter will remove all of the distortion and hiss above 3400Hz (it allows the lows to pass).”
Original source: Ask the Engineer: Fix Your Phone Tape