I recently finished a cassette tape restoration project of a previously unheard Rolling Stones gig from 1971. The cassette shell was damaged causing resistance so the tape wouldn't run free enough to play in a cassette player. I had to transfer the tape to a new shell where it played perfectly apart from the original damage to the cassette which was mainly at the start of the tape.
The original transfer was quite dull sounding and low in level which isn't good news when it comes to a cassette recording as the music I wanted to preserve was down in the noise floor. I decided first to lift the tonal balance of the recording by applying a shelving EQ to raise all of the upper frequencies equally. The first thing that was noticeable was how much hiss there was in the recording it was as loud as the music! I decided that the best approach would be to remove just enough hiss that it was bearable this way the original recording would be left as intact as possible.
With the hiss removed to an acceptable level I then set about removing any clicks and pops that would distract from the listening of the music. I even managed to remove some of the tape machine whine that had made it's way into the recording. Restoration work was mainly focussed on the quiet sections between tracks as this is where any unwanted distractions were more apparent due to the band not playing.
Below is a demonstration of a crunchy click being removed:
Below is a comparison of the original cassette transfer, the restored recording and the cassette recording just before the hiss was removed: