In another post we discussed how to record on high bias tapes in normal bias shells. One issue is to have the proper playback EQ so the person listening to your tape won’t find it too dull nor too bright. Many cassette decks have a button to choose either 70 (chrome) or 120 µS (normal) EQ on playback, but this button was done away with on many later generation decks.
Tape EQ selector
High bias tapes are traditionally recorded with 70 µS EQ. If you play them back with 120 µS EQ they will have a 4.5 dB high frequency boost .
If you cannot choose the recording EQ on the machine you can adjust the audio input by applying a high-shelf EQ filter to cut 4.5 dB:
EQ compensation when recording high bias tapes with 70 microsecond chrome EQ but playback will be at 120 microsecond normal EQ
This would explain why pre-recorded CrO2 tapes are recorded at 120uS instead of 70uS. So they can be played back on any regular recorders or players that are set to play normal (Type One) tapes by default at 120uS since they don’t have Bias or EQ setting switches. That way the 4.5 dB high frequency boost if recorded at 70uS isn’t an issue. I have a bunch of very good CrO2 or similar pre-recorded tapes I got from my Mom before she died. They even say Dolby on them and are dark colored tape. They sound very nice, but I use the 120 uS Normal setting for them to play back for this reason mentioned.