Tangent as you mentioned boosted high frequencies on tapes...
It's possible (likely) that those prerecorded cassettes had boosted high frequencies because they were intended to be played on a deck that supports Dolby B noise reduction, and will do the reverse operation to get the level back to where it's supposed to be.
Dolby B noise reduction didn't actually reduce noise at the source. Instead:
- During recording: Boost the volume of high frequencies (where tape hiss is most audible)
- During playback: Apply the inverse.
When you reduce the treble during playback, you're reducing the hiss along with it, but the original signal (which was boosted before) ends up at the intended level. This improves the signal-to-noise ratio in the high frequencies.
This is similar to the RIAA equalization curve used for vinyl records.
- During mastering: Reduce bass, boost treble.
- During playback:The RIAA phono preamp applies the inverse curve—boosting bass and reducing treble.
IIRC the reasons for the RIAA curve aren't just about improving signal-to-noise ratio, but something about the physical limits of vinyl.
It's possible (likely) that those prerecorded cassettes had boosted high frequencies because they were intended to be played on a deck that supports Dolby B noise reduction, and will do the reverse operation to get the level back to where it's supposed to be.
Dolby B noise reduction didn't actually reduce noise at the source. Instead:
- During recording: Boost the volume of high frequencies (where tape hiss is most audible)
- During playback: Apply the inverse.
When you reduce the treble during playback, you're reducing the hiss along with it, but the original signal (which was boosted before) ends up at the intended level. This improves the signal-to-noise ratio in the high frequencies.
This is similar to the RIAA equalization curve used for vinyl records.
- During mastering: Reduce bass, boost treble.
- During playback:The RIAA phono preamp applies the inverse curve—boosting bass and reducing treble.
IIRC the reasons for the RIAA curve aren't just about improving signal-to-noise ratio, but something about the physical limits of vinyl.