I recently started transferring to pc some recordings I made 45 years ago on a simple portable cassette tape recorder. Playing piano, it seems when I pounded a little too loud on the keys, the recorded volume goes completely to almost zero but comes back up to the proper recording level in a second or so. I don't know technically what to call this...drop out? Anyway, it mars a lot of places in some of my recordings. Is there a way to fix this digitally? Thank you...hope what I've said makes sense.
Sounds like some sort of AVC (Automatic Volume Control). Otherwise known as a rudimentary compressor on the input to prevent recording overload. It kicked in on the attack transient of the piano note and didn’t recover quickly enough not to kill the beginning of the sustain. It’s called ‘pumping’.
Piano is a very dynamic sound - big initial transient followed by a much lower sustain level. Very easy to overload a recording input with piano. Or to get unfortunate effects from applying compression.
This may be difficult to fix effectively i.e. not to make it sound worse than the original! You could try using a compressor with 'expander' settings.
Like Laurence mentioned, it sounds like recorder adaptively changed the gain of the microphone as you were recording to prevent it from clipping.
You might have luck repairing the audio with a program called iZotope RX. I believe that it has a preset designed to fix sudden drops in gain.
Since simple recorders like this didn't generally have a recording level adjustment, they usually included Automatic Gain Control, which is a type of dynamic range compression, to prevent overloading the tape when recording loud sounds.
As long as the recorded audio level doesn't drop down below the noise floor on these passages, it should be possible to recover them (albeit with a good amount of noise).
You may find a software solution* (essentially a dynamic range expander with some automation) to fix these spots automatically.
If there are only a few occurrences, it may be easier to edit them in a DAW and manually apply gain envelopes to boost these passages (with a gradual fade down to correspond to the decay time of the AGC).
*Specific software recommendations are off-topic for this site.