I just found some music on a re-recorded second generation old audio cassette tape.
The music is distorted on some parts of every track and has low fidelity. I want to improve the quality overall to make it sound the best highest quality possible, by removing as much distortion as possible if not all, and fixing the low fidelity.
Can anyone give me some tips and advice how to go about this? Like good programs and settings etc..?
Nothing posted by Schizomorph is wrong - if it's busted, it's busted...
However, one small ray of light is azimuth/stereo balance correction in software. Back in the old days I had a bit of freeware (and the source code) by Airwindows - one of the most amazing, if just a little esoteric, sound designers on the planet - that could do this task [non-realtime], but it's long gone & the best current 'switch it on & it works' software I could suggest would be Izotope RX [various prices from expensive to zomg wtf] but it can do the job.
If you can wade through the plethora of plugins, some free, some commercial, at Airwindows to see what Chris has been up to for the past 25 years... you may be pleasantly surprised. Phase Nudge sounds interesting, but I didn't see a full azimuth correction plugin yet... there's a lot to go through...
Caveat - the guy is a genius... & perhaps not all that simple to comprehend. Watch some of his videos for an idea of what he & his plugins do.
Note, he doesn't make pretty interfaces, he just labels the sliders.
I'm afraid you can't re-construct information (in the waveform) that has been distorted. The one side you can improve is playback quality by using a decent tape player with a good head.
Tape distortion flattens the peaks, introducing odd harmonics. If there was only one frequency, you could filter those out but since it's a complex waveform we're talking about (your music), those harmonics change frequencies continuously depending largely on the base frequency. In addition, there is no way of knowing how the peak of a waveform fluctuated before it was flattened.
Just to add some encyclopedic knowledge, sound is recorded on tape by aligning the magnetic particles trapped inside it. The absolute maximum amplitude that can be recorded is when all those particles are facing the same way. That's why when you try to record a signal that is too hot, there isn't enough magnetic particles in the tape to make up the required magnetic field and the result is clipping at some maximum value.
Hopefully some of the artifacts you are hearing are the result of an old playback head and not a damaged or distorted tape.
If you can do do so, as opposed to the suggestion made in the answer by Schizomorph, it might make sense to try employing the same recorder you made the recording with for playback even if you have a somewhat better quality recorder available.
The reason is that you lose high frequencies when the tape head is not identically aligned to the way it was when recording. Of course you also lose high frequencies with a bad recorder but that happens more when recording than when doing playback, so the original recorder needs to be significantly lower in quality to be worse suited for playback than a different one, and then the probability of bad head alignment goes up as well, giving the actually worse original recorder a bigger thumbs-up. Whether it's big enough is worth testing out.
