ef37a wrote:I really don't see why any tape format should lose HF over time unless exposed to a magnetic field?
It's entropy innit? The signal recorded onto the tape via the orientation of magnetic particles degrades as those particles gradually attempt to revert back to their non-oriented state.
This happens on all tape formats, but is generally worse for thinner, slower tapes, and higher magnetic flux levels. So use standard-play tapes rather than long play, higher tape speeds where possible (open-reel, not cassette, obviously), and don't thrash the record meters to force heavy saturation!
Although gradual self-erasure affects all frequencies, it is the higher frequencies that are affected most quickly and most obviously, although keeping the tape cool helps to minimise the problem for long-term archives. The magnetic field embedded within adjacent layers of tape can also advance the self-erasure process (as well as leading to pre/post echos in some cases).
Moreover, the self-erasure issue with cassettes tends to appear worse than it really is because of the almost ubiquitous use of Dolby B, such that a slight loss of HF from the tape is further reduced by the mistracking action of the Dolby B decoder.
I also reiterate my point? Unless the tape had reference tones recorded on it (as studio Dolby A tapes always should) how could you know the HF was down on 5 years ago?
The obvious answer is to compare the (cassette) tape with the source. I have often done exactly that in my previous career, and can state that self-erasure really isn't a myth... although it might not be quite the huge problem some would claim!