Hugh Robjohns wrote:
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!
I'm not sure either way on tape self erasure. I understood that higher energy tapes were harder to erase just as they're harder to magnetise. So a Type II or Type IV tape would be less subject to accidental erasure than a Type I.
I also thought that multiple plays
in a machine tended to gradually erase tapes hence the qualifier with calibration tapes to check them after a certain number of plays, not necessarily after a certain time. Also the need to properly demagnetise a tape machine to minimise the tendency to gradually erase tapes after multiple plays.
It's wavelength related too, only indirectly to frequency. So a loss of 3 db at 10kHz on a cassette would be equivalent on a pro reel to reel tape to a loss of 3 db at 80kHz which of course no one would notice.
I agree any serious test needs to be done with careful measurements. The thing with tape is that there are multiple causes for loss of the highs, not just self erasure. So , a tiny speck of dirt on a repro head, a worn or misaligned head, in various planes, can be enough to compromise the highs. It's often not appreciated that just playing a tape back at its optimum is not always easy.
Here's an example of a cassette recording (Maxell UDR Type I) I made off the TV in 1976. It's been played many times over the years but it seems to have survived reasonably well. Of course this cassette recording off TV cant hope to compete with the original BBC videotape's audio quality. The vision isn't my recording, only the audio. https://youtu.be/KYFVx4gpe9c