DC-capable audio tape player

Analog audio tape players so far (and wire players before them) have used playback heads with electromagnetic coils for picking up the signal from the tape. That works, and is low-tech enough to have been cheap to make decades ago, but it has one problem: it picks up high frequencies at a higher amplitude than low frequencies, necessitating the use of an equalizer when recording and/or when playing back. DC (zero frequency) signals cannot be picked up at all. This is because the head works on the principle of electromagnetic induction, and electromagnetic induction works based on a change over time in the magnetic field, which DC doesn't provide, and which low frequencies provide less of than high frequencies do. The solution is to make a playback head with a modern DC-capable magnetometer sensor (e.g. Hall effect, magnetoresistance) in it rather than just a coil of wire. This should enable pickup of signals all the way down to DC with good response.

Now why do I want to be able to pick up DC sounds from a tape, given that it's pointless to send DC to a speaker? Well, I don't. But I do think it would be nice to be able to reproduce those low frequencies, even down into the infrasound range (which can be played by specialized speakers, and can be perceived by humans at high enough volume), without having to resort to disproportionately amplifying those frequencies and thereby losing dynamic range. Even with that equalization, with regular (rust) tape in a Compact Cassette, the lowest reproducible frequency is about 50 Hz, with chrome tape about 200 Hz, both well above the generally-used human ear lower frequency limit of 20 Hz, let alone the infrasound range. I presume these limits are due to the limited strength of magnetization in the tape, resulting in a limited rate of change of magnetic field in the playback head. With a DC-capable play head, the rate of change of magnetic field is irrelevant, so, even with only weak magnetization, if the head can pick it up, the head can pick it up regardless of frequency.

Could also be useful for archival or forensic transfer of audio from magnetic tape, by running the tape extremely slowly (more slowly than would produce a usable signal in an inductive head) to extract the signal at very high resolution.

N/A [2019-10-07]