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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.
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||The low-speed archival extraction idea is interesting.
||If you're set on an analog method, why not just upshift everything by 1000 Hz ? Then, DC encodes as a 1000Hz tone, and 20kHz as 21kHz, which is still within the available dynamic range. Frequency-shift multiplexing has been used for decades in telephony applications, and is simple and well-understood.
||Or use a conventional recorder and just run quickly towards
||But it's a good idea [+].
||// If you're set on an analog method, why not just upshift everything by 1000 Hz ? //
||Because existing tapes weren't recorded that way. I'm not trying to define a new format here (though I do
have a couple of those to post at some point), just make an innovation in playback of an existing format.
||Score is showing as 0 currently :|