How do the sound pressure values correspond to the magnetic domains on the tape? Is the format analog or digital?
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\$\begingroup\$ Sound pressure is nothing to do with magnetic domains on a tape or any other magnetizable material. \$\endgroup\$– Andy akaCommented Oct 15, 2014 at 20:51
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\$\begingroup\$ Analog. I'm not sure whether the magnetic domains are amplitude modulated or frequency modulated. Typical casette is 4 analog tracks, side A left, side A right, side B left, side B right. Some low cost demo tape mixers use all 4 tracks together for low-budget music master demo tape production. \$\endgroup\$– MarkUCommented Oct 15, 2014 at 20:54
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9\$\begingroup\$ @Andyaka: what do you think sound waves are other than pressure in the air? In fact, there is an exact physical correspondence between air pressure changes and magnetic changes on digital or analog recording tape. \$\endgroup\$– dodgethesteamrollerCommented Oct 15, 2014 at 23:15
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1\$\begingroup\$ @Andyaka That is the very definition of Analogue: "An analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves." \$\endgroup\$– Ajedi32Commented Oct 16, 2014 at 13:29
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1\$\begingroup\$ @Andyaka Now I'm confused. The definition of Analog is the justification for link between magnetic domains and sound pressure: tapes are analog, therefore, when used to store audio, their magnetic domains are analogous (related) to sound pressure. What exactly are you confused about? \$\endgroup\$– Ajedi32Commented Oct 16, 2014 at 18:14
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2 Answers
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It's analogue. A high-frequency 'bias' carrier has the incoming audio signal superimposed onto it, and the result is sent to the recording head, which is just an electromagnet.
The purpose of the bias is to drive the magnetic material around and around its hysteresis curve, so as to avoid the non-linear part of the curve close to the zero-crossing.
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2\$\begingroup\$ Hit the nail on the head, +1 \$\endgroup\$ Commented Oct 16, 2014 at 6:34
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Standard audio cassette tapes are analog. There are some digital formats, though, including DAT tapes.
Standard tape heads are much too large to address individual domains, so the signal is basically encoded as the average domain orientation in all the domains under the head at a given moment. I believe the fact that the domains are discrete is the source of the well-known tape hiss that characterizes analog magnetic recordings.
I believe the way recording works is the domains are first scrambled by a high frequency signal on the erase head, then written with the audio signal directly on the main record/playback head. Many recorders will add a large, high frequency signal to the audio signal during recording to help reduce distortion in the much smaller audio signal. Playback involves simply amplifying the signal from the read head.
answered Oct 15, 2014 at 20:53
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\$\begingroup\$ As I understand it, the high frequency signal is there to provide a kind of bias, to that the audio signal can "ride" over the non linear region, of the magnetic hysteresis curve. The high frequency is better than a DC bias, because it prevents the tape heads from becoming magnetized in one polarity or another. Yet cheaper portable tape recorders before casettes would often just use a DC bias, and for the earse head just use a permanent magnet! :-) \$\endgroup\$– RandyCommented Oct 16, 2014 at 2:34
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\$\begingroup\$ There is only one domain, the time domain. There is no 'scrambling' of 'domains' but there is a biasing technique to get the signal into the linear region of the tape transfer function. This is applied to the record head, not the erase head. Too much obfuscation here, of a basically simple process. \$\endgroup\$ Commented Oct 16, 2014 at 9:40
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2\$\begingroup\$ By domain I am referring to individual, countable magnetic domains, not the time or frequency domain. \$\endgroup\$ Commented Oct 16, 2014 at 22:19