Digital player for "analog" cassettes

Even though audio tape is referred to as an "analog" medium, the individual magnetic domains are almost all in one of two states, just as most of the points in a newspaper photograph will either be black or white, and for basically the same reason: while it is in theory possible to produce grays by printing a 'medium' amount of ink on an area, it's very difficult to produce a consistent gray that way; printing a black and white pattern of dots costs some resolution, but makes it much easier to maintain consistency. Similar issues exist when recording signals on a magnetic tape.

Normal analog playback devices work by measuring the average polarization of the magnetic domains. The behavior is roughly analagous to copying a newspaper photograph by making a somewhat 'blurry' scan of it. This approach will yield decent, albeit not stellar, results if the photograph is in perfect condition. If the paper is distorted or discolored anywhere, however, or if the ink has faded in some areas, such defects will show up in the copied image.<p>

What I would suggest doing with audio tapes would be more akin to taking a high-resolution scan of the newspaper photograph and using the known regularity of the halftone pattern as a spacial and density reference. Any defects caused by deformation, discoloration, or fading could be corrected provided such distortions were not so severe as to prevent the scanner from seeing where the dots were. Even when the distortion was so severe as to prevent reading the dots, this would often be detectable, and this could be used to generate a mask for areas to be "reconstructed".<p>

What I would propose doing would be to have a player for analog tapes operate on this same principle. Measure the domains indidually and use such measurements to adjust playback speed and volume in real-time.

To begin playback, the player measures the bias frequency and amplitude when running the tape at 1.875"/sec; it also starts putting measured signals into a buffer. Once the player has enough data captured (probably about a second), it could then start playing the buffer, normalizing it for a fixed bias frequency and amplitude. If, in part of the tape, the bias frequency were to drop 1% and then increase back to normal, the player would output the samples thus recorded slightly faster than it had read them off the tape (since the reason for the change would most likely have been that the tape stretched slightly there). Likewise, if there was a spot where the bias signal dropped in amplitude 10%, the player would boost the signal on playback so as to keep a uniform bias signal (since that would most likely indicate a bad spot on the tape).

Using this approach, it should be possible to get high quality playback even of tapes which have been damaged or, in some cases, which were recorded on marginal equipment or tapes. Indeed, it may be possible to make 20-year-old tape sound better than it did when it was new.