Print-through

Print-through (sometimes referred to as bleed-through) is a generally undesirable effect that arises in the use of magnetic tape for storing information, in particular music.

Print-through is a category of noise caused by contact transfer of signal patterns from one layer of tape to another. Print-through can take two forms: 1) thermo-remanent magnetization induced by temperature, and 2) anhysteretic magnetization caused by an external magnetic field.

The former is unstable over time and can be easily erased by rewinding a tape and letting it sit so that the patterns formed by the contact of upper and lower layers begin to erase each other and form new patterns with the repositioning of upper/lower layers after rewinding. This type of contact printing begins immediately after a recording and increases over time at a rate dependent on the temperature of the storage conditions.

The audibility of print noise caused by contact printing depends on a number of factors: 1) the amount of print due to conditions of time and storage; 2) the thickness of the base film that acts as magnetic barrier (thin C-90 cassette tapes are more susceptible than studio mastering tapes that use a base film four times thicker); 3) the stability of the magnetic particle used in the tape coating; 4) the speed of the tape (the wavelengths of the prints shift so that higher speeds move printed signal closer to the range where the ear is more sensitive); the dynamics of the musical program (very quiet passages adjacent to sudden loud signals can expose the print signal transferred from the loud signal); and the wind of the tape (A-winds for cassettes with the magnetic layer facing outward have stronger print signals after a loud signal--"post-print"--than B-winds used in modern open-reel recorders that have stronger "pre-print" signals preceding a loud passage. echo.

Tape speed is a factor because of the shift in wavelengths. For example, the strongest print signal on a C-60 cassette running at 1.875 ips is about 426 Hz (605 Hz for a C-90), while an open-reel tape recorded at 7.5 ips would have its strongest signal at 630 Hz if the tape were a professional tape with a 1.5 mil base film or 852 Hz if the tape were a consumer version with a base film of 1.0 mil thickness.

The cause of print-through is due to an imbalance of magnetic and thermal energy in the magnetic particle. Once the magnetic energy is only 25 times greater than the thermal energy, the particle becomes unstable enough to be influenced by flux energy from the layer above or below the tape. The amount of magnetic energy depends on the coercivity of the particles, their shapes (long, thin particles make stronger "magnets"), the ratio of ideally shaped particles to defective particles, and their crystalline structures. Metal particles, although very small, have very high values of coercivity and are the most resistant to print-through effects because their magnetic energy is seldom challenged by thermal energy. Particles fractured by excessive milling prior to coating will increase levels of print depending on their ratio compared to their well-formed neighboring particles.

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Wikipedia