Here is a chart from a Service Manual of a MiniDV HDV camera, showing the periodic inspection list. This includes cleaning and degaussing the rotary drum.
It actually understandable that a service manual would recommend this, the manual is no place for controversy. But the concept of routine degaussing has its roots deep in history when low residual ferrous materials were difficult to make, and recorder electronics were far less sophisticated.
Consider the type of field a demagnetizer creates:
an AC field with a zero average polarization. Consider what we have a tape machine that could present a polarized steady-state field, or an AC field with strong DC component, required for magnetization. In analog audio recorders, the strongest field present is the bias and erase field, again,
an AC field with zero average polarization. We have the actual audio signal,
an AC field with zero average polarization. And the field presented by recorded tape, same thing,
an AC field with zero average polarization. All of these would
demagnetize, not magnetize.
In a video deck, the carrier is an AC field, as is the erase field, again
a degaussing mechanism in and of itself. Magnetic materials used in tape heads and guides in any magnetic recorder must, by design and necessity, have a neutral magnetic state with low retentivity. This means that even if you try to deliberately magnetize one of these components, after a period of time it will revert to a neutral state. If not, they would need demagnetizing every few minutes!
The point there is that
routine degaussing is unnecessary, and because it can actually increase (temporarily) residual magnetism, even if the user is "doing it right", it is ill advised
without monitoring the indicating effects of residual magnetism to see if you have helped or hurt with your efforts. I'm not saying "don't do it ever", I'm saying "don't do it blind" because it's probably not necessary, and you could be making things worse in the short term.
The article on video tape formulas for HDV was interesting, but please don't extract from it that degaussing will improve your CNR. What should have been apparent from the article was that HDV, and DV tape is very different from audio tape in formulation and required characteristics, and less apparent but still included (in Table 1) was the fact that to improve CNR, you have to increase coercivity, which makes the tape
even less susceptible to the effects of low level residual magnetism by making a tape that requires higher flux levels to record on it.
Video and digital tape is not used to record linear, analog signals. In fact, to reduce dropout and improve CNR, it is formulated to be deliberately used in a highly non-linear region. There is simply no way, apart from a catastrophic failure, that video tape path components could have high enough residual fields to affect CNR significantly. Surface imperfections and head contamination are far more significant issues, by many orders of magnitude.
MiniDV HDV (High definition MiniDV) has dropouts and errors. Moving from Standard Definition to High Definition has made it worse where the tape had to be re-designed for better quality to improve dropouts and error rates. The higher end tapes are now made with advanced metal evaporated II process (smaller particles) and dual layer. This is to reduce errors and dropouts.
Very true, but in reducing dropout and error rates by using advanced formulas, the coercivity was increased making the tape less sensitive to magnetism.
Visual dropouts or glitches do happen on digital tape and it does ruin a shoot. Next time you watch TV and see a video glitch, it may not be a transmission hit, but a tape hit in recording.
Again, very true. But don't confuse dropouts or glitches with the effects of residual magnetism. They are not at all the same mechanisms, and only distantly related.
By these newer technologies you have a higher output with lower noise. The improved CNR (carrier to noise ratio) reduces the errors. Can demagnetizing the head after 500 hours of use help error rate? If others notice a reduced hiss of the noise floor on analog tape then I think so. If one is not using the most expensive digital master tape, then I think demagnetizing will have a bigger impact.
You seem to assume that usage time has a proportional relationship with residual magnetism. I hope I've laid that to rest in the above comments, but if not, I'll say that residual magnetism in any magnetic recorder is an anomaly caused by defects in materials, design flaws, or electronic failures (the latter being the most probable). It's not a result of use, wear, or time.
You really cannot compare the improvement on noise in analog tape by demagnetizing with reduced errors in a digital video system. The mechanisms are very different, as are the energy levels involved. If you did introduce a very strong DC magnetic field into a digital video system to the extent that it raised the noise floor and thus the error rate, you would be producing a lot of very short term errors that would most likely at some level exceed the system's ability to error correct or error conceal, and the signal would completely collapse. Tape error correction is based primarily on the statistical probability of errors in size and frequency that tape produces. A decrease in CNR would be unexpected by this system, and over-run it's capabilities fairly quickly. Fortunately, with the extremely high coercivity levels of digital video tape (that is the point of it), it would be impossible without the use of a strong permanent magnet, to introduce enough DC field to raise the noise floor.
Noise caused by a DC field in analog tape is an entirely different mechanism, because the recording method is entirely different. Analog tape depends on a more or less linear relationship between the recorded magnetic pattern and the analog signal. Because tape is highly non-linear, high levels of AC bias signals are used to push the low audio signals through the non-linear range where the tape doesn't want to accept a new magnetic state. An extraneous DC field increases noise by introducing gross asymmetry to the composite record signal. The noise that results is largely the analog of variations in the tape's specific coercivity, or non-linearity levels, reproduced as a fluctuating DC component. It's character is of predominantly low frequency noise, with elevated hiss only in extreme cases. (Hiss in recording does come from other sources).
Video recording doesn't concern itself with overcoming the non-linearity of the tape, but rather takes advantage of it to extend CNR. The video signal (or data) is recorded with an RF carrier at a high enough level to "force" the tape to accept a new magnetic state. It doesn't matter if this signal is linear or not, because to recover it all we need is its frequency. The analog tape noise mechanism simply doesn't exist in digital or video systems.
Hey, all I was trying to do was shed some light on the analog tape routine demagnetizing myth. I didn't mean this to be a term paper.
If you get nothing else from all of this, please get this: for analog recorders, if you insist on demagnetizing, please check for the results with proper instrumentation. If you don't, you can just as easily make it worse. With instrumentation, you'll probably find it's not required. In rare cases, you might find you have a serious problem to fix. For video recorders, frankly, I'd keep that degausser out of the machine. One slip, and you could introduce a problem you that you'll have no ability to analyze (I don't have a bit-error rate counting device in my tool kit, do you?).
Cheers,
Jim
