Biasing in Tape Recording
磁带录音中的偏置
A music signal alone cannot be used to produce a faithful tape recording of a sound because the magnetization of the tape is so sensitive to its previous magnetic history, even the effects of the signal recorded just ahead of it. A high frequency bias signal is typically applied to the tape through the tape head along with the music signal to remove the effects of this magnetic history. This large bias signal (typically 40 to 150 kHz in frequency) keeps "stirring" the magnetization so that each signal to be recorded encounters the same magnetic starting conditions. The necessity for biasing has its origin in the magnetic property called hysteresis - the magnetic material tends to hold onto any magnetization it receives and must be actively driven back to zero to start over. Magnetic emulsions made with chromium dioxide require a larger biasing signal to make use of their wider dynamic range, so modern recorders have different bias settings for iron oxide, chromium dioxide, and metal tapes. With optimum biasing, the recorded magnetic image is proportional to the signal current applied to the record head.
单独的音乐信号无法用来产生忠实的磁带录音，因为磁带的磁化对其之前的磁历史非常敏感，甚至其前面刚刚记录的信号效果也会产生影响。通常，通过磁带头将高频偏置信号与音乐信号一起施加到磁带上，以消除这种磁历史的影响。这个较大的偏置信号（通常频率为40到150 kHz）会“搅动”磁化，使得每个要记录的信号都遇到相同的磁起始条件。偏置的必要性源于一种称为磁滞的磁性特性——磁性材料倾向于保留它所接收的任何磁化，必须主动将其驱动回零以重新开始。使用氧化铬制成的磁性乳液需要更大的偏置信号来利用其更宽的动态范围，因此现代录音机为铁氧化物、氧化铬和金属磁带设有不同的偏置设置。在最佳偏置条件下，记录的磁性图像与施加到录音头的信号电流成正比。
One could become almost philosophical about the process by which one can produce the sound of a beautiful symphony from a "ribbon of rust". Biasing is critical to this process of imposing the design of the symphony upon a medium which would tend to have random variations because of thermal energy and a kind of "inertia" in the form of hysteresis that resists the production of an undistorted image of the music. Biasing plays the role of "scrambling" or "stirring" the medium dynamically so that each element of the musical image you intend to impose will have the same "blank slate" upon which to write. The scrambling process of biasing also increases the sensitivity for the recording of a clean signal, i.e., you can record with a much smaller input signal because the medium is more susceptible to the imposition of the pattern you intend.
人们可以几乎哲学地思考将一首优美的交响乐之声从‘一根锈带’中产生出来的过程。偏置对于这一过程至关重要，因为这一过程是将交响乐的设计施加于一种倾向于因热能而产生随机变化的介质上，而这种介质还有一种‘惯性’形式，即滞回，它会抵抗产生一个未失真的音乐图像。偏置的作用类似于‘搅乱’或‘搅拌’介质，使其动态地变化，使得你想要施加的音乐图像的每一个元素都有一个相同的‘白板’来书写。偏置的搅乱过程也提高了记录清晰信号的灵敏度，即你可以用一个远小的输入信号来记录，因为介质对你想要施加的模式更加敏感。
|
