The time code information is not directly recorded on the tape, because it will cause confusion of the time code information read at different tape speeds: in actual use, the SMPTE/EBU time code is recorded by means of biphase modulation, that is the logic "1" in the time code information is the double frequency of the logic "0", and there is a transition (representing the clock signal) at the beginning of each bit period, and a jump is added in the middle of the bit period when the logic "1" is there is no transition in the middle of the logic "0".
The advantages of using two-phase modulation for time code recording are: (1) The clock signal can be separated from the time code without a separate clock signal. (2) Independent of signal polarity. (3) No DC component. For time signals at any frame rate, the actual time code signal varies between approximately 1-2 kHz, which is in the middle of the average frequency range of the recording channel. The SMPTE/EBU standard limits the rise and fall times of the pulse edges in order not to cause side effects such as crosstalk and distortion in the harmonic components of the square wave signal of the time code.
After this limitation, the audio bandwidth required for time code recording is approximately 10 dB. For multi-channel recorders, the time code information is recorded in one channel up, but for the two-channel recorder, the time code is recorded in Special time code channel on the channel. The track position and width of the audio signal and the time code signal of the NAB track format are usually recorded by a separate recording head to avoid crosstalk. The heads of the two-channel recorders of recordable time codes produced by different manufacturers are arranged in different order, so that certain error time compensation is required.
Normally, the recording level of the time code is about 10 dB lower than the standard level to avoid crosstalk problems. In addition, the time-coded recording channel cannot use a noise reduction system because it causes the time-recorded signal to produce varying degrees of distortion.
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