Analog Tape Machine Alignment (4)
Analog Tape Machine Alignment (4)
(Continued from previous article)
The value of standardizing tape formats is the ability to play a tape recorded on one machine on any other machine of the same format. When you do this, you would want that other playback machine to sound the same as the one you recorded on (i.e. have the same frequency response and reproduce level). To make sure that different machines will sound the same, they should be aligned to the same standard (things equal to the same thing are equal to each other).
If we call the alignment tape letter "B" and two different tape machines letter "A" and "C", then:
If A = B,
and C = B,
then A = C
Putting in words, if we make machine A equal to alignment tape B, and machine C also equal to tape B, then machine A will be equal to machine C.
So you see, by establishing an accepted alignment standard and using it to calibrate all your machines (level set and frequency response), you can guarantee interchange-ability of tapes from machine to machine.
The overall level alignment of a tape machine is set using a 1 kHz signal. Level adjustments in both reproduce and record modes are done with a 1 kHz tone as the reference. You will find a 1 kHz tone in three places on this tape: one near the beginning, or head of the tape, one in the frequency response tones section, and one near the end of the response tones (after the 20 kHz tone). It makes no difference which one you use.
High Frequency Alignment
It is at high frequencies where the effects of head wear and gap width become critical factors affecting machine response. A signal level alignment done at 1 kHz will not tell us enough about the response at these higher frequencies, so another frequency in addition to the 1 kHz is needed. After doing a level alignment at 1 kHz we then do a high frequency reproduce alignment using a 10 kHz signal. You do not readjust the level, but instead adjust the high frequency equalization circuit to bring just that frequency range into calibration.
Low Frequency Alignment
The low audio frequencies consume the most energy in the record process and can very substantially effect the record response by causing harmonic distortion and tape saturation. To completely align a machine's electronics therefore, you will need to do a low frequency alignment at a frequency around 100 Hz. To do this alignment you must first make sure you have the correct type of alignment tape for this procedure.
The Fringing Effect
When you record a 100 Hz signal with a full width recording head (full track recording), the signal will be across the entire tape width (edge bands notwithstanding). Now, when you "read" this tape with a full width reproduce head of exactly the same width as the recorded path, you will be able to accurately measure the magnetic energy on the tape.
If however, you are using a full track recording but reading it with a multitrack head assembly (a common situation in studios), then what happens is that the magnetic energy in the spaces between the tracks of the individual reproduce heads (the guard bands) will be picked up by the heads on either side of the space, adding to the amount being read directly in front of the head. This is called the fringing effect. In other words, the low frequency output level would be read as more than the 1 kHz level even though this is not the way it is on the tape. The unwary engineer would, upon seeing the high level, adjust the low frequency trim to reduce the level. This would have the effect of making the low frequency response of the head deficient.
Compensated Alignment Tapes
There are alignment tapes that are recorded full track, but with the low frequency tones recorded specifically for playback on multitrack heads. These tapes are called compensated alignment tapes. When using these tapes you can make the low frequency adjustment in the reproduce alignment process. Be careful, you must be using a tape that is compensated for the exact head format that you have, i.e., a 24-track compensated tape with a 24-track head assembly, or a 2-track compensated tape for a 2-track head stack. Usually, the voice announcement on the tape will tell you what compensation if any is on the tape.
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