ANALOG TAPE 101
Part Three, Bias Magic
Page 1, 2, 3

 

If you are reasonably sure that the machine was never misaligned, then the “factory” bias setting should be better than attempting to readjust bias and potentially make things worse. If you’re not happy with the sound, the “Good Vibrations” section will be revealing. But first these cautions…

I do not recommend using high-output tape on narrow-format machines. Mechanically, they are heavier, less pliable and more likely to accelerate headwear. In their heyday, Tascam’s MS-16 and Model 38 would become sluggish over time using the recommended tape (Ampex 456, 3M 226). Having the reel motors rebuilt will improve performances and extend the life of the motor drive amplifiers. Electronically, some narrow-format machines are not capable of full erasure. Also, as some machines absolutely must be run with the built-in noise reduction, there really is no need or no point to pushing the levels.

Good Vibrations:
The Magician’s Secret

I saved this technique for last because it’s more complicated. If you are uncertain about how much over-bias is required, or just want to satisfy your own curiosity, record a 40Hz sine wave at 30 ips. Listen to what happens as the bias is varied. (Hint: If the fundamental is filtered with an equalizer, you’ll hear only the distortion artifacts; the changes will be more revealing.) Adjust to minimize the fuzz and low-frequency harmonic distortion. (Congratulations! You’ve just used your ears as a distortion analyzer.) Switch to 15 ips and then 7.5 ips, making the adjustment each time and notice that the “window of relative cleanliness” becomes wider as the speed is cut in half. Also, try this on a digital machine and notice how there is no obvious distortion.
At 30 ips, the narrow window is perfect for getting the most precise adjustment, but it is important to perform this “test” over several channels. Now, switch the oscillator to 20 kHz, note the level on the VU meter (A), then reduce the bias until the signal level peaks, again noting the new output level (B). Average the results. The difference in dB from (A) to (B) is the optimized over-bias. Use this amount to align all channels.

Phew, now we can get on with the Record calibration.

Record Level: 1 kHz
As mentioned, Input and Record level calibration are inconsistently implemented on professional and semipro machines alike. On some pro machines, the two might interact. Older Tascam machines have separate meter calibrations for Input and Repro, as well as level adjustments. Read the manual. Make the adjustment at 1 kHz so that Input and Repro both read “0 VU” (PAR, if applicable).

Record EQ: 10 kHz
For this adjustment, it is important to confirm that the oscillator is flat, so check input after switching to 10 kHz, then go into Record and adjust the Record EQ so that 10 kHz also reads “0 VU.” Sweep up to 15 kHz and check the level—it should be within ±1 dB—and again at 20 kHz, where ±2 dB is acceptable. Of course, flat is the goal. The head condition is most suspect if the level drops as the frequency goes up. Channel-to-channel inconsistencies point to the electronics. Swap cards with the power off. Test. Then swap channels back to confirm. (Sometimes reseating is a temporary fix.)

Low-Frequency Playback EQ
Accurate low-frequency playback adjustments can only be made after recording a frequency sweep, starting as high as 500 Hz. A slow sweep, from 200 Hz down to 20 Hz, will show the most critical range to be within the last two octaves (below 80 Hz). Along the way, note the value and frequency of the peaks and dips. Ultimately, the last major peak and dip should be adjusted so they appear symmetrically above and below “0 VU.” Then, find the lowest frequency that falls on “0 VU,” note and record a section of it after the bass sweep.

Master Tones
It is customary to create a tone reel for a project, especially for a Mix Master, so that mastering engineers can adjust their decks to match the original record machine. In addition to 1 kHz, 10 kHz and 15 kHz, the bass sweep and final bass tone are extremely helpful.

Narrow Escape
Narrow-format machines offer little in the way of Record EQ adjustment, and there is no low-frequency playback compensation for worn heads. One trick I’ve used is to note the level of the nastiest low-frequency head bump, then set the 10kHz Playback EQ (from the test tape) to that level. It is cheating, but only enough to minimize mistracking of the noise reduction system—dbx noise reduction multiplies low-frequency errors by 2, so a 1.5dB bump becomes a 3dB bump.
Back in the day when I aligned many a Fostex E-16, the extra work of adjusting the bias by the PAR method paid off because channel-to-channel frequency response and phase were more accurate. The difficulty with most Fostex machines is that the bias adjustment is a continuously variable cap. Without a ‘scope or a meter, it is easy to misalign one of these machines. By being meticulous about the bias adjustment on all machines, you may discover track-to-track anomalies that could either be caused by aging components or a head near the end of its life. The last narrow-format tip concerns tape thickness. Many of these machines perform better with 1-mil tape rather than 1.5-mil tape. Unfortunately, it’s not so easy to find Ampex/Quantegy 457 or 3M 227 tapes these days; however, Quantegy 407 is still manufactured.

Tail Out
The warmth for which analog tape is famous comes from the composite of its idiosyncrasies—saturation and head bumps. Aside from noise, the negatives are mostly from mechanical problems that cause speed variations—wow and flutter and tape path wanderings that affect phase at high frequencies. While I may tackle these topics in a future article, I first must recover from Alignment 101. I hate to think what would happen if I had office hours.

Eddie recently had his head demagnetized after returning from a vacation in NYC, where his brain had been saturated with noise, smells and images. E-mail [email protected] with comments and suggestions.

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Reprinted with permission from Magazine, October, 2000
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