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 youre 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, Tascams 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.
The Magicians Secret
I saved this technique for last because its 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, youll hear only the distortion artifacts; the
changes will be more revealing.) Adjust to minimize the fuzz and low-frequency
harmonic distortion. (Congratulations! Youve 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.
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).
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 levelit should be within ±1 dBand
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.)
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.
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-format machines offer little in the way of Record EQ adjustment,
and there is no low-frequency playback compensation for worn heads. One
trick Ive 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 systemdbx 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, its not so easy
to find Ampex/Quantegy 457 or 3M 227 tapes these days; however, Quantegy
407 is still manufactured.
The warmth for which analog tape is famous comes from the composite of
its idiosyncrasiessaturation and head bumps. Aside from noise, the
negatives are mostly from mechanical problems that cause speed variationswow
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
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.
to Page 2; Page
Reprinted with permission
Magazine, October, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved