4.1 - What does it mean to "align" a tape machine?
Description
This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
4.1 - What does it mean to "align" a tape machine?
There are a number of standard adjustments on any analogue tape
machine, which can roughly be broken up into mechanical and electronic
adjustments. The mechanical adjustments include the head position
(height, skew, and azimuth), and sometimes tape speed. Incorrect head
height will result in poor S/N and leakage between channels, because
the tracks on the head do not match up exactly with those on the tape.
Incorrect tape skew will result in level differences between channels
and uneven head wear, because there is more pressure on the top of the
head than the bottom (or vice versa). Incorrect azimuth will result
in loss of high frequency response and strange skewing of the stereo
image. Tape speed error will result in tonal shifts, although on many
machines with capstan speed controlled by crystal or line frequency,
it is not adjustable.
Electronic adjustments include level and bias adjustments for each
channel. Some machines may have bias frequency adjustments,
equalization adjustments for playback and record emphasis,
pre-distortion adjustments, and a varied bevy of adjustments for
noise reduction systems.
Alignment is relatively simple, and the same general method applies
from the smallest cassette deck to the largest multitrack machine.
First, put a test tape on the machine. Use a real reference tape,
from the manufacturer, from MRL, or a similarly legitimate lab. DO
NOT EVER use a homebrew test tape that was recorded on a "known
good" machine. You will regret it someday. Spend the money and get
a real test tape (and not one of the flaky ones from RCA).
1. Speed adjustment (if necessary). Play back a 1 KHz reference
tone and, using a frequency counter, adjust the tape speed for
proper frequency output. There are strobe tapes available for
this as well, but with cheap frequency counters available, this
method is much easier.
2. Head height and skew adjustments. Better see your machine's manual
on this one, because I have seen a variety of ways of doing this.
3. Azimuth adjustment. I find the easiest way to do this is to take
the left and right outputs and connect them to the X and Y inputs
of an oscilloscope, and play back a 1 KHz reference tone, while
adjusting the azimuth until a perfectly-diagonal line appears.
You can do this by ear if you are desperate, but I strongly
recommend the lissajous method, which is faster and more
accurate. On multitrack decks, use the two tracks as close as
possible to the edge of the tape. Now you have the playback head
azimuth set... put a 1 KHz source into the record input, with a
blank tape on the machine, and adjust the azimuth of the record
head for the proper diagonal line.
4. Playback eq adjustment (if necessary). This is a case of playing
back various test tones at different frequencies, and adjusting
the response curve of the deck to produce a flat output. You can
also do this by playing back white noise and using a third-octave
spectrum analyzer of great accuracy to adjust for flat response.
Again, this is one to check your deck's manual for, because the
actual adjustments vary from one machine to another, and you will
want to use the test tape once again.
5. Record eq adjustment (if necessary). How this is done (and
whether you want to do it after biasing the tape) depends a lot
on your deck.
6. Bias adjustment. There are a lot of ways to do this. My
favorite method is to use a white noise source, and adjust the
bias until the source and tape output sound identical. Some
people prefer to use a signal generator and set so that the
levels of recorded tones at 1 KHz and 20 KHz are identical. I
find I can get within .5 dB by ear, though your mileage may
differ. [Ed. note: Many tapes have recommended overbias
settings, and many decks will also provide a chart that
correlates the amount of overbias against available tape
formulations. -Gabe]
7. Record level adjustment. I use a distortion analyzer, and set
the level so that at +3 dB, I get 3% distortion on the output.
Some folks who are using very hot tape set the machines so that a
certain magnetic flux is produced at the heads given a certain
input, but I find setting for a given distortion point does well
for me. If you don't have a distortion analyzer, use a 1 KHz
tone source and set so that you have the onset of audible
distortion at +3 dB, and you will be extremely close.
[Ed. note: The traditional way to do this is to align the repro
side of the machine using a calibration tape, and then to put the
machine into record. Monitoring off the repro head, the operator
then aligns the record electronics until the output is
flat. -Gabe]
At this point, you will be pretty much set. Whether you want to do
this all on a regular basis is a good question. You should
definitely go through the complete procedure if you ever change
brands of tape. Checking the mechanical parameters on a regular
basis is a good idea with some decks (like the Ampex 350), which
tend to drift. Clean your heads every time you put a new reel on,
and demagnetize regularly. [Scott]
Continue to:
My Books
