WE ARE FAMILY
Remember how quickly the low frequency response rolls off at 30 IPS
on quarter-inch (half track) stereo tape? The same type of response
is true for narrow-format machines at 15 IPS! Head wear further exaggerates
the bass response making the bumps bigger, the troughs deeper and the high
frequencies droopier. More important than electronic alignment is
the condition of the head surface. If your narrow format machine
sounds tired, have the head relapped first. The machine may not even
need an alignment.
In addition, narrow format machines rarely provide the luxury of a read-after-write
head. By contrast, two-inch twenty-four track machines can monitor playback
while recording. This not only facilitates alignments, but also yields
better performance because the record and playback heads are each optimized
for their respective jobs. ( The typical one-inch sixteen track may have
separate record and playback heads, but both are technically playback heads.)
A KICK IN THE PANTS
On machines with a dedicated playback head, especially at 30 IPS, a
record test is highly recommended. Instruments such as kick drum are particularly
vulnerable to saturation at high speeds. While switching from input to
repro, note how its low frequency character changes with dynamics.
When tape saturation is employed for effect, use a limiter to keep from
exceeding the maximum level for the sound you want. (Start with a medium
attack and a slightly faster release.) Application permitting, less trouble
will be encountered if the kick resonance falls between 75 Hz and 85 Hz.
In addition, avoid adding bottom to kick or bass instruments while recording.
Let the tape and the head do their thang and make EQ adjustments on the
THE WHOOLY MAMMOTH
The warmth associated with analog tape has as much to do with what happens
at high frequencies as it does at the opposite end of the spectrum. If
youíre in the habit of pushing signals into the red, be particularly careful
with instruments such as tambourine. On older machines equipped with VU
meters, the average level will seem to be about "-5VU" but the peaks be
off the scale and in the +10VU to +14 VU range. LED displays do not suffer
from mechanical inertia.
ALIGNMENT and HEAD WEAR COMPENSATION
With the exception of head wear and different tape formulations, tape
machines donít go out of alignment unless something is wrong or someone
is dangerous with a screwdriver. Pro machines were designed to be aligned
so that users, taking advantage of this capability, can choose the operating
level, tape formulation, speed and optional noise reduction on a per-project
basis ó hence the generous array of adjustments.
Test tapes are full-track recordings with "no compensation for multitrack
reproduction." The important reference tones are 1kHz and above. Accurate
low frequency playback adjustments can be made after recording a frequency
sweep very slowly from 500 Hz down to 20 Hz making recognizable stops along
the way to note the peaks and dips. Youíll need this information in a moment.
Note: In addition to speed, be sure to choose the correct equalization
(AES, IEC or NAB) for your machine. Also, demagnetize your tools and the
headstack before playing the test tape.
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 Iíve used (on heads that are not yet toasted) is to find the nastiest
low-frequency head bump ó note the level ó and set the 10 kHz reference
tone (from the test tape) to that level. It is cheating, but otherwise,
the noise reduction system will mistrack and exaggerate the response errors
by a factor of two.
In addition to the head bump, it is also helpful to find the nearest
trough. Machines with low-frequency playback EQ should be set so that these
two anomalies are equally above and below 0VU. Then, pick the frequency
in between that falls on 0VU, record it for at least 30 seconds and make
a note on the box.
INVERSE NARROW-FORMAT TIPS
Hint: If youíre smart youíll start at number 8. Tip: If you accidentally
turn an adjustment that does "nothing," put it back in place.
Note: Noise reduction is not bad ó it should
be used on narrow format machines ó but it is level and frequency
Donít attempt a record alignment. Most sonic problems are likely to be
related to head wear on the playback side. Without a dedicated playback
head, any attempts at record alignment will simply throw the machine more
out of whack.
Normal head wear can be corrected by resurfacing (lapping) while severe
wear can damage the head beyond repair. Replacement is expensive,
so donít put it off. See the back pages of EQ for a lapper near you.
Do not attempt a record calibration on machines not equipped with a dedicated
playback head. You will be sorry! Donít be fooled by the record EQ adjustments,
they donít do anything. In additionÖ
Do not use bias to affect record EQ. Bias is optimized for minimal distortion.
All narrow format machines require more "overbias" than pro machines.
2 ½ to 3 dB overbias (using 10 kHz @15 IPS) is typical for a Pro
Machine, 4 ½ to 5 dB overbias is the norm for machines mit da skinny
For the terminally curious, make a record test ó with the noise reduction
off ó using an oscillator set to 1 kHz. Donít freak when all of the LEDís
donít line up. A segment plus or minus means little. Use the oscillatorís
level control to find an average level that satisfies most of the meters.
On playback, confirm that the signal is at the same level.
If the edge tracks are down at mid or high frequencies, clean the heads.
If the improvement is negligible or inconsistent, itís lap time.
Own an alignment tape and adjust playback only. Most 15 IPS machines use
If youíve never done an alignment, it may be better to leave well enough
alone. Otherwise read the manual and have someone show you not just the
procedure, but also the pitfalls. It may even be necessary to pay a technician,
but this is exactly what bias is about. Bias is a radio frequency that
is like currency to the tapeís magnetic particles ó theyíll make music
if you pay cash up front.
Adjusting Bias is like tuning an old fashioned AM radio. There is a
generally accepted "window of correctness" where the signal comes through
loud and clear. Underbias and the signal will be bright on top and mushy
on the bottom. Excessive bias makes dull and dirty recordings.
Table One shows typical overbias values for reference purposes. (Contact
the tape or the machine manufacturer if unsure.) The amount of overbias
is not global ó it is not a magic number that works for all speeds and
machines. The frequency used to make the adjustment and the amount of signal
reduction (overbias) are speed specific. Change the speed and either the
frequency or the amount must change as per Table One. Also note that at
7 ½ IPS, high-frequency tests must be made 10 dB below 0VU (20 dB
below for cassette decks).