1: How tape speed affects frequency response
Tape machines dont
go drastically out of alignment unless something is wrongsuch as
intermittent parts, bad tape or poor mechanical stabilitybut dont
expect ruler-flat response like digital. Figure 1 shows the performance
variables of a Studer B67 configured as a 1/4-inch 2-track, the format
equivalent of a 2-inch 16-track. To show the relationship between speed
and track width, consider the following: Compared to 2-inch 16-track,
24-track machines running at 30 ips have difficulty with frequencies less
than 60 Hz, the same as narrow-format machines such as the Fostex E-16
running at 15 ips. This is a head-related issue. Only so much metal and
wire can be crammed into a small space.
3: Bias vs. Distortion curves for EMTEC SM900 (Courtesy EMTEC Magnetics)
2 (click to view chart) is a half-inch cross section of tape. Left-to-right
track density increases starting with 1/2-inch 2-track. The narrow
formats are to the right of professional 24-track. Wider tracks yield
more signal, less noise, better low-frequency response at higher speeds
and more headroom across the frequency spectrum.
Bass bumps and the natural saturation at extreme low and high frequencies
is a form of peak limiting, giving analog tape a characteristic smoothness
or that dreaded and overused word warmth. For years, recording
engineers have been experimenting with speed (no comments), track width,
elevated levels and noise reduction in an attempt to find the best match
for the material being recorded. The 15 ips 2-track on 1/4-inch tape was
once the norm, and noise reduction was a popular option. In the late 70s,
30 ips, elevated levels and no-noise were the trend until
the 80s ushered in the 1/2-inch format, but that still wasnt
quiet enough for purists who would rather not use noise reduction.
It should be noted that there is nothing wrong with noise reduction
(NR) other than the additional electronics required. Be a purist if you
want, but sometimes noise gets in the way. The fault of NR
is that it does not tolerate frequency-response anomalies. Narrow tracks
have exaggerated head bumps and reduced headroom at low frequencies. Headwear
further antagonizes response at both ends, increasing low-frequency bumps
and decreasing high-frequency response. This combination is deadly when
NR is added, and narrow-format machines cannot be operated without it.
All machines have playback and record adjustments in the midrange at 1
kHz, but narrow-format machines often have no low-frequency (headwear
compensation) adjustment and minimal high-frequency record EQ adjustment
range. Tempting as it may be, bias should not be used to manipulate record
EQ because its purpose is to minimize distortion. See Fig. 3, BASF EMTEC
Bias vs. Distortion curves.
THE LAP DANCE
It is important to emphasize that head surface condition is a prerequisite
to an electronic alignment. If the high frequencies waver or increase
in level after applying some drag to the supply reel, look closely as
the tape passes through the guides as well as between the capstan and
pinch roller. Any up-and-down motion can cause the signal level to vary.
More likely, a worn head has diminished tape-to-head contact. Increasing
tape tension is not an option, especially when considering high head-replacement
Seven-Point Alignment Procedure
1.Check Playback Level (1 kHz)
2. Check Azimuth (8 and 16 kHz)
3. Check High-Frequency PB Level
4. Check Bias
5. Check Record Level (1 kHz)
6. Check HF EQ Level (10 kHz)
7. Record a Bass Sweep
On a three-head deck, record a bass sweep while monitoring via playback
head. Align until peaks and dips fall on equal sides of 0VU,
then select a low frequency that falls on 0VU. Print that
tone, and note it on the box. Include the bass sweep if the tape becomes
a Mix Master.
On a two-head deck, record a bass sweep. Check playback, noting the
peaks and dips, then select a low frequency that falls on 0VU.
Print that tone and note it on the box.
1 (click to view) compares 24-track head prices from the manufacturer
with those of a third-party source. Dizzy? Worn heads can be resurfaced
or re-lapped, in this case for about 25% of the cost of a
new head. To get the most head life, the machine must be well-maintained
to minimize normal headwear. Poor mechanical alignment can cause the heads
to wear unevenly; the good end is sacrificed to match the
bad end in the lapping process. Head restoration specialists can determine
head life, so have those heads checked before purchasing a used machine.
BASIC MAINTENANCE FOR ALL MACHINES
Use 99% (anhydrous) isopropyl or denatured alcohol on a cotton swab
to clean the heads. Do not use rubbing alcohol, which is 30% water. To
clean rubber parts, use only water-based cleaners such as Windex or Fantastic,
dilute if necessary and wipe off the excess with a damp cloth. Formula
409 works well on ceramic capstan shafts but dont let liquid drip
into the capstan bearing. Do not use cleaners designated for
rubber parts or pinch rollers unless you are certain they are water- based.
Some are solvents that can damage or prematurely age a pinch roller, and
remember, parts are getting more difficult to find and more expensive
with each passing year.
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Reprinted with permission from Mix Magazine, August, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved