Analog Tape 101-- PART ONE
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Figure 1: How tape speed affects frequency response

Tape machines don’t go drastically out of alignment unless something is wrong—such as intermittent parts, bad tape or poor mechanical stability—but don’t 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.

Figure 3: Bias vs. Distortion curves for EMTEC SM900 (Courtesy EMTEC Magnetics)

Figure 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 wasn’t 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.

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 prices.

Condensed 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.

Table 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.

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 don’t 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