Joining the Reel World

Mar 1, 2007 12:00 PM, By Eddie Ciletti

Playback Alignment

The remaining task before calibrating the machine is to choose the playback reference level. As tape, heads, and electronics have evolved, so too has the ability to record at higher levels, lowering the noise floor in the process. You don't have to peg the meters to record loud — they're expensive to replace. Simply pick an operating level based on the capabilities of your machine and the brand of tape you plan to use on it.

The reference level is specified in nanowebers per meter — 185 nWb/m was an early popular standard. (A nanoweber is the quantity used to express the magnitude of magnetic flux.) The standard or reference level has changed over the years as tape formulations have improved by having greater headroom and lower noise. Just as 0 VU is a reference (and not the lack of signal), so too is 250 nWb/m.

Elevated levels generally increment in 3 dB steps. The level on tape will be referred to as “+x over y,” where “+x” refers to the number of dB over the reference level “y.” For example, +3 dB over 185 nWb/m is 250 nWb/m. To avoid confusion, always know and state the reference level; don't just say “+6.”

On narrow-format machines, level calibration should be done according to the manufacturer's specification (using the modern equivalent tape formulation). Internal noise reduction reduces the need to hit the tape harder.

Test tapes typically have a 1 kHz tone for checking playback level and a 10 kHz tone for checking high-frequency playback level. (A 1 kHz tone is in the middle of the audio spectrum and least affected by most anomalies.) Low-frequency response is typically adjusted after recording a tone sweep from 250 Hz down to 20 Hz. Because test tapes are recorded in full-track mono, do not adjust to the prerecorded low-frequency tones unless they have been “compensated for multitrack reproduction.” (Full-track tapes will show exaggerated low-frequency response when played on a multichannel machine.)

Playback Azimuth

Tones at 8 and 16 kHz are provided on test tapes for coarse and fine adjustment of the azimuth. This adjustment optimizes the head's high-frequency response and compatibility with other machines. While adjusting azimuth, note any level discrepancies that may exist between 8 and 16 kHz. (A 10 kHz tone is used to set playback EQ, and the two frequencies on either side can be used to evaluate and get in the ballpark with the HF response.) Fig. 5 shows the typical locations where you would find adjustment screws.

Playback azimuth adjustment is tricky because the wavelengths of 8 and 16 kHz are so small. (The reason I suggested using pink noise during the record test is that any artifacts will be obvious to the ear.) The easiest, most accurate way to adjust record- and playback-head azimuth is by mixing all tracks from the test tape to mono (for the record and playback heads, respectively). The alternative is to route two neighboring tracks to either an oscilloscope or a mixer (summing to mono) while monitoring on a VU meter. (Each individual signal should read -6 VU and sum to exactly 0 VU.)

Record Alignment

If you correctly set the bias as described earlier in this article, you can now adjust the record level. Set the machine to monitor input, then apply a 1 kHz tone at the machine's reference level: +4 dBu or -10 dBV as required by either XLR or RCA connectors, respectively. (This translates to -18 dB Full Scale in the digital domain.)

Some machines have an Input Calibrate adjustment that may interact with the record level calibration. Enter record, switch to repro, and adjust the record level for 0 VU. Check input and tweak the Input Calibrate control if necessary. Once satisfied, set the oscillator for 10 kHz, confirm the input is still 0 VU, then monitor the repro head while tweaking the record EQ adjustment.

Next, on a 3-head deck, record a bass sweep from 250 Hz down to 20 Hz while monitoring the playback head. Align the low-frequency EQ until peaks and dips fall on equal sides of 0 VU, then select a low frequency that falls on 0 VU. Print that tone to tape and note it on the tape box. Include the bass sweep at the beginning of the tape if it will be used for a mix master.

On a 2-head deck, record the bass sweep, then rewind and check playback. Because many narrow-format machines have neither a bass EQ adjustment nor VU meters, all you can do is note the frequencies at which there are peaks and dips. Select a low frequency that falls on 0 VU, print that tone to tape, and note it on the tape box.

Extreme bumps above 0 VU can cause noise-reduction mistracking, which results in artifacts such as pumping. Having the heads lapped can cure exaggerated bass bumps on all machines.

Tape Storage: Tail Out

Tapes should be stored after being played to the end, which is known as tail out. This serves two purposes. As it plays, the tape is packed evenly and smooth so that dust and humidity won't damage the exposed edges.

In addition, louder audio signals will saturate the magnetic particles on the tape, making it easy to magnetize the surrounding layers, causing pre- and postecho. Postecho is more acceptable and is often masked by the original sound.

Reel Gone

Once you've done all the dirty work, it's time to play. Get to know the sonic characteristics of your machine. For example, record a kick drum while monitoring the playback head, slowly increasing the record level until you notice saturation. Then compare the input level — what's being sent to tape — with what's coming back. Saturation will affect the observed and perceived levels, and how much of it you want is your call. Don't forget that you can also send an entire mix to your “new” analog recorder.

Owning and maintaining a tape machine is a labor of love. The reward is knowing what your machine can do.

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Eddie Ciletti has several tape machines in his audio cave, known as the Prototype, and he's secretly looking for a disc-cutting lathe.

RESOURCES FOR ANALOG RECORDERS

This list of third-party vendors should be helpful if you own an analog reel-to-reel deck.

Athan Online (www.athan.com) rebuilds motors and sells replacement pinch rollers.

ATR Services, Inc. (www.atrservice.com) offers Ampex machine restoration, recording tape, test tapes, parts, and classes.

JRF Magnetic Sciences (www.jrfmagnetics.com) offers head restoration, heads, and test tapes and does custom work.

Magnetic Reference Laboratory, Inc. (http://home.flash.net/~mrltapes) makes and sells calibration tapes.

MDI PrecisionMotorWorks (www.precisionmotorworks.com) offers machine restoration, rubber parts, and rollers and rebuilds motors.

Terry's Rubber Rollers and Wheels (www.terrysrubberrollers.com) sells pinch rollers for professional, consumer, and specialty machines (such as the Maestro Echoplex and the Roland RE-201 Space Echo).

BAKING The sad reality is that the glue, also known as the binder, that secures the iron particles to the plastic tape absorbs moisture over time and eventually becomes more like rubber cement. Tapes with degraded binder will shed their oxide onto stationary surfaces such as heads, guides, and lifters.

The good news is that baking the tape at a low temperature (130 to 140 degrees Fahrenheit) eliminates the moisture and reactivates the binder. The length of time required to bake the tape depends on the tape width, type, brand, and condition and the number of reels being baked.

Tape baking is typically done before archiving previously recorded material. It is not recommended for recycling old tape for new projects. For more information, visit www.tangible-technology.com/tape/baking1.html.