Calibration tapes for analog audio recording
If you are new to professional analog audio magnetic-tape re- cording, the first and most important step you can take to ensure high-quality recording is to make sure that the recorder you’re using is properly calibrated. And, to calibrate the recorder, you’ll need a calibration tape.
Read the manual
First, refer to your recorder’s operation and maintenance manual. All professional tape recorders have the same basic adjustments, but the adjustment procedures and the location of the adjustment controls usually differ from one machine to the next. Before you begin tweaking, have the correct test tape on hand, know how to perform the adjustments and when to call a more experienced technician.
Second, understand the basic recording parameters. While tape width may seem obvious and easily measured, most recorders can be set up for any combination of widths, speeds, levels and equalizations. As such, the recorder’s model number alone may not be of much help and will require some investigation. If you’re already using a calibration tape from Magnetic Reference Lab (MRL), Ampex, BASF (Emtec), Standard Tape Lab or another supplier, the label and the voice announcement will provide all of the details. If the tape has deteriorated, the MRL part numbers are still valid. For all other tape types, contact MRL for the equivalent part number.
When choosing the tape’s internal (magnetic) operating level, also called reference fluxivity, you must base your choice on several considerations. These considerations include the type of program level meter you’re using — standard volume unit (VU) meter or peak program meter (PPM) — the type of blank tape you’re using, whether you’re using noise reduction, and if you’re using compression. Fluxivity is commonly stated in nanowebers per meter (nWb/m). For older professional tapes and consumer tapes, 200 nWb/m is typical. For general studio usage, 250 nWb/m is best. For the highest output mastering tapes or when you’re using audio compression, 500 nWb/m is best. If the calibration tape is not at the desired reference fluxivity but is otherwise correct, you can easily set your reproducer for a different reference fluxivity.
Test signals and calibration tapes
In addition to level, azimuth and preliminary frequency response, a multifrequency calibration tape will include 13 spot frequencies best suited for first-time calibration and reproducer troubleshooting.
While multi-frequency tapes are only available in single-speed versions, shorter tapes are less expensive to purchase, quicker to use (for touch-up purposes) and may be available as two-speed versions. These shorter tapes provide at least two tones required to calibrate a tape reproducer: 1kHz to set reproducer gain (also called reproducer level) and 10kHz (used first to adjust the mechanical azimuth of the reproducing head, and then to set the high-frequency reproducer equalization control). An optional 100Hz tone is really too high to accurately set the low-frequency reproducer equalizer response, but it does provide a quick test that the low-frequency response of the reproducer has not failed. Some tape reproducers do not even have a low-frequency adjustment control.
Tape speed and equalization
There are several tape recording speeds available, and each has a corresponding equalization. Equalizations are known by the names of the organizations that have standardized them. These organizations have changed over the years, resulting in some confusion.
For new recordings made at 3.75in/s, the same equalization is used everywhere. It is standardized by both the NAB and the IEC, so we call it “NAB and IEC.” For 7.5- and 15in/s, the equalizations used are IEC2 (mostly used in the United States and formerly referred to as NAB) and IEC1 (mostly used in Europe and formerly referred to as IEC, CCIR or DIN Studio).
For the 15in/s speed used on narrow-format recorders (eight and 16 tracks on ¼-inch tape, 16 and 24 tracks on 1-inch tape) the IEC1 (IEC and CCIR and DIN Studio) equalization is usually used. And, for 30in/s, the equalization used everywhere for new recordings is AES, also called IEC2. (Note: During the early years of tape recording — 1948 through, roughly, 1968 — some of the equalizations were changed several times, especially at the slower speeds, as new-and-improved tapes were developed.)
Jay McKnight is president of Magnetic Reference Lab.
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