February 11, 2013, by David Miles Huber & Robert E. Runstein
Go here for the first installment of this article.
The Magnetic Tape Head
Most professional analog recorders use three magnetic tape heads, each of which performs a specialized task:
The function of a record head (Figure 1) is to electromagnetically transform analog electrical signals into corresponding magnetic fields that can be permanently stored onto magnetic tape.
In short, the input current flows through coils of wire that are wrapped around the head’s magnetic pole pieces.
Since the theory of magnetic induction states that “whenever a current is injected into metal, a magnetic field is created within that metal” … a magnetic force is caused to flow through the coil, into the pole pieces and across the head gap.
Like electricity, magnetism flows more easily through some media than through others. The head gap between poles creates a break in the magnetic field, thereby creating a physical resistance to the magnetic “circuit.” Since the gap is in physical contact with the moving magnetic tape, the tape’s magnetic oxide offers a lower resistance path to the field than does the nonmagnetic gap.
Figure 1: The record head.
Thus, the flux path travels from one pole piece, into the tape and to the other pole. Since the magnetic domains retain their polarity and magnetic intensity as the tape passes across the gap, the tape now has an analogous magnetic “memory” of the recorded event.
The reproduce or playback head (Figure 2) operates in a way that’s opposite to the record head. When a recorded tape track passes across the reproduce head gap, a magnetic flux is induced into the pole pieces.
Since the theory of magnetic induction also states “whenever a magnetic field cuts across metal, a current will be set up within that metal” … an alternating current is caused to flow through the pickup coil windings, which can then be amplified and processed into a larger output signal.
Note that the reproduce head’s output is nonlinear because this signal is proportional to both the tape’s average flux magnitude and the rate of change of this magnetic field.
This means that the rate of change increases as a direct function of the recorded signal’s frequency. Thus, the output level of a playback head effectively doubles for each doubling in frequency, resulting in a 6-dB increase in output voltage for each increased octave.
Figure 2: The playback head.
The tape speed and head gap width work together to determine the reproduce head’s upper-frequency limit, which in turn determines the system’s overall bandwidth.
The wavelength of a signal that’s recorded onto tape is equal to the speed at which tape travels past the reproduce head, divided by the frequency of the signal; therefore, the faster the tape speed, the higher the upper-frequency limit. Similarly, the smaller the head gap, the higher the upper-frequency limit.
The function of the erase head is to effectively reduce the average magnetization level of a recorded tape track to zero, thereby allowing the tape track to be re-recorded.
After a track is placed into the record mode, a high-frequency and high-intensity sine-wave signal is fed into the erase head (resulting in a tape that’s being saturated in both the positive- and negative-polarity directions). This alternating saturation occurs at such a high speed that it serves to confuse any magnetic pattern that existed on the tape.
As the tape moves away from the erase head, the intensity of the magnetic field decreases, leaving the domains in a random orientation, with a resulting average magnetization or output level that’s as close to zero as tape noise will allow.
Equalization (EQ) is the term used to denote an intentional change in relative amplitudes at different frequencies.
Because the analog recording process isn’t linear, equalization is needed to achieve a flat frequency-response curve when using magnetic tape.
The 6-dB-per-octave boost that’s inherent in the playback head’s response curve requires that a complementary equalization cut of 6 dB per octave be applied within the playback circuit (Figure 3).
In addition to the nonlinear changes that occur in playback level relative to frequency, another discrepancy in the recording process exists between the amount of magnetic energy that’s applied to the record head and the amount of magnetism that’s retained by the tape after the initial recording field has been removed.
As Figure 4a shows, the magnetization curve of tape is linear between points A and B, as well as between points C and D.
Signals greater than A and D have reached the saturation level and are subject to clipping distortion. Signals falling within the B to C range are too low in flux level to adequately magnetize the domains during the recording process.
Figure 3: A flat frequency playback curve results due to complementary equalization in the playback circuit.
For this reason, it’s important that low-level signals be boosted so that they’re pushed into the linear range. This boost is applied by mixing an AC bias current (Figure 4b) with the audio signal.
This bias current is applied by mixing the incoming audio signal with an ultrasonic sine-wave signal (often between 75 and 150 kHz).
The combined signals are amplitude modulated in such a way that the overall magnetic flux levels are given an extra “oomph,” which effectively boosts the signal above the nonlinear zero-crossover range and into the linear portion of the curve.
In fact, if this bias signal weren’t added, distortion levels would be so high as to render the recording process useless.
Figure 4a & 4b: The effects of bias current on recorded linearity: (a-top) magnetization curve showing distortion at lower levels; (b-bottom) after bias, when the signal is boosted back into the curve’s linear regions.
The output signal of a professional ATR channel can be switched between three working modes:
In the input (source) mode, the signal at the selected channel output is derived from its input signal.
Thus, with the ATR transport in any mode (including stop), it’s possible to meter and monitor a signal that’s present at a channel’s selected input. In the reproduce mode, the output and metering signal is derived from the playback head.
This mode can be useful in two ways: It allows previously recorded tapes to be played back, and it enables the monitoring of mate- rial off of the tape while in the record mode. The latter provides an immediate quality check of the ATR’s entire record and reproduce process.
The sync mode is a required feature in analog multitrack ATRs because of the need to record new material on one or more tracks while simultaneously monitoring tracks that have been previously recorded (during a process called overdubbing).
Here’s the deal … using the record head to lay down one or more tracks while listening to previously recorded tracks through the reproduce head would actually cause the newly recorded track(s) to be out of sync with the others on final playback (due to the physical distance between the two, as shown in Figure 5a).
To prevent such a time lag, all of the reproduced tracks must be monitored off of the record head at the same time that record tracks are being laid down onto the same head. Since the record head is used for both recording and playback, there is no physical time lag and, thus, no signal delay (Figure 5b).
To Punch Or Not To Punch
You’ve all heard the age-old adage … “$%& happens.” Well, it happens in the studio—a lot! Whenever a mistake or bad line occurs during a multitrack session, it’s often (but not always) possible to punch-in on a specific track or set of tracks.
Instead of going back and re-recording an entire song or overdub, performing a punch involves going back and re-recording over a specific section in order to fix a bad note, musical line—you name it.
This process is done by cueing the tape at a logical point before the bad section and then pressing Play. Just before the section to be fixed, pressing the Record button (or entering record under automation) will place the track into record mode.
Figure 5a and 5b: The sync mode’s function. (a-top) In the monitored playback mode, the recorded signal lags behind the recorded signal, thereby creating an out-of-sync condition. (b-bottom) In the sync mode, the record head acts as both record and playback head, bringing the signals into sync.
At the section’s end, pressing the Play button again causes the track to fall back out of record, thereby preserving the section following the punch.
From a monitor standpoint, the recorder begins playback in the sync mode; once placed into record, the track switches to monitor the input source. This lets the performers hear themselves during the punch while listening to playback both before and after the take.
When performing a punch, it’s often far better to “fix” the track immediately after the take has been recorded, while the levels, mic positions and performance vibe are still the same.
This also makes it easier to go back and re-record the entire song or a larger section should the punch not work.
If the punch can’t be performed at that time, however, it’s generally a good idea to take detailed notes about mic selection, placement, preamps and so on to recreate the session’s setup without having to guess the details from memory.
As every experienced engineer/producer knows, performing a punch can be tricky. In certain situations, it’s a complete no-brainer…for example, when a stretch of silence the size of a Mack truck exists both before and after the bad section, you’ll have plenty of room to punch in and out.
At other times, a punch can be very tight or problematic (e.g., if there’s very little time to punch in or out, when trying to keep vocal lines fluid and in-context, when it’s hard to feel the beat of a song or if it has a fast rhythm). In short, punching-in shouldn’t be taken too lightly … nor so seriously that you’re afraid of the process.
Talk it over with the producer and/or musicians. Is this an easy punch? Does the section really need fixing? Do we have the time right now? Or, is it better just to redo the song?
In short, the process is totally situational and requires attention, skill, experience and sometimes a great deal of luck.
- Before committing the punch to tape, it’s often a wise idea to rehearse the punch, without actually committing the fix to tape. This has the advantage of giving both you and the performer a chance to practice beforehand.
- Some analog decks (and most DAWs) will let you enter the punch-in and punch-out times under automation, thereby allowing the punch to be surgically performed automatically.
- If you’re recording onto the same track, a fudged punch may leave you with few options other than to re-record the entire song or section of a song. An alternative to this dilemma would be to record the fix into a separate track and then switch between tracks in mixdown.
- The track(s) could be transferred to a DAW, where the edits could be per- formed in the digital domain.
Tape, Tape Speed, And Head Configurations
Professional analog ATRs are currently available in a wide range of track- and tape-width configurations.
The most common analog configurations are 2-track mastering machines that use tape widths of 1/4 inch, 1/2 inch, and even 1 inch, as well as 16- and 24-track machines that use 2-inch tape.
Figure 6 details many of the tape formats that can be currently found. Optimal tape-to-head performance characteristics for an analog ATR are determined by several parameters: track width, head-gap width and tape speed.
In general, track widths are on the order of 0.080 inch for a 1/4-inch 2-track ATR; 0.070 inch for 1/2-inch 4-track, 1-inch 8-track, and 2-inch 16-track formats or 0.037 inch for the 2-inch 24-track format.
As you might expect, the greater the recorded track width, the greater the amount of magnetism that can be retained by the magnetic tape, resulting in a higher output signal and an improved signal-to- noise ratio. The use of wider track widths also makes the recorded track less susceptible to signal-level dropouts.
The most common tape speeds used in audio production are 15 ips (38 cm/ sec) and 30 ips (76 cm/sec).
Figure 6: Analog track configurations for various tape widths.
Although 15 ips will eat up less tape, 30 ips has gained wide acceptance in recent years for having its own characteristic sound (often having a tighter bottom end), as well as a higher output and lower noise figures (which in certain cases eliminate the need for noise reduction).
On the other hand, 15 ips has a reputation for having a “gutsy,” rugged sound.
A form of deterioration in a recording’s quality, known as print-through, begins to occur after a recording has been made.
This effect is the result of the transfer of a recorded signal from one layer of tape to an adjacent track layer by means of magnetic induction, which gives rise to an audible false signal or pre-echo on playback.
The effects of print-through are greatest when recording levels are very high, and the effect decreases by about 2 dB for every 1-dB reduction in signal level. The extent of this condition also depends on such factors as length of storage, storage temperature and tape thickness (tapes with a thicker base material are less likely to have severe print-through problems).
Because of the effects of print-through, the standard method of professionally storing a recorded analog tape is in the tails-out position.
—Professional analog tape should always be stored tails-out (on the right- hand take-up reel).
—Upon playback, the tape should be wound onto the left-most “supply reel.”
—During playback, feed the tape back onto the right-hand take-up reel, after which time it can again be removed for storage.
—If the tape has been continuously wound and rewound during the session, it’s often wise to rewind the tape and then smoothly play or slow-wind the tape onto the take-up reel, after which time it can be removed for storage.
So why do we go through all this trouble? When a tape is stored tails-out (Figure 7), the print-through will bleed to the outer layers, a condition that causes the echo to follow the original signal in a way that’s similar to the sound’s natural decay and is subconsciously perceived by the listener as reverb instead of as an easily-audible pre-echo.
It’s very important for the magnetic recording heads and moving parts of an ATR transport deck to be kept free from dirt and oxide shed.
Oxide shed occurs when friction causes small particles of magnetic oxide to flake off and accumulate on surface contacts.
This accumulation is most critical at the surface of the magnetic recording heads, since even a minute separation between the magnetic tape and heads can cause high-frequency separation loss.
For example, a signal that’s recorded at 15 ips and has an oxide shed buildup of 1 mil (0.001 inch) on the playback head will be 55 dB below its standard level at 15 kHz.
Denatured (isopropyl) alcohol or an appropriate cleaning solution should be used to clean transport tape heads and guides (with the exception of the machine’s pinch roller and other rubber-like surfaces) at regular intervals.
Figure 7: Recorded analog tapes should always be stored in the tails-out position.
Magnetic tape heads are made from a magnetically soft metal, which means that the alloy is easily magnetized … but once the coil’s current is removed, the core won’t retain any of its magnetism.
Small amounts of residual magnetism, however, will build up over time, which can actually partially erase high-frequency signals from a master tape.
For this reason, all of the tape heads should be demagnetized after 10 hours of operation with a head demagnetizer. This handheld device works much like an erase head in that it saturates the magnetic head with a high-level alternating signal that randomizes residual magnetic flux.
Once a head has been demagnetized (after 5 to 10 seconds), it’s important to move the tool to a safe distance from the tape heads at a speed of less than 2 inches per second before turning it off, so as to avoid inducing a larger magnetic flux back into the head.
Before an ATR is aligned, the magnetic tape heads should always be cleaned and demagnetized in order to obtain accurate readings and to protect expensive alignment tapes.
Backup And Archive Strategies
In this day of hard drives, CDs and digital data, we’ve all come to know the importance of backing up our data. With important music and media projects, it’s equally important to create a tape backup copy in case of an unforeseen catastrophe or as added insurance that future generations can enjoy the fruits of your work.
Backing Up Your Project
The one basic truth that can be said about analog magnetic tape is that this medium has withstood the test of time.
With care and reconditioning, tapes that have been recorded in the 1940s have been fully restored, allowing us to preserve and enjoy some of the best music of the day. On the other hand, digital data has two points that aren’t exactly its favor:
- Data that resides on hard drives isn’t the most robust of media over time. Even CDRs (which are rated to last over 100 years) haven’t really been proven to last.
- Even if the data remains intact, with the ever-increasing advances in computer technology, who’s to say that the media, drives, programs, session formats and file formats will be around in 10 years, let alone 50!
These warnings aren’t slams against digital, just precautions against the march of technology versus the need for media preservation.
For the previously listed reasons, media preservation is a top priority for such groups as the Recording Academy’s Producers & Engineers Wing (P&E Wing), as well as for many major record labels—so much so that many stipulate in their contracts that multitrack sessions (no matter what the original medium) are to be transferred and archived to 2-inch multitrack analog tape.
When transferring digital tracks to an analog machine, it’s always wise to make sure that the recorder has been properly calibrated and that reference tones (1 kHz, 10 kHz, 16 kHz and 100 Hz) have been recorded at the beginning of the tape.
When copying from analog to analog, both machines should be properly calibrated, but the source for the newly recorded tones should be the master tape.
If a SMPTE track is required, be sure to stripe the copy with a clean, jam-sync code.
The backing up of analog tapes and/or digital data usually isn’t a big problem … unless you’ve lost your original masters. In this situation, a proper safety master can be the difference between panic and peace.
Just as it’s important to back up your media, it’s also important that both the original and backup media be treated and stored properly. Here are a few guidelines:
- As stated earlier, always store the tapes tails-out.
- Wind the tapes onto the take-up storage reel at slow-wind or play speeds.
- Store the boxes vertically. If they’re stored horizontally, the outer edges could get bent and become damaged.
- Media storage facilities exist that can store your masters or backups for a fee. If this isn’t an option, store them is an area that’s cool and dry (e.g., no temperature extremes, in low humidity, no attics or basements).
- Store your masters and backups in separate locations. In case of a fire or other disaster … one would be lost, but not both (always a good idea with digital data, as well).
This article is excerpted from Huber & Runstein’s book Modern Recording Techniques, Seventh Edition. For another installment, click here.