The Basics Of Magnetic Tape Recording (Parts, How It Works)

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  • Learn the basics of tape recording.
  • What are the parts of a tape machine?
  • What approaches can you take with editing?

A Revox B77 tape machine.

Before the days of digital audio workstations and quick editing with a click of the mouse, recording engineers would use magnetic reel-to-reel tape recorders. Learning to use a tape machine is not a primary goal for many aspiring audio engineers, but it’s beneficial to learn its basic concepts at the very least so you can appreciate where the technology in your DAW came from.

In this article, you will learn…

  • The basic science of magnetic recording.
  • The parts of magnetic tape.
  • The parts of the tape machine transport mechanism
  • An explanation about AC bias and why it’s important.
  • The monitoring options from the tape machine.
  • The features on remote auto-locate boxes.

How Does Magnetic Recording Work?

Magnetic recording is based on the principles of Faraday’s law of electromagnetic induction. Take a piece of magnetic material, wrap wire around it to form a coil, then bend the material into a donut shape. Just don’t let the very ends touch – make sure you leave a gap.

In this setup, if the current is run through the wire, a proportional magnetic field is produced at the gap. If a changing external magnetic field is moved across the gap, it induces a proportional varying current in the coil. These principles are used by tape heads for recording and playback.

Image courtesy of Aaron Coe, The Cutting Corporation.

The Tape

When using your tape machine, you are using the unique medium of magnetic tape to record and play back sound. The parts of magnetic tape include the magnetic coating, the base (aka substrate), and the back coating (this was added as tape evolved).

The magnetic coating is a mix of magnetic particles, lubricants, stabilizers, the binder, and magnetic pigment. Magnetic particles retain the magnetic orientation as the tape passes the headstack. They start out as tiny iron oxide pieces and are then crushed into powder.

Magnetic pigment is added to give the magnetic particles their color. Stabilizers and lubricants slow the tape’s deterioration and help it slide along the tape path.  The binder adheres the magnetic particles to the base.

The tape’s base is the sheet where the coating mix is applied. Different materials have been used throughout time: paper, cellulose acetate (CA), polyvinyl chloride (PVC), polyester aka polyethylene terephthalate (PET or PE), and polyethylene nap phthalate (PEN). The base is also covered in thin layers of primers. The material used here can significantly affect the tape’s performance.

The back coating was added to improve tape handling. Often made of carbon black, it improves winding properties, removes electrostatic charges, and allows the tape to be packed tight enough so it doesn’t fall apart.

For open reel tape machines, the tape will be wound around a tape reel’s hub, forming a tape pack, with the tape being protected by flanges on the sides.

Blank tape has unaligned magnetic particles and domains. Unmagnetized particles point in random directions. Their individual magnetic fields cancel each other out so there’s barely any magnetic flux (force). After audio is sent to the record head, its flux polarity induces the tape’s magnetic particles to align accordingly. The strength of the record head’s field determines how many tape particles align in a given direction at any point in time.

Understanding The Basic Tape Machine Parts

the path of tape

Transport Mechanism

We’ll just focus on the important parts to start out with. Let’s start with the transport mechanism. It’s made up of a capstan, idler arms, a pinch roller, tape guides, servos, brakes, and tape lifters.

The capstan(s) are rotating steel cylinders that are powered by a motor to move the tape past the tape heads. It works with the pinch roller to maintain constant tape tension.  Some machines have a pinch roller, a free rotating wheel that presses against the capstan while the tape is playing and this is usually made of rubber.

Tape guides, near the tape heads, are stationary posts that keep the tape at the proper height.

The supply reel holds the tape before it passes the tape heads when playing, recording (non-reverse) or fast-forwarding. The supply servo/motor zips the tape back in fast rewind and maintains tape tension during fast forward. The take-up reel works the opposite way with its own motor – tape ends up here after passing the tape heads. Both of these reels have brakes to quickly (but gently) stop the tape.

During any fast winding (fast forward or fast rewind), tape lifters move the tape away from the tape heads so audio won’t be heard as it zips past them. Also, it saves the tape heads from additional wear. Once fast winding is stopped, by pressing stop or play, the tape lifters will retract so that the tape is once again touching the tape heads.

There is something called cue mode (also known as stop editing), usually a button that says “cue”, that retracts tape lifters, so the tape touches the heads even while fast winding. It allows one to quickly hear the recording to get an idea of how far along they are in the tape. Sometimes a control knob makes the tape move forward or backward at varying slower speeds.

Other editing modes include dump editing. Dump editing will allow one to play the tape, but without winding tape to the take-up reel. Instead, larger tape sections can be played right into the garbage!

Meanwhile, spot erase functions allow very specific parts to be erased on designated channels.

The transport mechanism is controlled by the transport controls (play, fast-forward, rewind, stop) on the tape deck and on the remote controller (if connected).

AC Bias

A very high-frequency alternating current (AC bias) is sent to the erase and recording heads. Magnetic materials have magnetization or B-H curves (magnetizing forces vs. resulting magnetism/domain alignment). Magnetic material that hasn’t been magnetized yet has an almost linear initial magnetization response, but prior-magnetized materials do not.

Tape’s B-H curve is very non-linear due to hysteresis. A reversal of the magnetic polarity will result in its magnetization value crossing over the zero line. But hysteresis means the applied magnetic force has to reach a threshold before the tape’s particles start changing magnetic alignment. So there is remaining magnetism on the tape after it leaves the tape head’s range of influence, meaning the reproduced signal without AC bias is nonlinear and may be distorted.

To prevent this, AC bias is added to the audio signal before it reaches the record head’s amplifier. The AC bias frequency should be at least five times the audio’s maximum frequency and be recorded at more than 10 times the audio’s signal strength.

ac bias max frequency

For the most part, the repro head doesn’t respond to most of the super high-frequency AC bias signals but does respond to the original audio. At lower levels, the repro may respond to the bias, necessitating the use of a bias trap to prevent bias tone from getting through.

After AC bias is applied, the audio recorded onto the tape has a more linear magnetization function due the audio signal riding on top of the bias. Now, anytime the audio signal has to reverse magnetic polarity, it won’t have to cross the zero line where the hysteresis occurs.

Increasing the AC bias amount increases the tape machine’s output to max sensitivity. Further increasing AC bias past this will decrease the output’s sensitivity.

Many people prefer to overbias the tape machine during calibration. If the machine is aligned for the best distortion characteristic, there will be an uneven high-frequency response.  Aligning for the max high-frequency response yields a lot of unwanted distortion.

The AC overbias setting is a compromise of balancing these two extremes alongside the tape speed, tape-coating’s relative dimensions, record head gap distance, and wavelength of the audio being recorded.

Tape Heads

These C-shaped metal pieces determine what happens to the magnetic particles in the tape. There are three heads that make up the headstack: erase, record/sync, and repro. Since the middle tape head has both the ability to record and playback, it’s referred to as the record head when it’s recording and the sync head when it’s playing back.

The erase and repro (reproducing) heads are self-explanatory in their functions. Each tape head has a narrow gap which the tape goes past.

When recording or erasing, the erase and record heads each have a coil of wire wrapped around a metal core. Current is run through the wire, generating a magnetic field at the gap. When monitoring from the sync or repro head during playback, the gap detects a moving magnetic field, generating a current in the wire. Smaller and cheaper tape machines may only have 2 tape heads: one erase and one record/playback head. For now, we’ll address a setup for a 3-head tape machine.

Blank tape has unaligned magnetic particles and domains. They point in random directions since they are unmagnetized. All of their individual magnetic fields cancel each other out so there’s a super small amount of magnetic flux. After audio is recorded on it, the record head’s flux polarity causes (via induction) the magnetic particles to align accordingly.

Image: Audiothing

The erase head does exactly as the name implies. Its gap is wider than the ones found on the erase and record/sync heads and sometimes it even has two gaps. A very high frequency is fed into it while recording so the magnetic polarities of the tape’s particles rapidly flip back and forth.

When tape leaves the erase head’s magnetic field’s reach, the tape’s particles, on record enabled channels, will have random magnetic polarities and be unmagnetized. Some cheaper tape machines use a permanent magnet instead to erase the incoming tape while recording.

The record head’s gap is wider than the repro head’s. A current (both the recorded audio and the AC bias) is fed through its coil of wire. Louder audio means more current is sent to the record head, resulting in a stronger magnetic field at the gap and more magnetization of the tape particles.

The record head’s magnetic field induces a residual changing magnetic pattern, as tape moves past its gap, on record enabled channels in record mode. It’s proportional to the magnetic flux created at the record head’s gap. The record head’s magnetic field strength determines how many particles become aligned in a given direction at any point in time. That is until the magnetic field induced onto the tape is so high that the tape reaches saturation (unable to become further magnetized).

Tape with prior-recorded audio (aligned magnetic particles), has magnetic flux. As it moves past the repro or sync head, the flux’s rate of change gets detected by the tape head, and then is induced as a proportional voltage in the coil of wire.

Output Select

We need a selection to control where in the signal flow to monitor back from. Usually, there are three points in the signal flow.

Choosing the input option monitors the audio at the input point- the signal before it reaches the record head, listening to the audio before it reaches the tape. Simple enough so far…

Selecting the sel-sync/sel-rep option corresponds with the signal at the record/sync head (they are one tape head). What is heard depends on the record status (safe or armed) and whether the tape machine is playing, stopped, or recording.

If record is armed but playback is stopped, the audio heard is actually the same signal as if in input mode. But if record is armed and the tape is playing, the audio heard is the playback audio picked up by the sync head. And if recording, record safe channels monitor playback audio via the sync head and record ready channels monitor at the input point.

This is why there is a separate repro head – it allows one to monitor what has been recorded to tape instead of the audio entering the machine at the input point. This is especially helpful while laying down initial tracks.

Repro monitoring is designed exclusively for playback. The repro and record heads have different physical and electronic properties which affect the sound quality of the playback. The record head is optimized and designed for recording, not playback. It also has a limited response to lessen the crosstalk between channels when overdubbing.

If in “record safe” mode, playing or stopped, audio is monitored from the signal picked up by the repro head. This also occurs if in “record ready” and playing, or if actually recording audio. If in record ready and stopped, the signal is monitored at the input point.

There are switches to select these three modes for each individual channel. They’re used during overdubbing and mixing. There are also master selection switches that override the individual channel selections to choose the monitoring for all channels simultaneously.

Editing Block/Splicing Rail

This rectangular aluminum or steel block, usually mounted over the tape heads, is in the shape of one concave groove that holds the tape in place. The thin, deeper grooves across the large flat groove are for splicing.

Different angles can be offered on different blocks. 83 degrees is used for hard cuts on 1” and 2” tape. 87 degrees is used for hard cuts on ¼” tape, ½” 2-track tape, or ½” mono tape. Some editing blocks have an additional line groove for even softer cuts such as 30 or 45 degrees for soft cuts which are used for non-musical edits.

Remote Control & Auto-Location

Some tape machines have a remote auto-locator box. Usually, there’s a tape time timer/readout, in hours-minutes-seconds on the tape machine. Then there’s a locate time readout on the remote auto-locate box. The counters can be electronic or mechanical – both give numerical readouts to keep track of relative locations on the tape.

If the machine has a separate counter for the tape time and the locate time, most likely they have separate buttons that will reset the current tape time as the designated zero starting time, as well as a button that resets both to zero. There are also often ways to reset the locate readout to be the same as the tape readout.

A locate time function can quickly wind the tape back to a user-specified point on the tape. The time may be stored by pressing a “set” button at the desired time during playback, or it may be entered with a numerical keypad (all this depends on the specific tape machine you are using).

Many tape machines have a rehearse feature that enables practice punch-ins without destructively recording. Once audio is truely recorded onto magnetic tape, there’s no undo button to get it back. Consult the machine’s manual on how to rehearse any sort of punch in/out to avoid accidental erasures.

Varispeed

Varispeed, or variable speed, is exactly what it sounds like, varying the tape speed during playback. Slower speed means audio plays back at a lower pitch, and higher speed plays back at a higher pitch. This is used to correct tunings during overdubs, alter tempos, change a song’s key, or change the song’s running time.

You can also play with the varispeed for creative effects. Ween and Beck made extensive use of this technique in the 90s, recording their voices at vastly different speeds for a bizarre effect.

Conclusion

Recording with analog tape machines may not be the primary method anymore, but they are still commonly used to add the unique “tape sound” to things. This means nonlinear frequency response changes, tape compression, and saturation of 3rd order harmonics. Many professionals also route their projects through tape machines during the mixing or mastering stages.

Once you get a grasp of the fundamental concepts, you can take the deep dive into more of its amazing and inherent quirks and use tape machines in your own compositions!

(If you also want the “tape sound” in your DAW, check out our roundup of the 5 best tape emulation plugins on the market.)