Thursday, July 22, 2010
The Analog Tape Recorder: An Introduction
What every engineer should know about analog recorders, excerpted from Huber & Runstein's "Modern Recording."This article is the first half in our series on the analog tape recorder, excerpted from Huber & Runstein’s book Modern Recording Techniques, Seventh Edition. For the second half, click here.
From its inception in Germany in the late 1920s and its American introduction by Jack Mullin in 1945 (Figure 1), the analog tape recorder (or ATR) had steadily increased in quality and universal acceptance to the point that professional and personal studios had totally relied upon magnetic media for the storage of analog sound onto reels of tape.
With the dawning of the project studio and computer-based DAWs, the use of two-channel and multitrack ATRs has steadily dwindled to the point where no new analog tape machine models are currently being manufactured.
In short, recording to analog tape has steadily become a high-cost, future-retro, “specialty” process for getting a certain sound.
This being said, the analog recording process is still highly regarded and even sought after by many studios as a special sonic tool … and by others as a raised fist against the onslaught of the “evil digital empire.”
Without delving into the ongoing debate of the merits of analog versus digital, I think it’s fair to say that each has its own distinct type of sound and application in audio and music production.
Although professional analog recorders are usually much more expensive than their digital counterparts, as a general rule, a properly aligned, professional analog deck will have a particular sound that’s often described as being full, punchy, gutsy and “raw” (when used on drums, vocals, entire mixes or anything that you want to throw at it).
In fact, the limitations of tape are often used as a form of “artistic expression.” From this, it’s easy to see and hear why the analog tape recorder isn’t dead yet … and probably won’t be for some time.
To 2-Inch Or Not To 2-Inch?
Before we delve into the inner workings of the analog tape recorder, let’s take a moment to discuss ways in which the analog tape sound can be taken advantage of in the digital and project studio environment.
Before you go out and buy your own deck, however, there are other cost-effective ways to get “that sound” on your own projects.
For example:
- Make use of plug-ins that can emulate (or approximate) the overdriven sound of an analog tape track.
- Rent a studio that has an analog multitrack for a few hours or days. You could record specific tracks to tape, transfer existing digital tracks to tape or dump an entire final mixdown to tape. For the cost of studio time and a reel of tape, you could inject your project with an entirely new type of sound (you might consider buying a single reel of multitrack tape that can be erased and reused once the takes have been transferred to disk).
- Rent an analog machine from a local studio equipment service. For a rental fee and basic cartage charges, you could reap the benefits of having an analog ATR for the duration of a project, without any undue financial and maintenance overhead.
A few guidelines should also be kept in mind when recording and/or transferring tracks to or from a multitrack recorder:
- Obviously, high recording levels add to that sought-after “overdriven” analog sound; however, driving a track too hard (hot) can actually kill a track’s definition or “air.” The trick is often to find a center balance between the right amount of saturation and downright distortion.
- Noise reduction can be a good thing, but it can also diminish what is thought of as that “classic analog sound.” Newer, wide tape width record- ers (such as ATR Services’ ATR-102 1-inch, two-track and the 108C 2-inch, eight-track recorder), as well as older 2-inch, 16-track recorders, can provide improved definition without the need for noise reduction.
Magnetic Recording And Its Media
At a basic level, an analog audio tape recorder can be thought of as a sound recording device that has the capacity to store audio information onto a magnetizable tape-based medium and then play this information back at a later time.
By definition, analog refers to something that’s “analogous,” similar to or comparable to something else.
An ATR is able to transform an electrical input signal directly into a corresponding magnetic energy that can be stored onto tape in the form of magnetic remnants.
Upon playback, this magnetic energy is then reconverted back into a corresponding electrical signal that can be amplified, mixed, processed and heard.
The recording media itself is composed of several layers of material, each serving a specific function (Figure 2).
The base material that makes up most of a tape’s thickness is often composed of polyester or polyvinyl chloride (PVC), which is a durable polymer that’s physically strong and can withstand a great deal of abuse before being damaged.
Bonded to the PVC base is the all-important layer of magnetic oxide. The molecules of this oxide combine to create some of the smallest known permanent magnets, which are called domains (Figure 3a).
On an unmagnetized tape, the polarities of these domains are randomly oriented over the entire surface of the tape.
The resulting energy force of this random magnetization at the reproduce head is a general cancellation of the combined domain energies, resulting in no signal at the recorder’s output (except for the tape noise that occurs due to the residual domain energy output).
When a signal is recorded, the magnetization from the record head polarizes the individual domains (at varying degrees in positive and negative angular directions) in such a way that their average magnetism produces a much larger combined magnetic flux (Figure 3b).
When the tape is pulled across the play- back head at the same, constant speed at which it was recorded, this alternating magnetic output is then converted back into an alternating signal that can then be amplified and further processed for reproduction.
The Professional Analog ATR
Professional analog ATRs can be found in 2-, 4-, 8-, 16- and 24-track formats. Each configuration is generally best suited to a specific production and postproduction task.
For example, a 2-track ATR is generally used to record the final stereo mix of a project (Figures 4 and 5), whereas 8-, 16- and 24-track machines are obviously used for multitrack recording (Figures 6 and 7).
Although no professional analog machines are currently being manufactured, quite a few decks can be found on the used market in varying degrees of working condition.
Certain recorders (such as the ATR-108C 2-inch, multitrack/mastering recorder) can be switched between tape width and track formats, allowing the machine to be converted to handle a range of multitrack, mixdown and mastering tasks.
The Tape Transport
The process of recording audio onto magnetic tape depends on the transport’s capability to pass the tape across a head path at a constant speed and with a uniform tension.
In simpler words, a recorder must uniformly pass a precise length of tape over the record head within a specific time period (Figure 8).
During playback, this relationship is maintained by again moving the tape across the heads at the same speed, thereby preserving the program’s original pitch, rhythm and duration.
This constant speed and tension movement of the tape across a head’s path is initiated by simply pressing the Play button.
The drive can be disengaged at any time by pressing the Stop button, which applies a simultaneous breaking force to both the left and right reels.
The Fast Forward and Rewind buttons cause the tape to rapidly shuttle in the respective directions in order to locate a specific point.
Initiating either of these modes engages the tape lifters, which raise the tape away from the heads (definitely an ear-saving feature).
Once the play mode has been engaged, pressing the Record button allows audio to be recorded onto any selected track or tracks.
Beyond these basic controls, you might expect to run into several differences between transports (often depending on the machine’s age). For example, older recorders might require that both the Record and Play buttons be simultaneously pressed in order to go into record mode; while others may begin record- ing when the Record button is pressed while already in the Play mode.
On certain older professional transports (particularly those wonderful Ampex decks from the 1950s and 1960s), stopping a fast-moving tape by simply press- ing the Stop button can stretch or destroy a master tape, because the inertia is simply too much for the mechanical brake to deal with.
In such a situation, a procedure known as “rocking” the tape is used to prevent tape damage.
The deck can be rocked to its stop position by engaging the fast-wind mode in the direction opposite the current travel direction until the tape slows down to a reasonable speed … at which point it’s safe to press the Stop button.
In recent decades, tape transport designs have incorporated total transport logic (TTL), which places transport and monitor functions under microprocessor control.
This has a number of distinct advantages in that you can push the Play or Stop buttons while the tape is in fast-wind mode without fear of tape damage.
With TTL, the recorder can sense the tape speed and direction and then automatically rock the transport until the tape can safely be stopped or can slow the tape to a point where the deck can seamlessly slip into play or record mode.
Most modern ATRs are equipped with a shuttle control that enables the tape to be shuttled at various wind speeds in either direction.
This allows a specific cue point to be located by listening to the tape at varying play speeds, or the control can be used to gently and evenly wind the tape onto its reel at a slower speed for long-term storage.
The Edit button (which can be found on certain proffessional machines) often has two operating modes: stop-edit and dump-edit.
If the Edit button is pressed while the transport is in the stop mode, the left and right tape reel brakes are released and the tape sensor is bypassed.
This makes it possible for the tape to be manually rocked back and forth until the edit point is found.
Often, if the Edit button is pressed while in the play mode, the take-up turntable is disengaged and the tape sensor is bypassed.
This allows unwanted sections of tape to be spooled off the machine (and into the trash can) while listening to the material as it’s being dumped during playback.
A safety switch, which is incorporated into all professional transports, initiates the stop mode when it senses the absence of tape along its guide path; thus, the recorder stops automatically at the end of a reel or should the tape accidentally break.
This switch can be built into the tape-tension sensor, or it might exist in the form of a light beam that’s interrupted when tape is present.
Most professional ATRs are equipped with automatic tape counters that accurately read out time in hours, minutes, seconds and sometimes frames (00:00:00:00).
Many of these recorders have digital readout displays that double as tape-speed indicators when in the “varispeed” mode.
This function incorporates a control that lets you vary the tape speed from fixed industry standards.
On many tape transports, this control can be continuously varied over a ±20% range from the 7 1 2 , 15 or 30 ips (inches per second) standard.
Stay tuned for the next part of the series where we’ll discuss cleaning, alignment, archiving, and equalazation.
This article is the first part in a series on the analog tape recorder, excerpted from Huber & Runstein’s book Modern Recording Techniques, Seventh Edition For the second half, click here.
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