What is Magnetic Tape and How Does it Store Data?
Magnetic tape is a storage medium consisting of a thin magnetizable coating on a long, narrow strip of plastic film. It is used for magnetic recording of analog audio, digital data, and video. Data is stored by magnetizing the tape coating in a pattern that represents the audio, video, or digital information.
The key advantages of magnetic tape include:
– High storage density
– Low cost per bit of data stored
– Non-volatile storage (data remains without power)
– Can store data for 30+ years
A typical magnetic tape cartridge used for data backup today can store anywhere from a few hundred gigabytes to 30+ terabytes of uncompressed data on a single cartridge. LTO-9 tapes, released in 2021, have a native data capacity of 18 TB per cartridge.
| Tape Format | Release Year | Native Capacity |
|---|---|---|
| LTO-5 | 2010 | 1.5 TB |
| LTO-6 | 2012 | 2.5 TB |
| LTO-7 | 2015 | 6.0 TB |
| LTO-8 | 2017 | 12 TB |
| LTO-9 | 2021 | 18 TB |
How Magnetic Tape Recording Works
During the recording process, the tape is moved past a recording head at a constant speed. The recording head generates a magnetic field that magnetizes the tape particles, aligning their magnetic domains in a pattern that represents the data being recorded.
To play back the recorded data, the process is essentially reversed. The magnetized tape is moved past a playback head, which detects the varying magnetic field from the tape and converts it back into an electrical signal representing the original recorded data.
Key components in a typical magnetic tape drive include:
– Supply reel to hold the tape cartridge
– Takeup reel to collect tape as it passes the heads
– Magnetic read/write heads to record and read data
– Tape path guides and tension arms
– Capstan and pinch rollers to control tape speed
– Erase head to erase old data
The Role of the Erase Head
Before new data can be recorded onto a section of magnetic tape, any old data already on that section needs to be erased. Otherwise, the new and old magnetic patterns would interfere with each other. This is the job of the erase head.
The erase head is an electromagnet that applies a high-frequency alternating magnetic field to the tape, which demagnetizes it and removes any previously recorded signal. It is positioned before the read/write heads in the tape path.
There are two main types of erase heads:
1. AC erase heads
2. Permanent magnet erase heads
AC Erase Heads
AC erase heads use an alternating current to generate the magnetic field that demagnetizes the tape. The AC current alternates the polarity of the magnetic field at a high frequency, typically in the range of 50 kHz to 200 kHz.
Advantages of AC erase heads include:
– Thorough erasure of old data
– No physical contact with tape required
– Can be optimized for different tape types
Disadvantages include:
– Require an AC bias signal which adds complexity
– Consumes more power than permanent magnet heads
– Slight noise added to tape from erase process
Permanent Magnet Erase Heads
Permanent magnet erase heads use a strong permanent magnet, rather than an electromagnet, to demagnetize the tape. As the tape passes by the permanent magnet, its strong fixed magnetic field randomizes the magnetic domains on the tape, effectively erasing any previously recorded data.
Advantages of permanent magnet erase heads include:
– Simpler design with no AC bias required
– Lower power consumption
– No added noise
Disadvantages include:
– May not erase as thoroughly as AC heads
– Makes physical contact with tape which can cause wear
– Fixed field strength cannot be optimized for tape type
Erase Head Configurations
Erase heads can be configured in different ways relative to the read/write heads in a tape drive. The two most common configurations are:
- Fixed erase head before read/write heads
- Movable erase head that can be positioned after read/write heads
Fixed Erase Head Before Read/Write Heads
In this configuration, the erase head is in a fixed position before the read/write heads in the tape path. As tape moves forward, it passes the erase head first, which demagnetizes it, then passes the write head which records the new data.
This allows data to always be written to freshly erased tape, preventing any interference from old data. It is a simple, reliable configuration used by many tape drive designs.
However, it has the drawback that data cannot be erased after being written, as the tape has already passed the erase head. To erase recorded data, the tape must be rewound past the erase head again.
Movable Erase Head
Some tape drives use a movable erase head that can be repositioned via a solenoid to either before or after the read/write heads. This allows erasing the tape either before writing new data, or after reading to securely erase sensitive data that was just read.
When the erase head is moved after the read/write heads, this configuration is sometimes referred to as a “read-after-write” or “RAW” head configuration. Data is securely erased immediately after being read.
Advantages of a movable erase head include:
– Ability to erase after reading or before writing
– Secure erasure of sensitive data
– Reduces need to rewind tape
Disadvantages include:
– More complex head mechanism
– Possible head positioning errors
– Slower than fixed head design
Tunnel Erase Heads for Higher Density
As tape drive manufacturers strive to increase the amount of data that can be stored on a tape, the data tracks written onto the tape become narrower and closer together. To achieve clean erasure of these high-density narrow tracks without affecting adjacent tracks, a special type of erase head called a tunnel erase head is used.
A tunnel erase head has a structure with a deep, narrow gap that concentrates the magnetic flux onto a small area of the tape. This allows it to thoroughly erase a narrow track without demagnetizing the tracks on either side. It gets its name because the tape essentially moves through a “tunnel” in the head structure.
Tunnel erase heads are used in high-end, high-density tape drives to enable recording of many parallel data tracks on the tape with minimal interference between tracks. They are more expensive and complex to manufacture than traditional erase heads.
Conclusion
The Magnetic tape erase head plays a crucial role in the recording process by demagnetizing the tape prior to writing, ensuring new data is written to a clean slate. Erase head design has evolved to keep pace with increasing tape densities and security requirements.
While AC bias erase heads were most common in early tape drives, modern high-density drives often use permanent magnet tunnel erase heads for their simplicity and ability to erase narrow tracks. Movable erase heads provide the flexibility to erase before or after writing/reading as needed.
Understanding the function and types of erase heads is important for designing reliable, secure, high-performance tape storage systems. As tape densities continue to increase, erase head technology will no doubt continue to advance as well.
Frequently Asked Questions (FAQ)
What is the difference between an AC erase head and a permanent magnet erase head?
An AC erase head uses an alternating current to generate the magnetic field that erases the tape, while a permanent magnet erase head uses a strong fixed magnet. AC heads are more thorough but complex, while permanent magnet heads are simpler but make physical contact with the tape.
What is a tunnel erase head?
A tunnel erase head is a special type of permanent magnet erase head with a deep, narrow gap structure that focuses the magnetic field onto a small area of tape. This allows erasing narrow tracks on high-density tapes without affecting adjacent tracks. The tape passes through the “tunnel” in the head.
Why are erase heads necessary in tape drives?
Erase heads are necessary to demagnetize the tape before new data is written. This ensures the new data is not corrupted by any old magnetic patterns already present on the tape. They create a clean slate for the write head to record the new data.
How does a movable erase head provide security benefits?
A movable erase head can be positioned after the read head to immediately erase sensitive data from the tape once it has been read. This provides a secure way to destroy data and prevent it from being read again or recovered later.
What are the advantages of magnetic tape for data storage?
Magnetic tape offers high storage density, low cost per bit, non-volatile data retention, and a long archival life compared to other storage mediums like hard drives. A single modern tape cartridge can hold up to 30TB of uncompressed data and retain it safely for 30 years or more. Tape remains popular for backup and archival storage.
