Background: Magnetic Recording |
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Magnetic recording, originally invented by Valdemar Poulsen in 1898, had been developed and refined in Germany before and especially during the Second World War. It works by converting the electronic signal to be recorded into a varying magnetic field, and causing this field to be "printed" into a magnetic material.
The electronic signal is converted into magnetism using a device called a recording head, which is essentially a coil of wire wrapped around an iron core. Whenever a varying current flows through a coil a magnetic field is produced, and the core intensifies this field by focusing it along itself.
Now, the core contains a tiny gap, and this causes
the magnetic field to "spill out" beyond the
core itself. If a magnetic material is pulled past this
gap while the signal is flowing through the coil, some of
the "spilt" magnetic field affects the material
and it becomes magnetised itself. If the recorded tape is rewound and moved past the head once more, the magnetic field held by the particles in the tape's coating induces a tiny electric current back into the coil, and this can be amplified back into a reasonable copy of the original signal. |
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The first working magnetic recorders used steel wire, but by 1936 proper tape had been developed. Tape is simply a flexible plastic strip with a magnetic material coated onto its surface. This coating is usually made up of magnetic particles suspended in an inert binder. A tape will hold a recording indefinitely, unless another magnetic field erases or overwrites it - or until the tape itself disintegrates with age.
Why recording TV is impossible
The very high frequencies used in a video signal - up to 5.5MHz - mean that the tape must be moved past the recording heads at very high speed. This is because the coating of a tape is made up of particles, and while different particles can be magnetised by different amounts, the strength of the magnetic field is constant within each particle. So in order to record a high frequency signal, the particles must be moved past the heads at least as fast as one particle per cycle, and preferably much faster. Now, in the 50's, the coating used on tapes was basically a powder made of ground rust, and this had very coarse particles by today's standards. This meant that tape speeds had to be high, and consequently tapes didn't play for very long.
There is also a second, more fundamental problem. The strength of a magnetic recording is directly related to the frequency of the signal - the higher the frequency, the stronger the signal induced in the head when played back.
For audio, where the highest frequencies are about 1000 times as high as the lowest (20Hz to 20,000Hz), this problem can be overcome by "biasing" the replayed signals to re-balance them - decreasing the strength of the high frequencies, and boosting the low frequencies. But video requires a frequency ratio, or dynamic range, of more than 100,000 times (50Hz to 5.5MHz); this means that if the highest frequency in the signal is recorded at a reasonable level, the lowest frequencies will be lost in the background hiss of the tape, while on the other hand if the low frequencies are recorded properly the highest ones will "overload " the tape, and be distorted out of all recognition.
There are two ways to address this problem. Firstly, the bandwidth of the recorded signal can be reduced, by ignoring the highest frequencies. This reduces the detail in the picture, which is acceptable for some applications, like closed-circuit security cameras, and many VTRs of the sixties were limited to 2.5MHz. But this is no use for broadcast TV, of course.
The second way to reduce the dynamic range is to modulate the signal onto a carrier of very high frequency. Since the frequency variations are then small when compared to the carrier frequency, the relative dynamic range is much lower, and if the recorder is set up to record the carrier well, the entire signal will be captured properly.
Of course, a high carrier frequency means an even higher tape speed. Early experiments with linear recording required a speed of about 40 metres per second, which is 150 kilometres (90 miles) an hour. These machines were not really practical - partly due to the sheer danger of being anywhere near them while they ran!
Tune On, Turn On, Drop Out
A common problem with magnetic tape recorders is drop-out. A drop-out is a flaw or damaged spot on the magnetic surface of the tape, which causes a momentary loss of signal. For audio, this may go entirely unnoticed, but on a video recording appears as a bright white dash or line on the picture.
Towards the end of the sixties, drop-out compensators began to appear on top-of-the-range VTRs. These work by storing the last picture line in a buffer; if the strength of the current line drops below a threshhold - implying that the signal has been lost as the head scans a drop-out - the machine switches to the stored line to replace the missing part. Since adjacent lines are in general nearly identical, this is usually an invisible repair.
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