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Recording video is the same as recording audio. An audio signal is a changing voltage that oscillates anywhere from 20 to 20,000 times per second. To make an audio tape recording, you turn the audio signal into an oscillating current. Pass that current through the head of an audio recorder (with modifications as described in Analog Audio Tape). Pass the tape across the head and you create an audio recording.
A video signal is also an oscillating voltage. The only difference is that to record standard analog video you need to record frequencies up to 4 MHz. Why so high? Let's say your picture is a bunch of closely-spaced vertical stripes where there are 704 stripes across the screen. This is the maximum resolution of NTSC analog video. As the electron beam scans that picture and crosses the bright and dark lines, the voltage produced will oscillate approximately 4,000,000 times per second. There's your 4 MHz.
Let's say we have an audio recorder with a tape speed of 7½ inches per second. This recorder has head gaps that give a high-frequency response of 20 kHz at that speed. If we try to record a frequency as high as 4 MHz, the head will magnetize the tape one way, but the tape won't move the magnetized area of tape out of the way before it is magnetized the opposite way. Any spot on the tape will just be magnetized back and forth a few times, which erases the tape[1]. No information can be recorded.
What we need to do is make the head gap incredibly small. Unfortunately, a smaller gap cannot impart as much magnetic flux to the tape, so the recording will not be strong enough to overcome tape and electronic noise. The only workable solution is to move the tap past the head at a faster speed. Now a spot is magnetized and is moved away from the gap before the magnetic polarity changes. To record NTSC video, all we have to do is move the tape at 11 meters per second past a 6-micrometer gap, iron out a few kinks in the system and we have a working video tape recorder.
As you can see, it will take an incredible amount of tape to record any reasonable length of time at that speed.
The solution is to move the head past the tape instead of moving the tape past the head. Put the head on a drum and spin that drum at 3,600 RPM (60 revolutions per second). Now, wrap the tape around the drum at an angle so that the head makes a diagonal track across the tape[2] that is about 200 mm long (62 mm diameter head and ½-inch tape).
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| A VHS drum. Notice it is canted so that the tape wraps around it at an angle |
Now we can record one whole field of video without moving the tape at all. Next, we move the tape so that when the head comes around again, it tracks across fresh tape to record the next field. With thin-enough heads (and a couple of other tricks), moving the tape at about 33.3 mm per second keeps the tracks sufficiently-separated to record NTSC video at reasonable quality[3].
Audio is typically recorded as with any audio recorder along the edge of the tape. The path of the tape around the drum is designed to leave space for that purpose.
Sony (Beta) and RCA (VHS) eventually developed different methods of hacking the audio signal into the video signal, providing high-fidelity stereo sound.
One of the tricks to keep the tracks as close to each other as possible is to use two heads on the drum. Each head records one field of video; one head records the odd scan lines and the other records the even lines[4]. The heads are angled to each other so that the orientation of the magnetic fields on the tape are at a right angle to each other. This allows the tracks to overlap significantly, yet the heads can separate the information.
To get the heads to track across the magnetic tracks on the tape, the heads have to move at just the right speed and the tape has to move at just the right speed. When the tape is stopped, such as to freeze the picture, the angles don't line up as they do when the tape is moving. The heads will cross from one track to the next. This causes a bit of visual noise seen as a horizontal band of snow on the screen. Four-head recorders have an extra pair of heads. The extra heads are aligned such that they cross the tracks at a different time than the main heads. Electronic switching between the heads eliminates the noise when the heads cross the tracks. This allows freeze frame and slow motion with clean images.
Standard erase heads lie at a right angle to the tape. When they erase, they start at the end of one track but across the middle of other tracks. This leaves bits of old tracks on the tape until about 200 mm of tape has passed the erase head.
Let's say you have a tape with one episode of Wheel of Fortune followed by an episode of Jeopardy. The next day you want to record over the episode of Jeopardy with a new episode of Wheel of Fortune. You line up the tape and start the recording at just the right time. When you play back the tape, right when the new recording starts you will see some color noise slowly move down the screen as the new show starts. This is the partially-erased tracks from the previous recording.
A flying erase head is on the drum with the other heads. This head can erase a single pair of video tracks. A recorder with a flying erase head will cleanly erase the tracks individually, making a clean transition to the new show.
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