1969 EIA(J) color under on 1/2" tape reels. The principal fault with helical scan was color, or rather the lack of color. The trick of just recording NTSC composite video did not work for slant tracks. One of the secrets that made video recording possible is the video signal is modulated on a carrier frequency, very much like when it is transmitted. On playback this is "received" and demodulated, very much like how your TV set works.
This is important because the frequency response of magnetic recording tapers off at higher frequencies. When FM modulated, duplicate sidebands of information occur both above and below the carrier frequency. On reception, the demodulator only needs one sideband to recover the video, but if both are present, they combine. (This factor is also used in broadcasting, TV video is 4.2 MHz which when modulated creates 2 sidebands for 8.4 MHz total bandwidth. But TV channels are packed together with only 6 MHz spacing, so 75% of the lower sideband is not transmitted.) The lower sideband is mostly recovered and its tapering response is compensated by adding to it what is left of the upper sideband, which levels out the overall playback response.
To get color out of quad, the FM carrier frequency was shifted up slightly from the original monochrome position of 5MHz to 7.9 MHz to eliminate a "herring-bone" pattern in the picture.
Tape speed determines the video frame rate in both quad and helical scan, the edit pulses on the control track are used to precisely time the capstan. But the only thing constant in the universe is change. It is the nature of a servo to always be slightly adjusting, faster/slower. In quad, these speed bumps are inconsequential to the pickup of video which is transverse to the tape motion, but in slant track, they contribute a lot of jitter, which fouls up the FM modulation scheme. For monochrome, this just makes picture details a little blurry, but color with the slightest phase jitter becomes undecipherable. This is comparable to how multipath spoils FM stereo reception in town, expecially when driving in traffic, and this is a case where plain old AM holds up swell.
Looking back at the diagram, there is a bench of low frequency where response is constant, up to about 1.5 MHz. This is just enough bandwidth to support color information. So the color is stripped from the NTSC composite where it exists as an AM modulated subcarrier at 3.58 MHz and downconverted to the center of this subband, or 629 KHz in the case of VHS, and recorded directly. The luminence is still FM modulated at its higher carrier frequency. Chart below shows color under frequencies for several popular formats.
*format requires metal tape
System Format/year AM Color Carrier FM Luminence Carrier Quad/1958 direct 7.9 MHz EIA(J)/1969 767 KHz 4 MHz U-matic/1971 688 KHz 4.5 MHz Betamax/1975 688 KHz 4 MHz VHS/1976 629 KHz 4 MHz Video 8 */1981 743 KHz 5 MHz S-VHS/1987 629 KHz 6 MHz Hi8 */1989 743 KHz 7 MHz
Armed with JVC's color under invention and Sony's experience in tape transport, there was just one more detail that blocked marketing. SMPTE already proved it would turn a deaf ear, so these clever Japanese fromed their very own group, EIA(J), based on the American EIA manufacturer's trade organization. And these standards were no joke. A large number of manufacturers (but not Ampex) eventually made EIA(J) videotape decks, and they all could interchange tapes. I had both Panasonic and Concord models and never had difficulty playing tapes in color. Besides low cost and tape interchange, EIA(J) also had another brand new benefit. The complete deck was the size of a suitcase (or the typical reel audio recorder) and was portable. Sure it still ran off AC power, and portable cameras were mostly B/W but for the first time television could be produced outside the studio.
These were intended for industrial use, no tuners/timers. Philips brought out the compact audio cassette in 1963, and proved without doubt to be a winner with the consumers. By this time no one expected consumers to embrace an open reel format for videotape either. Allied with Technicolor in the US, Akai brought out a quarter inch videotape in a cassette. As a tape format, it had everything right - small compact size, easy to handle tape cassette, color, tv tuner - and it totally flopped. The problem was not the format, but the TV set. Akai knew about the problem and tried to solve it by including a 4" color monitor with the VCR. But Americans wanted to watch TV at home on a Big screen, although a few travelling sales people and training departments bought them for the portable features.
next on history of videotape, 1971 The U-matic tape VCR (Video Cassette Recorder) system