What is a Video Tape Recorder? Page 2
By Richard Diehl, ©2011 Labguy's World dot com
The Ampex One Inch Helical Scan Format
Once the transverse recording method, which became generally known as the quadruplex format, was successful, engineers immediately worked at making simpler and less expenisve video tape recording systems. The answer lay in a simpler head scanning assembly. Instead of having the head wheel positioned perpendicular to the tape, the angle was relaxed and the head scanner was made much larger and made to turn much slower. In the above diagram, we see the Ampex one inch format of the mid 1960s. This system uses one video head and a rather large diameter scanner. Note that the tape is wrapped almost, but not quite, 360 degrees aroung the scanner. Observe also that the tape enters the scanner at the lower side and exits at the uper side. This is very important. As the scanner rotates, with the video head protruding along the rim, it prints a track on the tape at a shallow angle of approximetel three degrees. The scanner spins at sixty revolutions per socond, in time with the vertical scanning of the video. For each pass of the video head across tape, one field of video is recorded on tape. Simultaneously, the tape is moving along its longitudinal length at a speed that spaces these video tracks apart in a non-overlapping pattern. Audio is recorded along the bottom edge of the tape in the normal manner. On the upper track, a signal called control track is recorded. This works just like the sprocket holes on film to align the head with the original tracks upon play back.
Prototype Ampex Two Inch Helical Scan Video Tape Recorder circa 1960
Many variations of the helical scan system exist in the same way that there were many versions of records. Like 78s, 45s and 33-1/3 long play LPs. They all worked in a similar manner, yet were all different and mostly incompatible. Incompatibility was the next big headache for VTR manufacturers when they began producing small format video recorders for the industrial and educational markets. The helical scan method persisted well into the 1990s evolving into almost microscopic mechanisms that even recorded digital video signals. Above is a photo of an early Ampex prototype helical scan video tape recorder.
The arrangement of heads on the scanner drum varied greatly. There were systems based on one head, two heads, and even three or more heads. The one head system has the advantage of having the highest writing speed as the rotation rate is determined by the video frame rate as well as the scanner diameter. A one head scanner of a given diameter has twice the writing speed of a two head scanner of the same diameter because each head writes (or reads) on field of video in each pass across tape. So, in general, the more heads, the larger the scanner diameter. One unique system was developed very early on by Sony. It was called the head and a half system. Two heads were used. But, instead of being placed 180 degrees apart, they were spaced only a few degrees apart and rode on different planes. The first head recorded and played video in the normal manner.
Sony Head and a Half system circa 1962
But, as with all one head systems, there is a short loss of signal when the head must pass from one edge of the tape to the other. This is called the vertical drop out because it is placed such that it coincides with the video signal's vertical blanking time and hence should be invisble in playback. But, then elaborate means are required to replacing the small missing bit of video to prevent the TV from jumping, rolling or otherwise being disrupted by the non continuous signal. That is where the half head comes in. It is set such that when the first head is in crossover, this second head is still on tape and records a short track, spaced between the main tracks, that contains the entire vertical portion of the video signal. By precisely switching between these heads at the right moments, the machine produces a continues, uninterrupted video signal with all the advantages of the one head system. This method persisted in all broadcast one inch helical scan machines til they were discontinued in the late 1990s.
What about the consumer market?
Prototype of the Sony CV-2000 home video system circa 1964
That is fine for broadcasters, but what about the consumer side of the business? In 1964, Sony produced the first serious attempt at a consumer video tape recorder called the CV-2000. This system used two head helical scan, with reduced specifications as compared to a broadcast video tape recorder. This, like all VTRs by Sony, was called a Videocorder, a Sony trademarked name. In the photo above, is one of the original prototype CV-2000 Videocorders that was shown at Sony's New York office in Manhattan throughout the mid 1960s. It was rescued from a basement in Long Island and added to the Labguy's World collection in October of 2010.
The Sony CV-2000 home Videocorder, 1967
Following the introduction of Sony's line of videocorders, other Japanese manufacturers began bringing similar machines to market. In only a couple of years, there were at least a half dozen incompatible systems on the market using either half inch or one inch tape. One manufactruer's machine could not play the tape made on another manufacturers machine, even though they both used the same tape! To make things even more confusing, some systems were so poorly manufactured they could not play the recording made on an identical machine. This was not a true reflection of quality control. But, rather an indication of the precision required to make the machines in the first place. But, like all technologies, they got better with each generation of equipment.
The Sony DVK-2400 portable video system circa 1967
1967 and 1968 saw the introduction of the earliest portable video tape recorders. Many of these were record only and were limitied in their functions. A few even used a small hand crank to rewind the tape in the field. Tapes could only be played on a separate compitable VTR deck. These systems were manufactured by Sony, Panasonic, JVC/Nivico (as Craig in the USA), and Shibaden. Each portable VTR and mating camera only worked with its manufacturers' VTRs, but their versitility made them very popular. The arts community embraced the portables with enthusiasm. June Nam Paik and Andy Warhol were early adopters of this technolgy. Groups sprang up across the US, like the Videofreax and Spagheti City (CHECK THESE NAMES) and others.
The Sony AV-3600 EIAJ type 1 compatible Videocorder, 1969
The first breakthrough came in 1969 with the Type 1 standard, commonly know as EIAJ, in defference to the industry association that organized the Japanese manufacturers to achieve interchange compatibility. Sony introduced three compatible models that year. The AV-3400 portable system with full features, like rewind and playback on board. The AV-34600 basic Videocorder deck and the AV-3650 editing videocorder. Only two years later, color versions of these three decks were available as the AV-8400, AV-8600 and AV-8650 respectively. And of course, all the competitors had similar systems too. JVC made the finest of the color portables, but soon blew the entire reel to reel market away with VHS (Video Home System), just after Sony did the same with the Betamax home system. Other cassette systems were introduced just before or at the same time, like V-Cord (Sanyo), VCR (Philips), and EIAJ-2 video cartridge system. But, all of these other systems failed quickly in the market place.
A typical Cartrivision VCR built into a console color TV, 1972
U.S. manufacturers lagged far behind the Japanese at this same time. Many U.S. VTR start up companies failed, almost immediately in many cases, to penetrate the consumer market in any meaningful way. Cartrivision was the most notorious example of this. Ampex did however acheive moderate success with its one inch type A machines which in fact, evolved into the professional one inch type C broadcast format, after a failed partnership with Sony to trade patents. Cartrivision developed their clever system in the late 1960s. Bu, it was too cumbersome and unreliable in the end to be adopted by price concious consumers. Poor marketing, and really bad luck, also spelled the demise of this one and only all American format in the end. The Japanese eventual won the hearts of consumers with two formats almost simultaneously in mid 1970s. The first was Betamax and the second was VHS. But, that is an essay for another time.
The consumer video tape recorder was born slowly and painfully. Many attempts over about fifteen years were made to bring video recording systems to consumers. In 1976, the VCR made the product a household necessity. Where it remained until the early 2000s. Ultimately being superseded by digital technolgies undreamed in the 1970s. Today, our DVRs, camcorders, pen cams, spy cams, iPads and all the neat technology is taken for granted. And in terms of complexity, there is no copmparing them to the dinosaurs on this site. But, knowing abit about the history behind the things we take for granted has never hurt anyone. ~Labguy~
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Created: June 23, 2011, Last updated: July 11, 2011