Video recorder
Video recorders (abbreviated VCR from English Video Cassette Recorder ) are devices for recording and playing back audio and video signals (usually television images) on magnetizable tapes.
technology
mechanics
Mechanical record
Even in the early days of television , hobbyists tried to record the programs. Some managed to do this with the help of modified records . The low bandwidth that the former mechanical television required could be recorded this way. Some recordings used even narrower formats, such as 30 lines at 4 frames per second.
In the 1970s, the Telefunken company started a new attempt at mechanical recording, see image plate .
Magnetic recording
With the advent of wire recorders and tape recorders , attempts have also been made to record television images on magnetic tapes.
- Linear recording
- The British Broadcasting Corporation's attempt was called VERA and used two tracks to record a 405-line television signal. The frequency spectrum was divided into two parts, which were distributed to both tracks. At the same time, attempts were made in the USA to distribute the frequency spectrum over twelve tracks.
- The problem was that the high bandwidth led to very small "band wavelengths". Very high tape speeds had to be achieved to make these wavelengths long enough. Distributing the signal to multiple tracks partially solved this problem, but new problems of an electronic nature appeared. It proved difficult to split the signals and then put them back together again later.
- Analog hard disk recorders
- Hard disk recorders are a subtype of linear recording . Here the signal is recorded from a standing head onto a round, rotating disk. As a rule, the disc rotates once per field or frame. The head can usually also be moved in or out step by step in order to record several circles (tracks) with independent images. The advantages of this technique are the quick access to any image on the plate. Multiple disks can speed up this access.
- Hard disk recorders could record between half a minute and about 30 minutes, depending on their size. Smaller systems were used for slow motion in sports broadcasts. Larger systems could even take on typical offline editing tasks in the 1970s .
- In the photo sector, this technique was used with still video cameras . There, one image was recorded on a 3-inch floppy disk per revolution . Laser discs use similar techniques.
- Modern hard disk recorders have nothing in common with this technology (apart from the rotating magnetic disks).
- Other recording movements
- The problem of impractical high tape speeds has been solved by not only moving the tape but also rotating the recording head by mounting it on a cylinder and rotating it. For the recording, the linear tape speed was no longer important, but the relative speed between the tape and the head. This could be orders of magnitude greater than the tape speed, which led to recordable “tape wavelengths” even for video frequencies.
- Arcuate scan
- One of the first methods was introduced by the Ampex company in the early 1950s. Some experimental recorders had the heads mounted on the face of a rotating cylinder. The tape was then passed over this area with the heads, so that sickle-shaped tracks were created on the tape. This procedure did not work very well because it was not possible to make secure tape contact.
- Transversal Scan ( cross track recording )
- Another method mounted (mostly) four heads on the surface of a cylinder, the axis of rotation of which was parallel to the direction of travel of the tape. The tape was nestled against this cylinder, arching it transversely to the direction of travel of the tape. The tracks are almost perpendicular to the direction of travel of the tape. A track has a length that is just about the width of the tape. Since this was too short to be able to record an entire field, each field was usually divided into 6 to 20 tracks. These had to be reassembled neatly during scanning by a TBC, otherwise this would lead to regular interference bands or interference strips. This procedure worked quite well after initial problems had been solved and was used from 1956 for the quadruplex format and the BNC format.
- Helical Scan (helical scan recording )
- This method was most common on later VCRs. In contrast to the transversal scan, the cylinder's axis of rotation is almost perpendicular to the direction of travel of the belt. The resulting tracks are inclined only a few degrees against the direction of the tape. Together with a much larger head drum than with the transversal scan, this enables longer tracks that can record half or even full images, so that the head switch can be invisibly placed in the vertical blanking interval of the image signal. This also made freeze frame, slow motion and picture search possible. The heads mounted on the head drum have different azimuths, which leads to less crosstalk between adjacent tracks.
electronics
Modulation method
Video signals are much broader than audio signals. Both DC voltage and signals with a frequency of several megahertz occur. Such signals cannot be recorded magnetically directly (in the baseband), but have to be modulated onto a carrier (carrier tape).
- Amplitude modulation
- Early recorders tried to solve this problem with the help of amplitude modulation. The slightest fluctuations in the contact between the head and the tape led to fluctuations in the signal strength, which led to fluctuations in image brightness and contrast . Automatic readjustment turned out to be very difficult.
- Frequency modulation
- Charles Anderson came up with the idea of recording the signal in a frequency-modulated manner around 1954. This modulation method is largely insensitive to fluctuating signal levels, improves the signal-to-noise ratio, especially for the clearly visible lower video frequencies, and is still used today.
Color recording
Color television was new when the first VCRs came out. Therefore the color was ignored for now.
- Direct color
- Later, the already existing high bandwidth of the video recorder was used to record the entire television signal including the color signal. The problem with this was playback. Mechanical tolerances made the heads slide somewhat unevenly, resulting in a "jerking movement". This led to a phase shift in the high-frequency components. It was barely noticeable on black-and-white television, but it led to significant color errors and, in unfavorable cases, to total failure of the ink carrier. This is why so-called “timebase correctors” were built in. Initially, these were circuits made up of varactor diodes and coils, which could delay the signal in a small range over time. This procedure was later supplemented with a series of delay lines that were switched on or off depending on the desired delay. In the 1970s, this analog process was replaced by digital storage.
- Direct color recording was also used on laser discs .
- Reduced color under
- Since direct color required very high-quality mechanical components and a (at that time) more expensive timebase corrector, they looked for ways to produce color more cheaply. A simple possibility was to reduce the frequency of the color subcarrier (here red). Typically, its frequency is then:
- VHS , S-VHS , VHS-C : 627 kHz
- Video 2000 : 625 kHz
- Video 8 : 743 kHz
- It is recorded directly on the tape without being modulated again. The low frequency prevents interference from the brightness signal , which continues to be frequency-modulated.
- However, such low frequencies tend to crosstalk, which is why very densely packed formats such as VHS always have integrated noise reduction to minimize the color noise. With some devices this leads to a “bleeding” of the colors.
- Some formats (e.g. S-VHS ) use special additional heads for this color signal.
- Virtually all analog color home video recorder systems use this technique.
- Sequential color recording
- In some professional formats such as Betacam / Betacam SP or M and M-II format, the two color difference signals are recorded one after the other. For this purpose, both signals are pushed into a BBD memory and then read out again twice as fast and recorded one after the other on a track, compressed in time. This achieves a higher image quality without mutual interference between color and brightness signals.
Sound recording
- Longitudinal track tone
- Originally, the sound was recorded on separate linear tracks on the edge of the tape, similar to audio tapes . However, because of the generally lower linear tape speed in video recorders compared to tape recorders, this sets limits on the sound quality. Until the 1980s, however, this method was used exclusively in professional recorders, as the tape speed is higher here. The main advantage is that each individual track can be dubbed separately. Usually one to three longitudinal tracks are used for the sound.
- Helical ( Hi-Fi ) sound
- Later, the sound was recorded frequency-modulated on several frequencies in addition to the image signals in the helical track, mostly with special "Hi-Fi" heads with a larger head gap, for example 0.9 µm compared to the video heads with 0.3 µm (on VHS) .
- As a rule, one FM-modulated carrier per audio channel is recorded. With VHS 1.4 MHz for the left and 1.8 MHz for the right channel.
- The frequency response is greatly improved by using frequency modulation . However, dubbing is not possible with this recording process; this must be done on the additional audio longitudinal tracks (with limited quality).
- The video signal and stereo sound are written on the same track, with the stereo sound being recorded before the video signal. Due to the larger head gap of the audio head, the sound is magnetized deeper into the tape than the subsequent video signal. Excessive crosstalk between the two signals is prevented by the different azimuth angles of the heads (with VHS: −6 ° for video head 1 and + 30 ° for audio head 1).
- However, the head switch then has to work very precisely because, in contrast to the image signal, there are no blanking gaps in the audio signal in which the switch could be "hidden". Hi-Fi VHS recorders always record a linear audio signal in addition to the hi-fi audio signal so that the cassettes can also be played on VHS recorders without hi-fi capability.
- In systems such as Video 8 , hi-fi sound is a mandatory system component because it significantly simplifies the mechanics. The separate audio heads outside of the video head are saved here . This also makes the drive significantly smaller.
- Digital ( PCM ) sound
- In some formats, digital sound is also defined. This is usually recorded in PCM-coded form on the helical tracks.
Timecode process
When cutting ribbons, it makes sense to give each picture an individual number so that you can find it again.
- Splotches of color
- Before real timecodes existed, a very simple procedure was used. The editor played the tape and, at the right time, put a splash of color on the back of the tape. The tape was later cut at this point. Of course, this only works in direct cut and is quite imprecise.
- Longitudinal track time code
- Later, with the advent of digital technology , the time code signal was recorded as a sequence of tones on one of the audio tracks. The procedures were usually designed in such a way that reading at increased speed is also possible. In addition, the timecode can easily be changed afterwards.
- Vertical interval timecode
- You can also record the timecode in the vertical blanking interval; then it can also be read in the still image mode, and it is part of the image, which means that it can be transmitted over all conventional transmission links at no additional cost.
Digital systems
Digital video recorder systems use different methods to encode video and audio signals. If, for example, the analog signal is sampled with four times the color subcarrier frequency in the pulse code modulation process (PCM), this is known as the “composite” process. Often, however, the signal is split up into RGB or the color difference signals before coding . These signals are then usually processed further as PCM signals.
The PCM signals were originally recorded without data compression . This resulted in good quality, but required high recording speeds and large amounts of data to be handled. For your video recording in Full HD mode with a picture height H of 1080 lines, a picture width B of 1920 columns, a picture frequency F of 25 full pictures per second and a color depth T of 8-bit for each of the three primary colors (the three color channels red , green and blue: K = 3) there is a data transfer rate R of over one gigabit per second:
- R = B * H * F * K * T
- = 1920 x 1080 x 25 Hz x 3 x 8 bits
- = 1244 GBit / s = 155.5 MByte / s
Computers later became so powerful that data compression and decompression could be performed in real time . This made it possible to reduce the data rates and data volumes by a factor of 100 with almost no concessions to quality, since the redundant recording of several images of a film that differ little from one another is no longer necessary and single-colored areas do not need to be saved with full resolution.
The digital tape recorder systems are currently being replaced by hard disk recorders and digital audio recorders with permanent storage because the acquisition and maintenance costs are lower than for tape devices. Hard disk recording, but above all permanent storage recording, is also characterized by short access times and low wear.
Recording of data that is already available digitally
Data that is delivered to the user in digital form via the Internet or television reception can be stored directly on digital data storage devices , such as hard drives , memory cards or USB sticks , without transcoding . This makes it possible to record video streams or television programs without any loss of quality and to consume them as often as desired and with any number of interruptions or omissions (e.g. when skipping advertising blocks ).
Transition from decks to video recorders
In 1965 (when the price of a professional MAZ system was 250,000–300,000 DM ; today's purchasing power around 560,000 euros ), Philips presented a video recorder with magnetic tape on reels that cost just under 7,000 DM (around 14,200 euros today). The device was not intended for home use, but for (semi-) professional use (e.g. recording of operations in the hospital or rehearsals of actors , control of movement sequences in competitive sports , school television ).
Home video recorder
The first home video recorders hit the market in the early 1960s. An example of an early device was the Loewe Optacord 500 , which was presented to the public at the 1961 radio exhibition . The musicians John Lennon and Paul McCartney were among the most prominent owners of these early home video recorders, which at that time were virtually unknown for the home sector, from around 1965 onwards , who considered their devices to be prototypes of a technology still officially in development that had not yet gone into series production.
In the 1970s, the VCR formats (with the offshoots VCR Longplay and SVR ) and Video 2000 by Philips and Grundig were developed in Europe . However, these European formats found it difficult to establish themselves in the United States and Japan.
European developments were soon harassed by two Japanese systems: Betamax from Sony and VHS from JVC . The first recorder with the VHS recording format was the HR-3300 from JVC, which was introduced in autumn 1977.
Betamax offered a slightly better picture quality and better tape running properties than VHS, but it failed because the licensor Sony took levies from other manufacturers for the production of Betamax devices. Another reason was the playing time. Due to the larger housing of the VHS cassette, it was possible to record for two hours even when it was launched, which was enough for a feature film or a football game. Betamax initially only allowed one hour of game time. By the end of the 1980s, VHS had completely ousted the competing systems from the retail business, mainly due to a more clever licensing policy. This battle for market share, ultimately won by VHS, became known as the format war . It was repeated in a similar form from around 2005 as a competition between the formats HD DVD , VMD and Blu-ray Disc . All of them were developed as the successor to DVD, and Blu-ray eventually caught on.
Further formats, including professional ones, can be found in the article List of video tape formats .
In 1979 about 270,000 devices were sold in the Federal Republic of Germany. In 1981 there were already around 750,000 and in 1983 1.4 million. In 1985 there was a video device in around 7 million apartments, and thus in every fourth household.
The introduction of DVD as a playback medium and, since 2000, increasingly also as a recording medium for private users, is increasingly pushing tape-based video devices back. However, since many users still want to continue using their analog recordings, VHS recorders are still available to buy. Some of these devices also combine the various recording technologies (mostly a VHS drive and a DVD recorder in one device) and thus offer an uncomplicated way of copying from one format to another.
On July 22, 2016, Funai Electric , the last remaining video recorder manufacturer, announced that it would finally cease video equipment production this month. This is the end of a decade-long era.
Structure of a VHS drive with description
VCR drive from above
- Here is u. a. the head amplifier, which amplifies the FM signals coming from the video and audio heads, if present .
- In front you can see the head disk with the video and in the present case also audio heads. The top disk can be rotated and is driven by an electronically commutated external or disk rotor motor located on the same shaft. The head disk contains a rotary transformer consisting of concentric ferrite cores for each head ; the information is transmitted inductively via an air gap.
Possible number of heads on the backplate:- 2 heads : two video heads for standard play or combined for standard play and long play with mono longitudinal track sound
- 4 heads : two video heads for standard play or combined for standard play and long play and two rotating audio heads for hi-fi stereo sound. If dubbing, this is only in mono
- 6 heads : four video heads for standard and long play, as well as better still picture and two audio heads for hi-fi stereo; vertical erase head, therefore dubbing only in mono.
- 7 heads : four video heads for standard and long play, two audio heads for hi-fi stereo and a rotating erase head, better for video editing plus "standing" erase head, dubbing in mono
- The shaft motor has the task of pulling in the cassette and pulling the tape out of the cassette (using various "fingers" and additional mechanics) and looping it around the rotating drum. In addition, it takes over the control between the operating modes of the video recorder (play, fast forward and reverse, etc.). The so-called mod switch is used for this.
- The erase head erases all information on the tape when it is re-recorded.
- As with tape recorders, the pressure roller transports the tape by pressing it against a shaft.
- The sound head records the mono sound and plays it. The synchronization head (also called CTL head) is also built in. It is used to record sync pulses and thus to control the current angle of rotation of the head wheel with respect to the tape feed during playback. Next to the sound and CTL head is the erase head for the mono sound track (black), which is required for dubbing and video editing.
- The so-called capstan (capstan wave, also called capstan in tape recorders) is driven by a tape servo (not shown here). He transports the tape. After threading the tape, the pressure roller presses the tape against the capstan so that it can drive the tape.
- IR sensor to detect whether a cassette is inserted. 18 is the associated station.
- and 10. The pulleys wrap the tape around the head drum. They are height adjustable to adjust tracking. The tape is placed around the head drum so that it covers about half the drum circumference. The tape runs from above in the shape of an 'M', which is why the process is also called M-Loading .
- The supply mandrel picks up a reel of tape from the video cassette. While rewinding, it is driven by the tape servo.
- The take-up mandrel takes up the second reel of tape from the video cassette. During the playback of the tape and during the advance, this mandrel is also driven by the tape servo (capstan motor).
- Guide grooves of the cassette slot.
- A guide pin of the cassette slot.
- This plastic tab unlocks the cassette door that protects the tape.
- This lever pushes the 15 forward to unlock the cassette flap when the cassette is inserted. When the cassette goes down, this lever opens the cassette door.
- The cassette slot accepts the cassette.
- IR sensor to detect whether a cassette is inserted. It is mounted on a plastic pin. This plastic pin unlocks the tape reels on the video cassette.
- Depending on the direction of rotation of the band servo, this lever switches between winding or unwinding mandrel.
VCR drive from below
- This switch starts and stops the chute motor when a cassette is inserted or ejected.
- Gear for the cassette slot, it is similarly available on the other side of the cassette slot.
- Sensor for measuring the speed of the belt servo.
- Band servo rotor (capstan motor). Under this rotor there are coils arranged in a star shape. The motor is electronically commutated and speed controlled. The servo is connected to a switching lever via a belt, which switches the drive to the winding or unwinding mandrel.
- The head drum servo, like the band servo, is an electronically commutated external rotor motor . He drives the head disk.
- Gear for pulleys and bolts that wrap the belt around the head drum.
- A bolt and pulley from below.
- Brake lever for the band servo. This brakes the Capstan shaft and the flywheel connected to it, so that no tape loops occur when playback or search advance is interrupted.
Insertion and ejection of a VHS cassette
- The video recorder drive (a VHS drive in the picture ) has the task of receiving and pulling in the video cassette, threading and transporting the tape. High precision is necessary here. This applies in particular to the track position of the tape ( tracking ) and the tape transport by the tape servo motor that drives the capstan and the head drum motor (head drum servo ) that drives the head disk with the video heads. The speeds of all servomotors are controlled by a microcontroller .
See also
Web links
Individual evidence
- ↑ Markus Bautsch: Storage Requirements, Wikibook: Digital Imaging Processes , Chapter: Digital_Images , accessed on July 17, 2014
- ↑ Recordings via USB: Einfach preiswert , test.de , December 20, 2010, accessed on July 17, 2014
- ↑ a b These figures were based on the template: Inflation determined, have been rounded to a full ten thousand or one hundred EUR and refer to January 2020.
- ↑ alpha-retro: alpha-retro: 1965 - The record "The technical report". In: alpha-retro (series on ARD-alpha ). Retrieved on August 28, 2019 (the information cited is only contained in the film itself, not in the table of contents; the second part of the film reports on the production and use of magnetic tapes for sound and image recording).
- ↑ Jürgen Kniep: “No youth release!” Film censorship in West Germany. 1949-1990. Wallstein-Verlag, Göttingen 2010, ISBN 978-3-8353-0638-7 , p. 288.
- ↑ Last Japanese manufacturer of VHS recorders discontinues production Heise online, July 22, 2016