About servo systems
We have figured out how to record and play back the video signal. Now, we need to be able to find it on the tape. This is a job for the servo system. The servo system, at minimun, consists of a drum servo that controls video headwheel speed, and a capstan servo that controls linear tape speed.
In order to play back a scanned track on a piece of tape, we have to lay the track down at the right place on the tape during recording. The actual track path is determined by mechanical guiding of the tape past the head. The speed of the video headwheel must be held constant at the correct speed to produce consistent, repeatable tracks, and to preserve timing accuracy of the recorded sync information. (Sync, or synchronizing information in video is pulses present in the signal to denote the beginning of lines or fields. The timing accuracy of sync is very critical, as we shall see later.) The speed of the tape past the heads must be held constant to keep the tracks at the correct angle, and allow the video heads to scan down the middle of the tracks during playback. The servos serve to keep everything constant at the correct speeds during recording. The servo system also generates a signal that is recorded on a longitudnal track on the tape, called the control track. The control track serves as a refrence for the servos on playback, so the tracks can be found on the tape.
During playback, the servos are much busier. The drum servo is looking at the timing of vertical sync, and adjusting it's speed to keep it constant. The drum servo also has to adjust headwheel position to start scanning a video track at it's beginning. The capstan servo looks at the control track pulse, and adjusts tape speed to put the video tracks under the rotating video heads. Building a good servo system is more challenging than building a good video signal system. The servo system has to lock up quickly when playback is started, and then keep locked for the duration of the tape. If the drum servo becomes unlocked, the picture quality will deterioate, and the picture will appear to get narrower or wider, often alternately. If the capstan servo becomes unlocked, the picture will alternately go to noise and come back.If severe enough, the picture may break up entirely.
Early servo systems
Early servo systems required constant adjustment, and became tempremental even if the room temperature changed. Modern electronics fixed these problems, and today most servos are actually digital approximations of analog circuits. The advantage of the digital servos is that they can lock up faster, and they don't drift.
There are other servo systems in some videotape machines. Quadruplex machines have a vaccum guide shoe that forms the tape into a 'U'. This guide needs to be moved in and out to minimize timing variations that occur between scans. If uncorrected, guide position error leads to a condition called banding. Banding is seen in the picture as a series of horizontal stripes spaced evenly. It is most apparent in areas of saturated color. This error can be automatically corrected for by measuring the timing error in the horizontal sync, and adjusting the guide position to minimize it. This servo is called, appropriately enough, the guide servo.
Most better machines made today are capable of precision shuttle, slow motion, editing, etc. In order for these features to work properly, the tape must be at the proper tension at all times. The reel servo system does this by adjusting the speed of the supply and take-up reels to keep the tape properly tensioned. Until the advent of microprocessors, reel servos tended to be very complex. They were also very challenging to troubleshoot because some servos had half a dozen or so feedback loops!
Most tension servos do not really measure tape tension. They actually measure the position of a moving guide, which is under spring tension. The spring actually determines the proper tension. All the servo does is try to keep the guide in the same spot all the time. If the guide is in that spot, the tension is enough to balance the force of the spring, and tension is held constant.
One very interesting tension servo was used in the Ampex AVR-1 (Quad) and the IVC 9000 (Helical) VTR's. In these machines, the tape came off the reel and entered a Vaccuum Column. There, vaccuum pulled a loop of tape into a long box whose height was just high enough to fit the tape. A series of lights were mounted on one side of the box, and photocells on the other. The loop of tape would cover some of these, and tell the servo system how much tape was in the column. Enormously powerful reel motors could control tapr reel-off precisely enough to keep just the right amount of tape in the columns. A similar system was employed on the take-up side to control spooling of tape onto the take-up reel. The system worked extremely well, and was one of the few reel servo systems that allowed mixing many different sizes of reels. It never caught on because of it's mechanical complexity and the powerful vaccuum pump needed to evacuate the columns. This concept was borrowed from computer tape drives of the time.
Some professional videotape machines are capable of noiseless slow motion. Remember that the track pitch on the tape is a function of head scanning AND tape motion. Stop the tape motion, and you will scan two or more tracks with the video head. Wherever you cross tracks, you get a Noise Bar in the picture. This also occurs in shuttle, where the tape speed is too high, or in the wrong direction for proper tracking. In shuttle, you usually see several noise bars. But, how do they get rid of noise bars in slow motion?
In consumer machines
In consumer machines, there is a special circuit in the capstan servo that can center the track under the video head. In consumer formats, the heads can just about stay on a track in still, provided it is centered properly. The slower the normal tape speed, the better this works. This is why the slowest speeds on your beta or VHS machine produce the best still and slow effects.
In professional machines, the high tape speeds and the wide Guard Band (The unrecorded space between tracks, which is almost nonexistent in consumer machines.) make any kind of 'centering' scheme unworkable. (Although some early machines had a mechanical scheme of varying the angle of the tape across the heads in still, to give a better still picture.) So, a servo system was developed that measures the amount of signal coming from the video heads, and adjusts the position of the heads to follow the tracks. The position adjustment is done by mounting the head on a piezoelectric bimorph, which bends when voltage is applied to it.
By adjusting the amount of bend as the head scans the tape, the track can be properly scanned in still or even reverse play. Typical speed ranges over which a system like this is effective is 1 X reverse to 3 X forward playback. The servo system responsible for doing this is called Automatic Scan Tracking (AST)(Ampex) or Dynamic Tracking (DT)(Sony). This system is not seen in consumer machines because the bimorph assembly is extremely difficult to manufacture, and the electronics are very complex. A high voltage supply is also required (+/-250 volts) which must be connected to the rotating head via slip rings. The one exception to this is the European Video 2000 format, which has a head-moving servo. This consumer format also has a number of other unique features. AST was developed by Ampex engineers Dick Hathaway and Ray Ravizza.
An equally complex magnetically operated head-moving system is also used in some machines, notably some of the digital VTR's.
Recently, some VTRs have been appearing on the market that use digital reconstruction of the broken-up picture to create a slow motion similar to AST. While costing far less than AST-equipped machines, and also having similar control-feel, their picutre quality is not nearly as good.
JVC has recently released a consumer VHS machine with a feature called 'Dynamic Drum'. This feature is also proported to be present in some of their professional D-9 (Digital-S) machines. This system slightly tilts the entire video head drum assembly to improve signal recovery in non-standard tape speeds.