Specific VTR Problems and Their Solutions

To design and build a VTR meant not only to discover how to ensure the neces- sary head-to-tape speed and how to record a signal whose bandwidth was much larger than 10 octaves, but also how to find an adequate solution for several basic problems common to all VTRs:

• good tracking, that is, following recorded tracks correctly • stability of the time base of reproduced signals

• coping with unavoidable dropouts

As described in Section 10.1, the problem of ensuring a relatively high writing speed is solved by the introduction of rotary video heads. Such heads record on tape a series of parallel tracks, which can be almost perpendicular to the direction of the tape motion (or more simply to the tape edges) or be at any other angle. Considering that rotary video heads have to record video signals on tape that runs at a certain speed in front of these heads, video signals will be recorded on a num- ber of parallel slanted tracks and, not like analog audio, on one continuous track

10.4 Specific VTR Problems and Their Solutions 165

parallel to the direction of the tape motion and to the tape edges—a longitudinal

track. To correctly reproduce the recorded information, the playback heads have

to precisely follow the recorded tracks. Any tracking error, that is, any situation in which the playback head departs from the previously recorded track, will result in visible distortions of the reproduced picture because the playback head will not reproduce the useful information recorded on the video tracks but will scan the space between two adjacent tracks (the guard band) or simultaneously and partially scan two adjacent tracks, which results in indecipherable information for the reproduction circuits (see Figure 10.6).

To avoid such errors it is essential that tape and the video heads run at constant, precisely defined speeds, which have to be both absolutely identical during the recording and the playback processes and interlocked. Self-regulating electro- mechanical driving systems (servo systems) control these speeds and their mutual relationship. They receive a stable reference signal as well as the information on the speed (tape or head) to be regulated. The information is compared and the resulting data used for the correction of the controlled speed.

The operation of servo systems can easily be seen at the moment a VTR starts in the playback mode. After receiving the start command, the tape begins to run and the video heads to rotate, but the reproduced picture is unusable until the moment when the servo systems establish full control of these two speeds and ensure that video heads scan the previously recorded tracks correctly. This stabilization time, commonly called preroll, varies from a fraction of a second to several seconds in different models.

Time-base stability describes the overall stability of the reproduced video signal.

It indicates if the signal is stable over time (if the lines and fields follow their

Direction of sc an

Recorded_tracks

Direction of tape motion

Videohead path Guard band

standard rate with a high degree of stability) or if there is some oscillation around standard values.

In the case of VTRs, the lack of time-base stability is the result of several factors: slight but more or less unavoidable imprecision in the operation of the trans- port system, the instability of the signal-processing chain, and also the change of mechanical dimensions of magnetic tapes due to environmental conditions. As a result of all these factors, the reproduced video signal is not sufficiently stable to be forwarded as such to the downstream elements of a television chain. Do not be misled by the fact that, most frequently, a reproduced uncorrected video signal can be displayed without any apparent problem on a directly fed video monitor. The normal operation of the production, postproduction, transmission, and emission chain requires a considerably higher time-base stability than the one that would satisfy the demands of a video monitor.

Consequently, any professional VTR has to have not only a very precise and stable mechanical system and a similarly performing signal-processing chain, but also has to be equipped with specifically designed electronic circuits (called com-

pensation circuits) that can correct and reduce to an acceptable level the residual

time-base instabilities of the reproduced video signal.

If the compensation circuits are not switched on during the playback process, occasional shorter or longer, black or white disturbances in the reproduced picture will be noticed; they will look like scratches similar to the ones displayed during a film projection. These scratches, these momentary local imperfections of the repro- duced picture, known as dropouts, are the result of errors in the magnetization process during recording or in the reading process during playback.

When during any of these two processes a short inadequacy in the head-to-tape contact occurs, the resulting tape magnetization (during the recording process) or the head magnetic induction (in the playback process) will dramatically drop. That drop will be translated after demodulation as a loss of a part of one or of several television lines, as a spark, scratch, or dropout (see Figure 10.7).

Dropouts may be caused by a number of irregularities, which can be roughly systematized in three major groups:

1. The surface of the tape may not be ideally smooth. The active layer can occasionally contain small lumps (asperities), which will be detrimental to the head-to-tape contact. Since modern tapes have a very thin active layer, all imperfections of the base film will emerge at the surface of the tape and occasionally influence the head-to-tape contract.

2. In spite of all precautions and technical solutions, the tape surface will always be charged with static electricity that attracts dust and ash parti- cles, other debris, and some oxide particles that have detached from the active layer. These particles may, at any moment, appear between the head

10.4 Specific VTR Problems and Their Solutions 167

Dust or oxide particle

Video head Tape

Figure 10.7 One possible cause of a dropout.

and the tape and cause a momentary loss of contact that will in turn cause a serious drop in the transfer of the magnetic flux (see Figure 10.8). 3. The active surface of the magnetic tape can be scratched or damaged, and

part of its active layer destroyed; such deteriorations are quite common as a more or less unavoidable consequence of multiple passages of the tape

Video head Spacing loss necessary for dropout 0.81 µm Human hair 76.2 µm

1′′ video tape total thickness 29 µm

Finger print 15.7 µm Alcohol residue 33 µm Dust 38.1 µm Cotton fiber 152 µm Cigarette ash 7.6 µm

through the transport system of many recorders, or of the effects of the tape aging process.

Regardless of why they happen, dropouts always impair the overall quality of the reproduced picture. In extreme situations when the damage of the tape is seri- ous they may even lead to the complete loss of picture or continuity. To diminish the visual effect of these errors, special correcting devices called dropout compen-

sators were developed very early in the history of the development of videotape

recording techniques.

When a dropout appears in one of the lines, the compensator prevents that line from reaching the machine output and replaces it with the previous line kept in memory. As these are two adjacent lines, our eyes will not notice that they have identical content. Present-day digital videotape recorders use the same approach although with greater sophistication. When a dropout is detected, all pixels sur- rounding the missing ones are assessed; out of their values the most probable content of the missing information is computed, and the computed value is for- warded to the output. However, it would be dangerous and counterproductive to rely heavily on correction devices. It is strongly recommended to take all pos- sible preventative actions, mainly in the “ecology” domain, that is, keeping the tapes, the operating and storage areas, and the recorders themselves as clean as practicable so as to eliminate the causes rather than to correct the effects and consequences.

It is important to stress here that the design of a modern VTR not only has to solve all of the aforementioned problems but also has to ensure the availability of a number of operational features. Back in 1956, broadcasters were happy to see a machine that would simply facilitate continuous recording and immediate playback of a television program. Today no recorder can be accepted by the market if it does not offer large and flexible editing capabilities, stunt modes (slow motion, freeze-frame, and fast motion), a recognizable picture in shuttle speed, and so on. These features influence the design of a given recorder and sometimes influence even the basic definition of a new recording format.