Before development began, basic criteria for the VHD system were established as follows. First, both the videodisc and videodisc player should be compact. Second, the playing time must be long enough for a feature film. Third, two sound tracks should be available so that stereo sound or bilingual sound can be reproduced. Fourth, the operation must be simple and flexible, with a variety of playback modes available to the user through the VHD player. And finally, there should be a wide variety of software available. Another important consideration is the capability to play a digital audio disc on the same player used for the VHD videodisc.
In the VHD system, the disc rotation speed is 900 rpm in the NTSC standard and 750 rpm in the PAL/SECAM system. Two TV frames (4 fields) are recorded per revolution, with the sync aligned radially on the disc. The track pits are recorded on the disc in spiral form outside-to-inside with 1.35um of track pitch. The dimensions of the pit are about 0.8um in width and 0.3um in depth. The VHD videodisc system uses a grooveless capacitance pickup. Video and two
audio signals are recorded on the surface of the videodisc in the form of micropits. An electrode is attached to the rear of the stylus and can pick up capacitance variation between the stylus electrode and the conductive disc. Because no actual mechanical grooves are on the videodisc surface to guide the stylus, the pickup stylus is controlled by an electro-tracking servo system. The tracking signal is placed on both sides of the micropits of the video and
audio signals. During playback, the stylus electrode picks up the video and audio signals as well as two tracking signals in the form of capacitance variations (Fig. 1).
Encoding/Decoding System
One of the biggest differences between the videodisc medium and magnetic tape or magnetic discs is in the linearity of the recording signal. There is no linear portion on the non-magnetic videodisc. Rather, it is a two-state device with the properties of a hard limiter. If the two carriers go through the non-linear portion, such as the limiter circuit, the spurious products will be produced as a beat frequency of the two carriers. Therefore, it is important to use a single-carrier system with a symmetrical side-band spectrum to minimize even-order spurious
components.
Encoding
Figure 2 shows a simplified block diagram of the signal
processing for encoding the video and audio signals. The chrominance signal, 3.58 MHz in NTSC, of the incoming
composite video is converted down to 2.56 MHz. In the VHD
system, the frequency of the subcarrier, 2.56 MHz, was chosen to be an odd multiple (162.5 x fH) of one-half the line frequency, to minimize its visibility in accordance with well-known frequency interleaving principles. The two audio signals, after passing through the DE system,* are frequency modulated on carriers of 3.43 MHz and 3.73 MHz with a frequency deviation of +- 75KHz. The audio channel of the VHD videodisc system uses the DE system for both noise reduction and dynamic range expansion for better audio quality. The two frequency-modulated audio carriers are added to the converted video signal (luminance signal with
2.56 MHz chrominance signal). These three signals are frequency modulated on a carrier at 6.6 MHz. (6.1 MHZ sync tip and 7.9 MHz for peak white), and the output FM carrier is used to modulate the intensity of a laser beam passing through the first electro-optical modulator in the master cutting machine. Two pilot signals, fp1 = 511 KHz and fp2 = 716 KHz, will be made from the sync portion of the incoming video signal. There, fp1 and fp2 are used to modulate the second electro-optical modulator. The index signal, fp3 = 275 KHz, is also made from sync and recorded once in every
two frames in the vertical blanking period of the first field.
FM Spectrum
The main carrier is frequency-modulated by three signals, namely, 2.56 MHz chrominance, 3.43 MHz first audio
carrier, and 3.73 MHz second audio carrier. The FM spectrum on the VHD videodisc shown in Fig. 3 has an ideal symmetrical side-band distribution at both upper and lower sides of the main carrier.
fs: Chroma sub carrier 2.56 MHz
fa: Audio sub carrier 3.43 MHz
fb: Audio sub carrier 3.73 MHz
The two pilot signals, fp1 and fp2, at the low end of the spectrum are produced from the pilot signal track. Therefore, the fp1 and fp2 do not cause any even-order harmonic-distortion in the main carrier. Spurious components or moiré produced by the above FM spectrum will be so small that they can be ignored.
Figure 4 shows an oscillogram of a spectrum analyzer output of demodulated video for the "E-to-E" condition when a 100% saturated chrominance signal was used as the input signal. Note that the spurious components in the video passband are all below -50db, which produces a clean, moiré-free picture.
Decoding at Player
When the stylus passes across an information pit, the electrode on the stylus senses the capacitance variations (Fig.5). The stylus is connected to a resonant circuit which is tuned to approximately 910 MHz. Figure 6 shows the equivalent circuit of the stylus and pre-amplifier which convert the capacitance variation between the stylus and the conductive disc to an electrical signal. In Fig. 6a, the
capacitance change between the stylus and the disc will
vary the resonance frequency and cause amplitude modulation of the UHF at the frequency modulation rate. The response curve of the resonance circuit and its amplitude modulation output are shown in Fig. 6b. The AM output is output by using the circuit, Fig. 6c. The original FM-modulated signal on the disc will be reproduced by the AM-modulated UHF carrier. Figure 7 shows the block diagram of the signal processing for decoding of the video and audio signals. The
detected FM signals are fed to the preamplifier and then to the main FM detector through the bandpass filter (BPF). The main FM detector produces a video signal with 2.56 MHz and 3.43 MHz of the first audio carrier and 3.73 MHz of the second audio carrier. The demodulated video signal is fed to the chroma processing circuit and converts the chroma frequency from 2.56 MHz to 3.58 MHz. The chroma processing circuit also stabilizes the chroma signal. The 2.56 MHz chroma signal is removed from the demodulated video by a comb filter in order to maintain wide luminance bandwidth. The two audio carriers are fed to the individual FM demodulator. The demodulated audio signal will be reconstructed by the DE demodulator circuit.
Mastering
Figure 8 shows the optical path of the laser beam recorder (cutter). Using a master disc made of glass, master
recording is accomplished by using an optical cutting machine installed in a clean room. The smooth, flat glass disc is coated with ordinary photosensitive material.
While the disc is rotated at a speed of 900 rpm for NTSC (750 rpm, PAL/SECAM), it is irradiated by minute laser beams. Since the VHD videodisc system employs a constant angular velocity system, the track velocity changed from outside to inside. Therefore, the signal element at the innermost portion becomes shorter, relative to the outside. If the spot size of the laser beam is constant through the entire disc, from outside to inside, the duty cycle of the pits becomes asymmetrical, causing an even order of harmonic distortion of the playback FM signal. In order to compensate for this, the mastering machine varies the spot size of the laser beam to correspond with the diameter of the disc.
In Fig. 8 the argon laser beam is split, one half for the information signals, the other for the tracking signals. The recorded glass disc is then developed with interferometer thickness at a developing station.
Matrix (Electroforming)
The matrix or electroforming process on the VHD videodisc system is identical to the electroforming of the
conventional Long Play phonograph record. The overall matrix-process flow chart used for the VHD disc is shown in Fig. 9. A recorded glass master disc is silver-coated with an extremely thin layer (800Angstroms) that is sprayed onto the surface of the glass. The silvered-glass master is pre-plated at a low temperature and current density before being transferred to a tank for deposition at the normal speed. After plating, the metal master is separated from the glass master. The metal master is then nickel-plated for
"mother" forming. The stamper is produced from the mother in the same manner. Figure 10 shows the scanning electron microscope picture of the metal master (slant view).
Videodisc Pressing
The videodisc material is a PVC based compound which should have extremely fine particles, no degradation by
the cycle of high and low temperature, and good melt-flow properties. Figure 11 shows a flow-chart of the videodisc
replication process.
Physical process: The back of the stamper is ground before it is mounted to the molding machine to prevent any back-roughness from penetrating the surface of the disc.
Compound Process: The videodisc materials (resin, stabilizer additive, and carbon black) are mixed together in a "dry blending" machine. The carbon black is added to make the disc electrically conductive.
Compression Molding: The thermoplastic molding is produced by heat, pressure, and subsequent cooling from a pair of stampers in a suitable disc press. The VHD videodisc compression molding machine was converted from a conventional phonograph audio-record pressing machine. It uses a fully automated pressing process and is installed in a clean room. Figure 12 shows the scanning electron microscope picture of the pressed videodisc. No additional processing, such as rinsing, lubrication, or metal coating, is required after the pressing is made.
Conclusion
A most important consideration is to establish a standard for consumer products - a standard that results in high product performance - a product that can build and grow with today's technology. By understanding the capabilities and limitations of this process, in both software and hardware, the videodisc medium can be used to the fullest. The VHD family consists of many manufacturing elements from all over the world. Player and disc manufacturing companies are working together to produce a disc system with good interchangeability, and production processes with low material usage, low production cost, and above all, high quality.
[1] This contribution was received from Toshiya Inoue, JVC, Japan, Tsuneyoshi Hidaka, JVC, Japan, and Vincent Roberts, VHD Disc Manufacturing, USA, on July 29, 1982. Copyright
1982, by the Society Of Motion Picture and Television Engineers.
* The Dynamic range Expansion (DE) system is the noise-reduction system for the audio signal. The DE system not only improves the noise
performance of the audio signal, but also expands its dynamic range.
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