Apparatus for electromechanical recording of short wavelength modulation in a metal master

An unheated cutting stylus is positioned with respect to a metal master in order to cut a groove having a quiescent groove depth less than one micrometer while relative motion is established between the cutting stylus and the metal master. The cutting stylus is vibrated about the quiescent position thereof in response to a relatively high frequency signal while cutting the groove in order to cut an information track comprising short wavelength modulation (e.g., 0.6 to 1.6 micrometers) of groove depth in the bottom of the groove having a peak-to-peak dimension less than the quiescent groove depth. The cutting stylus vibration is effected by a piezoelectric element which is mounted directly between a pedestal and the cutting stylus by means of bonding materials. The pedestal, the piezoelectric element, and the cutting stylus are shaped to form a cutterhead structure having all external surfaces disposed in anti-parallel relationship to each other.

The present invention relates generally to method and apparatus for 
recording short wavelength modulation in a master; and, more particularly, 
concerns electromechanical recording method and apparatus advantageous in 
the formation of a high density information record, such as a video disc 
record. 
BACKGROUND OF THE INVENTION 
In certain high-density information record/playback systems, recorded 
information appears as an information track constituting relatively short 
wavelength variation (e.g., 0.6 to 1.6 micrometers) in the geometry of the 
groove bottom along its length. In one specific but nonlimiting 
embodiment, this short wavelength variation may encode, for example, a 
composite color video signal. There are several methods of encoding a 
composite color video signal in the information track comprising short 
wavelength variation. Illustratively, the method of encoding may be of the 
type shown in the copending U.S. Patent Application of E. O. Keizer, Ser. 
No. 441,069, filed Feb. 11, 1974, entitled, "COLOR PICTURE/SOUND RECORD 
AND RECORDING/PLAYBACK APATUS AND METHODS THEREFOR", and now U.S. Pat. 
No. 3,911,476. Pursuant to the Keizer method, a first (e.g., video) 
carrier is frequency modulated over a high frequency deviation range 
(e.g., of the order of 4.3 to 6.3 MHz) in accordance with a video signal 
including the luminance and the chrominance of a scanned image. A second 
carrier is frequency modulated over a low frequency deviation range (e.g., 
of the order of 716 KHz .+-. 50 KHz) in accordance with the audio signal 
accompaniment of the video signal. The once-modulated first carrier is 
duty cycle modulated in accordance with the once-modulated second carrier. 
A scanning electron beam apparatus responsive to the twice modulated first 
carrier records in the groove bottom of a pregrooved master having a 
coating of photoresist material short wavelength variation representative 
of the time variation of the recorded signal. 
Ordinarily, a stamper (having negative grooves) is obtained from the 
recorded master (having positive grooves) from which plastic disc records 
(also having positive grooves) can then be molded. To reconstruct the 
prerecorded signals, an appropriate relative motion is established between 
the grooved disc record and a groove-engaging signal pickup responsive to 
the spatial variation passing underneath. The signal pickup may be of any 
suitable variety (for example, a capacitance or a pressure type, etc.). 
Reference may be made to the U.S. Pat. No. 3,842,194, issued to J. K. 
Clemens, on Oct. 15, 1974, and entitled "INFORMATION RECORDS AND 
RECORDING/PLAYBACK SYSTEMS THEREFOR", for an illustration of a playback 
apparatus including a capacitance type of a signal pickup. Pursuant to the 
Clemens' system, the grooved disc record is provided with a thin deposit 
of dielectric material overlying a fine coating of conductive material on 
the base of the disc record (with respective thickness sufficiently small 
that the dielectric deposit and the conductive coating follow the contours 
of the groove and the groove bottom variation therein). A playback stylus 
has a groove-engaging tip incorporating a conductive electrode. The disc 
record is roatated at an appropriate speed in order to cause variation in 
the capacitance exhibited between the stylus electrode and the disc record 
conductive coating in accordance with the signal recorded in the groove 
bottom. A detector responsive to the capacitance variation reconstructs 
the prerecorded signal for audio/visual presentation on an ordinary 
television receiver. 
In the above-said type of video disc systems, in order to obtain adequate 
bandwidth for the signal recovered from the grooved disc record (e.g., 4.3 
to 6.3 MHz) during playback (1) the disc record is rotated at a relatively 
high playback speed (e.g., 450 rpm), and (2) the wavelength of the 
modulation in the disc record groove is relatively short (e.g., 0.6 to 1.6 
micrometers) as compared with conventional audio disc systems. 
Since the playback time is (1) directly proportional to the number of 
grooves per inch in the disc record, and (2) inversely proportional to the 
playback speed of the disc record, the higher playback speed (e.g., 450 
rpm) results in a larger number of grooves per inch in the disc record 
(e.g., 5,555 gpi) for a given playback time (e.g., 30 minutes from each 
side). In other words, in the aforesaid type of video disc systems, the 
groove convolutions are very closely spaced (e.g., 4.5 micrometers) in 
order to accommodate the information necessary for storing a video program 
of an acceptable quality and a reasonable playback time. The close spacing 
of the groove convolutions in the video disc type record (e.g., 4.5 
micrometers) results in a groove having a very small quiescent (without 
groove modulation) depth (e.g., 0.8 micrometer with a groove-apex angle of 
140.degree.). 
It has been determined from the noise spectra of the recorded master that 
the noise level in the signal recovered from the grooved disc record 
during playback is reduced as the wavelength of the noise components 
measured is reduced. In other words, for given recording level (the 
peak-to-peak dimension of the groove modulation), the signal-to-noise 
ratio for a given noise bandwidth improves as the wavelength of the groove 
modulation due to the recorded signal, and the corresponding noise 
components, is reduced. With electromechanical recording in a lacquer 
master, it has been possible to obtain a satisfactory signal-to-noise 
ratio (e.g., above 40 dB peak-to-peak video/rms noise) with a low 
recording level (e.g., 0.1 to 0.15 micrometers) when the wavelength of the 
groove modulation, due to the recorded signal, and the corresponding noise 
components, is kept relatively short (e.g., 0.6 to 1.6 micrometers). It is 
desirable to further improve the signal-to-noise ratio for a given 
recording level; e.g., to accommodate the unavoidable addition of noise 
during the video disc type record manufacturing operations. 
A variety of approaches to disc recording exist in the prior art. For 
example, electromechanical processes are known in the audio industry for 
recording groove modulation representative of an audio signal (e.g., 
having a bandwidth of 20 KHz) in a lacquer master. The following recording 
parameters are typical in the audio recording processes: (a) the number of 
grooves per inch -- 150 to 350; (b) the groove depth -- up to 50 
micrometers; (c) the wavelength of the groove modulation -- greater than 
10 micrometers; and (d) the peak-to-peak dimension of the groove 
modulation -- up to 80 micrometers. Further, the cutting stylus employed 
for recording audio groove modulation in a lacquer master is heated in 
order to obtain a satisfactory recording (e.g., a reasonably good 
signal-to-noise ratio). It has been found that the heated cutting stylus 
causes the lacquer material to flow while cutting a groove and modulation 
therein producing a burnishing effect (i.e., surface polish) in the 
grooves. 
A cutterhead for electromechanically recording a video signal in a lacquer 
master is described in U.S. Pat. No. 3,865,997, issued to J. B. Halter on 
Feb. 11, 1975, and entitled, "TRIANGULAR PIEZOELECTRIC TRANSDUCER FOR 
RECORDING VIDEO INFORMATION". In the above-said U.S. Pat. No. (3,865,997), 
the cutting stylus employed for recording groove modulation in a lacquer 
master for storing a video signal is also shown heated for satisfactory 
recording (i.e., reducing the surface noise). 
SUMMARY OF THE INVENTION 
A method for recording short wavelength modulation in a master by 
mechanical cutting, pursuant to the present invention, comprises: placing 
a metal master on a movable support in operating relationship with an 
unheated cutting stylus in order to effect relative motion between the 
metal master and the cutting stylus; positioning the cutting stylus with 
respect to the metal master in order to cut a groove having a quiescent 
groove depth less than one micrometer while the relative motion is 
established; vibrating the cutting stylus about the quiescent position 
thereof in response to a relatively high frequency signal while cutting 
the groove in order to effect modulation of groove depth in the metal 
master having a peak-to-peak dimension less than the quiescent groove 
depth. The electromechanical recording of short wavelength modulation in a 
metal master without application of heat to the cutting stylus provides 
superior recording characteristics as compared with other prior art 
recording technicques. 
According to a further feature of the invention, a cutterhead including the 
cutting stylus suitable for cutting a groove in a metal master having an 
information track therein comprising short wavelength modulation of groove 
depth is disclosed. The cutterhead comprises a piezoelectric element 
rigidly mounted directly between a pedestal and the cutting stylus by 
means of bonding material. The pedestal, the piezoelectric element, and 
the cutting stylus are shaped to form a cutterhead structure having all 
external surfaces disposed antiparallel to each other. Means are provided 
for energizing the piezoelectric element in accordance with a relatively 
high frequency signal. The pedestal is mounted in a manner that effects 
placement of the cutting stylus in operating relationship with the metal 
master during recording. The cutterhead, pursuant to the principles of the 
present invention, does not include means for heating the cutting stylus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a system for electromechanically cutting a groove in a 
metal master 10 having an information track comprising short wavelength 
modulation of groove depth. The metal master 10 is placed on a movable 
support 11 in operating relationship with a cutterhead 12 including a 
cutting stylus 13 in order to effect relative motion between the metal 
master and the cutting stylus. The cutting stylus 13 is positioned with 
respect to the metal master 10 in order to cut a groove 14 having a 
quiescent groove depth less than one micrometer while the relative motion 
is established. The cutting stylus 13 is vibrated in response to a 
relatively high frequency signal (e.g., up to 1.8 MHz) while cutting the 
groove in order to effect short wavelength modulation (e.g., less than 5 
micrometers) of the groove depth having a peak-to-peak dimension less than 
the groove depth. The electromechanical recording of short wavelength 
modulation in a metal master provides a high signal-to-noise ratio as 
compared with other prior art recording techniques. 
The formation of the relatively high frequency signal (the time variation 
of which is represented by the spatial variation in the groove bottom) 
will now be described with reference to FIG. 1 by way of an example only. 
Illustratively, a video camera 15 scans the image 16 for developing a 
video signal at the output thereof. The video signal may include 
components representative of the luminance and the chrominance of the 
scanned image 16. The output signal of the video camera 15 is slowed down 
(e.g., by a factor of 5) by a video slow down processor 39 in order to 
accommodate the bandwidth of the cutterhead 12. A video modulator 17, 
coupled to the video slow down processor 39, frequency modulates a 
slowed-down high frequency carrier over a high frequency deviation range 
(e.g., of the order of 4.3/5 to 6.3/5 MHz) in accordance with the 
slowed-down video signal. 
A microphone 18 picks up an audio signal accompaniment of the video signal 
from a speaker 19. The output signal of the microphone 18 is likewise 
slowed down (e.g., also by a factor of 5) by an audio slow down processor 
40 in order to synchronize the audio signal to be recorded with the 
slowed-down video signal. An audio modulator 20, coupled to the audio slow 
down processor 40, frequency modulates a slowed-down low frequency carrier 
over a low frequency deviation range (e.g., of the order of 716/5 .+-. 
50/5 KHz) in accordance with the slowed-down audio signal developed at the 
output of the microphone 18. An audio/video modulator 21 modulates the 
once-modulated, slowed-down, high frequency carrier in accordance with the 
once-modulated, slowed-down, low frequency carrier in a manner described 
subsequently in conjunction with FIGS. 8 through 15. A relatively high 
frequency signal at the output of the audio/video modulator 21 (e.g., up 
to 1.8 MHz) energizes the cutterhead 12 during the recording operation in 
order to effect short wavelength modulation of groove depth (e.g., 0.6 to 
1.6 micrometers) while cutting the groove 14 in the metal master 10 at a 
slowed-down recording speed (e.g., 450/5 rpm). It is contemplated that 
refined versions of the cutterhead 12 having relatively wide bandwidths 
could be developed which would make the real time recording feasible, and 
thereby eliminating the need for the video slow down processor 39 and the 
audio slow down processor 40. 
Several alternative modes of operation of the audio/video modulator 21 (in 
other words, several alternative methods of modulating the once-modulated 
high frequency carrier in accordance with the once-modulated low frequency 
carrier) will now be explored in conjunction with FIGS. 8 through 15. FIG. 
8, 10, 12 and 14 represent, respectively, the signals resulting from (1) a 
superposition, (2) an amplitude modulation, (3) a duty cycle modulation, 
and (4) a negative peak amplitude modulation, of a high frequency (e.g., 
video) carrier by a low frequency (e.g., audio) carrier. In FIGS. 8, 10, 
12 and 14, the high frequency carrier and the low frequency carrier are 
not shown frequency modulated, respectively, by a video signal and an 
audio signal accompaniment of the video signal for reasons of simplicity 
and clarity. FIGS. 9, 11, 13 and 15 represent, respectively, the spectra 
of the signals of FIGS. 8, 10, 12 and 14. In FIGS. 8 through 15, f.sub.h 
represents the high frequency carrier, while f.sub.l represents the low 
frequency carrier. As previously indicated, the signal at the output of 
the audio/video modulator 21 energizes the recording apparatus during the 
recording operation in order to effect a short wavelength modulation of 
groove depth. 
As can be seen from FIGS. 9, 11, 13 and 15, the duty cycle modulation of 
FIG. 13 requires the most bandwidth for the recording apparatus (2f.sub.h 
+ f.sub.l) in order to satisfactorily record the signal. As illustrated in 
FIGS. 8, 10, 12 and 14; (a) the superposition signal of FIG. 8, (b) the 
amplitude modulated signal of FIG. 10, and (c) the negative peak amplitude 
modulated signal of FIG. 14, require a reasonably good amplitude linearity 
for the recording apparatus in order to satisfactorily record the signal 
in contrast to the duty cycle modulated signal of FIG. 12 having a 
constant peak amplitude. 
On the one hand, the previously indicated scanning electron beam recording 
apparatus is inherently capable of a broad bandwidth (e.g., 12.6 MHz) 
typically required for satisfactory recording of video type signals. 
Therefore, the scanning electron beam recording apparatus is more suitable 
for recording signals obtained by duty cycle modulation (e.g., FIG. 12). 
ON the other hand, the electromechanical recording apparatus is capable of 
a good amplitude linearity over the range of recording levels (i.e., the 
cutting stylus 13 displacement) ordinarily encountered in video signal 
type recording (i.e., up to 1 micrometer). Thus, the electromechanical 
recording apparatus is more suitable for recording signals obtained by (1) 
the superposition method (e.g., FIG. 8), (2) the amplitude modulation 
method (e.g., FIG. 10), and (3) the negative peak amplitude modulation 
method (e.g., FIG. 14). 
Since the positive peaks of the signals illustrated in FIGS. 8 and 10 do 
not lie along a line parallel to the abscissa, and since the length of the 
playback stylus shoe is ordinarily less than the wavelength of the low 
frequency (e.g., audio) carrier, the playback stylus undergoes a vertical 
motion during playback of the recordings of the signals of FIGS. 8 and 10 
at the low frequency carrier rate. However, since the positive peaks of 
the signals illustrated in FIGS. 12 and 14 lie along a line parallel to 
the abscissa, and since the length of the playback stylus shoe is normally 
slightly greater than the longest wavelength of the high frequency (e.g., 
video) carrier, the playback stylus does not undergo a vertical motion 
during playback of the recordings of the signals of FIGS. 12 and 14 at the 
either frequency carrier rate. The vertical motion of the playback stylus 
during playback is undesirable for several reasons. First, the vertical 
motion greatly increases the wear of the playback stylus and the record 
medium. Second, the playback stylus may have a tendency to lose contact 
with the recording medium causing a signal dropout at the output of the 
signal pickup during playback. Although the playback stylus may be urged 
to remain in contact with the record medium during playback by increasing 
the stylus/medium contact pressure, the increase in contact pressure will 
result in additional wear of the playback stylus and the record medium. 
Third, the vertical motion of the playback stylus during playback results 
in variation in the spacing between the stylus electrode and the short 
wavelength groove modulation due to the high frequency carrier at the low 
frequency carrier rate. The variation in the spacing of the stylus 
electrode is undesirable because it varies the sensitivity and resolution 
capability of the signal pickup, thereby distorting the signal at the 
output of the pickup at the low frequency carrier rate. 
The signal obtained by the duty cycle modulation (e.g., FIG. 12) may be 
less desirable for the electromechanical recording apparatus because the 
duty cycle modulation signal requires the most bandwidth (e.g., 2f.sub.h + 
f.sub.l) for the cutter head 12. When the bandwidth of the 
electromechanical recording apparatus is increased, it is more desirable 
to increase the recording speed rather than consume the additional 
bandwidth to encode the low frequency (e.g., sound) carrier on the high 
frequency (e.g., video) carrier. Thus, it is noted that the negative peak 
amplitude modulation method (e.g., FIG. 14) is suitable both from the 
recording and the playback viewpoints. 
In the previously described specific but non-limiting embodiment, the 
innermost diameter required for a thirty minute playing time from a 12 
inch disc record, having 5,555 grooves per inch and rotating at 450 rpm, 
is approximately 6.6 inches. The shortest wavelength of approximately 0.6 
micrometer is cut at the innermost diameter of approximately 6.6 inches 
while recording the highest signal frequency of 6.3 MHz at the recording 
speed of 450 rpm. It has been found that for a recorded wavelength of 0.6 
micrometer, a peak-to-peak groove modulation of 0.1 micrometer (i.e., 
approximately 4 microinches) will provide an adequate signal-to-noise 
ratio. For example, a 60 dB signal-to-noise ratio was obtained for a 30 
KHz noise bandwidth with a peak-to-peak groove modulation of 0.1 
micrometer at 0.6 micrometer wavelength by using the presently disclosed 
electromechanical recording of short wavelength modulation in a metal 
master technique. This would translate into a 40 dB (rms signal/rms noise) 
signal-to-noise ratio for a 3.0 MHz bandwidth for the amplitude modulation 
type recording of a video signal. Refer to the U.S. Pat. No. 3,842,194 for 
an example of the amplitude modulation type recording of a video signal. 
In the case of the abovesaid frequency modulation type recording of a 
video signal, this would translate into a 56 dB signal-to-noise ratio 
(peak-to-peak video/rms noise) for a 3.0 MHz bandwidth. Refer to the U.S. 
patent application Ser. No. 441,069 for an example of the frequency 
modulation type recording of a video signal. It is noted that these 
signal-to-noise ratios have not been obtained by the electromechanical 
recording in a lacquer master technique at the same recording levels. 
Further, a groove modulation having a peak-to-peak dimension of 0.1 
micrometer (approximately 4 microinches) and a wavelength of 0.6 
micrometer will have a maximum slope at its zero crossing which can be 
computed as follows: 
##EQU1## 
The trailing edge of the cutting stylus 13 must have a slope greater than 
28.degree. in order to avoid interference with the previously recorded 
groove modulation in the region of the maximum slope at the innermost 
diameter of approximately 6.6 inches while recording the highest signal 
frequency of 6.3 MHz at the recording speed of 450 rpm. 
Reference will now be made to FIGS. 2 and 3. FIGS. 2 and 3 illustrate, 
respectively, a longitudinal-section (i.e., along the groove) and a 
cross-section (i.e., perpendicular to the groove) of the groove 14 cut in 
the metal master 10 by the cutting stylus 13. The included angle B of the 
cutting stylus 13 is the angle subtended between the cutting edges of the 
stylus. The clearance angle A of the cutting stylus 13 is the angle 
subtended by the trailing edge of the cutting stylus with the line of 
motion of the metal master 10. It has been found that it is more difficult 
to grind flaw-free cutting edges (when viewed under a magnification power 
of 10,000) as the clearance angle increases. The present diamond grinding 
technology permits grinding of flaw-free cutting edges with the clearance 
angle of up to 40.degree.. The stylus clearance angle of 40.degree. has 
been found to cut adequate peak-to-peak groove modulation (e.g., 0.1 
micrometer) at a short wavelength (e.g., 0.6 micrometer) in order to 
provide a satisfactory signal-to-noise ratio when the presently disclosed 
electromechanical recording of a short wavelength modulation in a metal 
master technique is used. 
The metal master 10 may be comprised of a thin deposit of a metal having a 
homogeneous and fine grain (e.g., nearly a single crystalline type) 
structure (e.g., copper) on a substrate disc (e.g., aluminum). 
Alternately, the metal master 10 could be a solid disc of a metal having a 
homogeneous and fine grain structure. In the preferred embodiment, a thin 
deposit of copper is electroplated on an aluminum substrate disc. The 
copper deposit is then faced off to make the surface to be recorded 
relatively flat. Materials having properties which can be processed to 
have a very fine (e.g., nearly a single crystalline type) crystalline 
structure (e.g., copper, speculum, certain aluminum alloys) are suitable 
for recording in metal. 
Typically, a stamper (having negative grooves) is obtained by 
electroplating process from the recorded master 10 (having positive 
grooves). Plastic disc records (also having positive grooves) can then be 
molded from the stamper by a suitable compression or injection molding 
process. FIGS. 4 and 5 illustrate, respectively, a longitudinal-section 
and a cross-section of a groove 22 of a disc record 23 manufactured from 
the recorded metal master 10. To reconstruct the prerecorded signal, an 
appropriate relative motion is established between the grooved disc record 
23 and a signal pickup 24 which is responsive to the spatial variation 
passing underneath. In the capacitance type signal pickup, the grooved 
disc record 23 is provided with a thin dielectric deposit (not shown) 
overlying a fine conductive coating (not shown) on the base of the disc 
record. The signal pickup 24 rides smoothly in the groove 22 during 
playback on the top of the positive peaks which are substantially aligned 
along a surface parallel to the line of motion of the disc record 23. The 
signal pickup 24 has a groove engaging tip incorporating a conductive 
electrode 25 having an exposed edge with an effective dimension along the 
groove 22 not exceeding one-half of the wavelength of modulation recorded 
in the groove 22 of the disc record 23 in order to obtain sufficient 
recorded signal resolution capability. The disc record 23 is rotated at an 
appropriate speed in order to cause variation in the capacitance between 
the electrode 25 and the conductive coating of the disc record 23 in 
accordance with the signal recorded in the groove 22 bottom. A detector 
(not shown) responsive to the capacitance variation reconstructs the 
prerecorded signal. 
FIGS. 6 and 7 illustrate, respectively, an elevation and a bottom view of 
the cutterhead 12 suitable for cutting a groove in the metal master 10 
having an information track comprising short wavelength modulation of 
groove depth. The cutterhead 12 includes a support 26, damping members 27, 
36 and 37, a pedestal 28, a piezoelectric element 29, a piezoelectric 
energizing means 30, and the cutting stylus 13. The piezoelectric element 
29 is rigidly mounted directly between the pedestal 28 and the cutting 
stylus 13 by means of bonding materials 31 (e.g., conductive) and 32 
(e.g., non-conductive). The pedestal 28, the piezoelectric element 29, and 
the cutting stylus 13 are shaped to form a cutterhead structure having all 
external surfaces disposed antiparallel to each other. To ensure a good 
bond strength between the piezoelectric element 29 and the cutting stylus 
13, a non-conductive epoxy bond 32 is provided at the interface. The 
cutting stylus 13 may preferably be made from diamond material for 
obtaining relatively flaw-free cutting edges and optimum dynamic 
properties (since it has the highest Young's modulus of the known 
materials) while providing a reasonable life for the cutting stylus. 
In order to apply the electrical signal representative of the high 
frequency signal to be recorded to the piezoelectric element 29, in this 
particular embodiment, (a) the pedestal 28, and (b) the bonding material 
31 interposed between the adjacent surfaces of the piezoelectric element 
and the pedestal, are made of conductive materials. A first wire 33 is 
electrically coupled to the conductive pedestal 28. A second wire 34 is 
electrically coupled to a fillet 35 of conductive material secured to the 
metalized surface (e.g., silver coated) of the piezoelectric element 29 
adjacent to the cutting stylus 13. 
The piezoelectric element 29 is responsive to the electrical signal to be 
recorded such that the vertical displacement of the cutting stylus 13 is 
in the same direction as the applied electrical field. The piezoelectric 
element 29 has sides approximately triangular shaped in cross-section 
conforming with the shape of the pedestal 28 upon which it is mounted. For 
a given largest linear dimension within the moving portion of the 
cutterhead 12, a triangular cross-section will provide a highest major 
resonant frequency of the various, simple, geometrical shapes which are 
most feasible to fabricate. While other cross-sections (such as circular, 
trapezoidal, etc.) could provide a satisfactory frequency response 
characteristic, a triangular cross-section is preferred because it 
provides a superior frequency response characteristic. Although the 
piezoelectric element 29 is illustrated with vertical sides (i.e., 
perpendicular to the line of motion of the metal master 10) in FIG. 6, the 
sides of the piezoelectric element may preferably be beveled to form a 
triangular truncated pyramidal structure similar in shape to the pedestal 
28. Further, the general triangular pyramidal structure of the cutterhead 
12 provides stiffness in the horizontal plane in order to prevent lateral 
resonances below the major vertical resonance of the cutterhead. Typical 
piezoelectric materials which are suitable for the piezoelectric element 
29 are of lead zirconium titanate type, and are available from the Clevite 
Corporation as PZT-4 or PZT-8 materials. 
The damping is provided to the cutterhead 12 by a member 27 (made of 
relatively rigid material) and the bonding materials 36 and 37 (made of 
relatively pliable material). Alternatively, the damping structure may 
include additional thin layers of pliable material separated by layers of 
more rigid material. The thin layers of pliable material may be of the 
silicon rubber or the cellulose of the type manufactured by the American 
Viscose Company, Markus Hook, Pennsylvania, under the name Viscoloid. The 
more rigid material may be made from Kapton available from the Dupont 
Corporation. The compliance of the members 27, 36 and 37 interposed 
between the pedestal 28 and the support 26 attenuates, and, therefore 
inhibits reflection of the propagating waves generated by the 
piezoelectric element 29 within the pedestal 28. The members 27, 36 and 37 
also decouple the moving portion of the cutterhead 12 from the resonant 
modes within the support 26 and other parts of the cutterhead suspension 
and the recording apparatus. The support 26 is desirably made of material 
having a high stiffness (e.g., aluminum or steel) in order to provide a 
proper termination for the damping elements 27, 36 and 37. 
In order to obtain a greater displacement of the cutting stylus 13 for a 
given level of energization of the piezoelectric element 29, it is 
desirable to maintain the surface of the piezoelectric element which is 
remote from the cutting stylus as stationary as possible, allowing the 
surface of the piezoelectric element which is adjacent to the cutting 
stylus to provide essentially all the displacement. In order to maintain 
the remote surface of the piezoelectrice element 29 as stationary as 
possible, the pedestal 28 should present a relatively large mechanical 
impedance to the motion of the piezoelectric element. Since the mechanical 
impedance of a material is proportional to the square root of the product 
of the density (.rho.) of the material and the Young's modulus (E) (Z = 
square root of .rho. .times. E), it is desirable to use a material having 
a relatively high density and a high Young's modulus for the pedestal 28. 
It has been determined that steel and tungsten are suitable materials for 
the pedestal 28. Although tungsten gives a slightly greater bandwidth, it 
is relatively difficult to work with. Steel provides a satisfactory 
compromise between the cutterhead bandwidth requirement and the 
workability. Other comparable materials may satisfactorily fulfill the 
above-said requirements. 
As can be seen from FIGS. 6 and 7, all external surfaces of the assembled 
cutterhead 12 are disposed antiparallel to each other in order to 
eliminate parallel transmission paths for the propagating waves between 
opposite external surfaces during recording. It has been found that the 
parallel transmission paths of the propagating waves increase the 
magnitude of the resonances (i.e., the Q parameter of the cutterhead). 
These resonances are detrimental to the performance of the cutterhead 12 
as a substantially uniform frequency response characteristic for the 
cutterhead is desirable. 
It is noted that, ideally, the motion of the cutting stylus 13 should only 
be perpendicular (e.g., vertical) to the motion of the metal master 10 
being recorded (e.g., horizontal) in order to prevent undesirable 
distortion of the signal recorded in the master. In practice, some 
horizontal motion of the cutting stylus 13 tip may be tolerated provided 
that the horizontal motion remains relatively small and nearly 
proportional to the vertical motion of the stylus tip (e.g., one-fifth) 
throughout the usable bandwidth of the cutterhead 12. In the case of an FM 
encoded video signal, slight waveform distortion of the signal recorded in 
the metal master 10 may be tolerated. However, timing errors caused by a 
variation in the relative horizontal motion, especially, in the 
longitudinal groove direction, with a change in the frequency of the 
recorded signal can cause problems (e.g., echos and ghosts in the 
reproduced picture). 
It has been determined that a cutterhead having a horizontal cross-section 
which is an equilateral triangle has external surfaces which are 
anti-parallel providing a relatively smooth frequency response 
characteristic; and, in addition, has a horizontal symmetry which is 
beneficial in reducing the horizontal motion of the cutting stylus 13 tip. 
The horizontal motion of the stylus tip can be reduced by approximately 
positioning the stylus tip under the centroid of the piezoelectric element 
29 when the cutterhead 12 is in the operative position. As can be seen 
from FIGS. 6 and 7, the altitude of the triangular surface of the cutting 
stylus 13 which is adjacent to the piezoelectric element 29 is less than 
the altitude of the adjacent piezoelectric element surface. The altitude 
of the adjacent triangular surface of the cutting stylus 13 is generally 
made slightly larger than two-thirds of the altitude of the adjacent 
surface of the piezoelectric element 29 so that the stylus tip is 
positioned just forward of the centroid of the piezoelectric element in 
the longitudinal groove direction. The forward positioning of the stylus 
tip compensates for the stiffness and mass loading effects of the cutting 
stylus 13 on the piezoelectric element 29 which tends to shift the 
zero-horizontal-motion point in the same forward direction from the 
centroid of the piezoelectric element. Alternately, the stylus tip can be 
positioned substantially under the centroid of the truncated pyramidal 
piezoelectric element 29 and additional loading (not shown) can be added 
to the remaining portion of the adjacent piezoelectric element surface in 
order to improve the balance of the load on the piezoelectric element. 
Where additional loading is provided for balancing the load on the 
piezoelectric element 29, adequate clearance should be provided for 
removal of the metal chip. 
The design of the cutterhead 12 is such that the frequency response is 
uniform to very low frequencies (e.g., audio frequencies), although the 
cutterhead is normally operated in the upper portion of its useful 
bandwidth over a range of about 1.3 decades (i.e., 9.3 MHz/0.716 MHz). The 
cutterhead operates in the fixed-free mode at the low frequencies, i.e., 
the surface of the pedestal 28 adjacent to the damping members 27, 36 and 
37 is relatively fixed. As the frequency of the signal which energizes the 
cutterhead 12 increases into the midband region, the mode of cutterhead 
vibration shift from the fixed-free mode to a pseudo free-free mode or 
terminated-free mode of vibration, i.e., the surface of the pedestal 28 
adjacent to the damping members 27, 36 and 37 vibrates. The displacement 
of the stylus tip for a given energization signal is not reduced 
appreciably as the cutterhead 12 shifts from the fixed-free mode to the 
pseudo free-mode of vibration. The pyramidal shape of the cutterhead 12 is 
helpful in providing a nearly uniform stylus tip displacement for a given 
energization signal in the midband and upper frequency band regions 
because the center of gravity of the cutterhead is relatively close to the 
surface of the pedestal 28 adjacent to the damping members 27, 36 and 37. 
The cutterhead 12 is geometrically proportioned and damped in order to 
reduce irregularities in the frequency response characteristic in the 
frequency range where the transition from the fixed-free mode to the 
pseudo free-free mode of vibration of the cutterhead occurs. 
As previously indicated, the recording styli employed for cutting groove 
modulation in a lacquer master for recording an audio or a video signal 
must be heated in order to obtain a sufficient signal-to-noise ratio 
during playback (i.e., in order to obtain a proper groove surface finish 
for reducing the surface noise). For example, refer to the U.S. Pat. No. 
3,865,997, issued to J. B. Halter on Feb. 11, 1975, and entitled, 
"TRIANGULAR PIEZOELECTRIC TRANSDUCER FOR RECORDING VIDEO INFORMATION". 
Pursuant to the principles of the present invention, the cutting stylus 13 
cuts short wavelength groove modulation (e.g., 0.6 to 1.6 micrometers) in 
the metal master 10 in a manner that provides a superior signal-to-noise 
ratio even though the cutting stylus is not heated. As the cutting stylus 
13 satisfactorily records in the metal master 10 without heating, means 
for heating the cutting stylus is not included in the cutterhead 12. 
Elimination of the heating means from the cutterhead 12 is a major 
breakthrough in the art of video type signal recording for the following 
reasons. 
First, it results in an increase in the bandwidth of the cutterhead 12 (e. 
g., approximately 1.8 MHz) which makes it possible to record short 
wavelength groove modulation (e.g., 0.6 to 1.6 micrometers) in the metal 
master 10 at relatively high recording speeds (e.g., 450/5 rpm). It may 
even open the door to real time electromechanical recording (e.g., at 450 
rpm) of video type signals. In order to satisfactorily record a high 
frequency carrier (e.g., 4.3 to 6.3 MHz) modulated by a broadband video 
signal (e.g., at rates up to 3 MHz) in real time, the cutterhead 12 must 
have a fairly flat frequency response characteristic extending to at least 
the highest first order sideband frequency (e.g., 9.3 MHz) of the highest 
instantaneous frequency of the carrier (e.g., 6.3 MHz). If, for example, 
the bandwidth of the cutterhead is one-fifth the highest first order 
sideband frequency of the carrier, the recording time would be five times 
the time required for the real time recording of the modulated high 
frequency carrier, and so on. This is so because the modulated high 
frequency carrier, and therefore the recording speed, would have to be 
slowed down by a factor of five (referred to commonly as five-times-down 
recording). Since the longer recording times are inconvenient, 
impractical, and costly, it is beneficial to reduce the recording time by 
increasing the bandwidth of the cutterhead 12. 
The bandwidth of the cutterhead 12 is limited by its major resonant 
frequency. As the major resonant frequency of the cutterhead 12 is 
increased by a reduction in the mass of the moving portion of the 
cutterhead (particularly near the stylus tip), the elimination of the 
heating means from the cutterhead reduces the mass of the cutterhead and, 
therefore, extends the cutterhead bandwidth. 
Of a greater importance is the elimination of the space required for the 
heating means on the cutting stylus 13 because it results in a smaller and 
lighter weight cutting stylus, whereby the cutterhead 12 bandwidth is 
further extended. 
Additionally, the elimination of the heating means from the cutterhead 12 
means that the cutting stylus can be directly mounted on the piezoelectric 
element 29, thereby eliminating the stylus mount (for example, of the type 
shown in the U.S. Pat. No. 3,865,997) from the cutterhead. The elimination 
of the stylus mount from the cutterhead 12 reduces the mass thereof, 
whereby the cutterhead bandwidth is still further increased. 
Second, the elimination of the heating means from the cutterhead 12 
simplifies the assembly, and reduces the cost of the cutterhead. Further, 
the elimination of the heating means from the cutterhead 12 makes the 
construction of small cutterheads feasible and more practical. 
Third, the higher temperature (e.g., approximately 350.degree. F) employed 
with a heated cutting stylus weakens the epoxy bond between the 
piezoelectric element and the cutting stylus, thereby reducing the signal 
level which can be recorded. Further, the weakening of such bond reduces 
the Young's modulus thereof, thereby impairing the frequency response 
characteristic of the cutterhead in the region of the higher frequencies 
of the recorded signal. Therefore, the elimination of the heating means 
from the cutterhead 12 improves the signal handling capability, and 
extends the frequency response characteristic of the cutterhead. 
Fourth, a higher temperature (e.g., approximately 350.degree. F) at the 
cutting stylus 13 is accompanied by a higher operating temperature for the 
piezoelectric element 29 which increases the runaway problem due to the 
self-heating of the piezoelectric element. Therefore, the piezoelectric 
element 29 drive level must be reduced in the region of the higher 
frequencies of the recorded signal. Thus, the elimination of the heating 
means from the cutterhead 12 permits use of the higher piezoelectric 
element drive levels. 
Fifth, a higher temperature (e.g., approximately 350.degree. F) at the 
cutting stylus 13 is also accompanied by a higher temperature of the 
damping structure (27, 36 and 37), thereby reducing its mechanical 
resistance. The reduction of the damping resistance is undesirable because 
the reduction causes the frequency response characteristic curve to be 
less uniform, and the peaks and dips of the curve to be more pronounced. 
Therefore, the elimination of the heating means from the cutterhead 12 
provides a more uniform frequency response characteristic for the 
cutterhead. 
Sixth, a hot cutting stylus will occasionally melt or burn the chips 
generated by the cutting of a groove in a lacquer master; especially, if 
the chips break and momentarily stay in proximity with the hot cutting 
stylus. This is undesirable as a hard residue may form on the cutting 
stylus close to its cutting edges which may impair the groove surface 
finish, thereby reducing the signal-to-noise ratio of the recording in the 
lacquer master. 
Seventh, a lower operating temperature of the cutterhead 12 due to the 
elimination of the heating means therefrom extends the life of the 
cutterhead. 
Further, a method for electromechanically cutting short wavelength 
modulation (e.g., 0.6 to 1.6 micrometers) in a metal master, pursuant to 
the principles of the present invention, can present many improvements in 
the quality of recording as compared with recording in other media (e.g., 
lacquer master, photoresist) in addition to the improvements stated above. 
One, the recording loss may be defined, for the present purposes, as a 
ratio of the peak-to-peak groove modulation (e.g., P in FIG. 2) to the 
peak-to-peak displacement of the cutting stylus 13 used for effecting the 
groove modulation. The recording loss in cutting short wavelength 
modulation (e.g., 0.6 to 1.6 micrometers) in a metal master is lower than 
that obtained in a lacquer master. For example, the recording loss is of 
the order of one dB in the metal master at 0.6 micrometer groove 
modulation wavelength which is approximately 7 dB lower than that obtained 
in the lacquer master. The recording loss in the metal master, vis-a-vis 
the lacquer master, is relatively low because of the lack of springiness 
(non-compliant behavior) of the metal master. It is believed that the 
electromechanical recording of short wavelength modulation in a metal 
master produces less recording loss at short wavelengths (e.g., 0.6 to 1.6 
micrometers) than any other presently available recording technique (e.g., 
lacquer master, magnetic tape, laser beam recording, electron beam 
recording). Since the electronic equalization of the recording for uniform 
response at the shorter wavelengths requires either a reduction in the 
recording level at the longer wavelengths or an increase in the recording 
level at the shorter wavelengths, the reduction of the recording loss at 
the shorter wavelengths extends the usable bandwidth, and/or increases the 
signal handling capability of the recording apparatus. 
Two, a stamper (having negative grooves) could be directly generated by 
electroplating process from the recorded metal master (having positive 
grooves). Plastic disc records (having positive grooves) can then be 
molded from the stamper by a suitable compression or injection molding 
process. However, two more generations of negative and positive matrices 
ordinarily intervene between the recorded master and the stamper used for 
pressing disc records in order to produce a sufficient number of stampers 
required for mass production of the disc records. Many more negative 
matrices (e.g., 20 to 40) can be made from a metal master vis-a-vis a 
lacquer master or an electron beam recording in a photoresist. 
Three, the surface noise in cutting short wavelength modulation (e.g., 0.6 
to 1.6 micrometers) in a metal master is lower than obtained in a lacquer 
master. The reduction in the surface noise in the metal master as compared 
with the lacquer master is believed to be due to a more homogenous 
composition and a more fine effective grain structure (e.g., a near single 
crystalline type) presented by the metal master vis-a-vis the lacquer 
master. The finer effective grain structure assures a reduction in the 
surface noise of the recording in a metal master relative to that obtained 
in a lacquer master, especially, in the region of the short wavelengths of 
the recorded signal. The reduction in the surface noise means that either 
the recording level may be reduced for a given signal-to-noise ratio or 
the signal-to-noise ratio may be improved for a given recording level. A 
reduction of the recording level means that smaller cutterheads having 
wider bandwidths are feasible. The wider bandwidths make possible higher 
recording speeds. Materials which can be processed to have a very fine 
(e.g., a near single crystalline) structure (e.g., copper, speculum, 
certain aluminum alloys, etc.) are suitable for recording short wavelength 
modulation in metal. 
Four, the system for electromechanically recording short wavelength 
modulation in a metal master presents certain processing advantages in 
terms of time and cost as compared with the electromechanical recording in 
a lacquer master. For example, in order to generate a stamper from a 
recorded lacquer master by the commonly employed electroplating process, 
the recorded surface of the lacquer, being non-conductive, must be 
metalized (e.g., silvering operation) in order to make it conductive to 
initiate the electroplating process. The metalizing operation is not 
required when the recording is made directly in a metal master. 
Five, the system for electromechanically recording directly in a metal 
master also presents certain advantages in terms of time and cost as 
compared with an apparatus for electron beam recording on a photoresist 
deposit overlying a pregrooved master. Basically in the electron beam 
apparatus, the master is pregrooved, coated with a photoresist deposit 
which is exposed by an electron beam in accordance with a signal to be 
recorded, the exposed photoresist is removed, (e.g., dissolved), and the 
recorded surface is metalized in order to generate a stamper, etc. 
Additionally, the operations involving the photoresist material are done 
under a special lighting (which does not affect the photoresist material), 
and a class 100 clean room is desirable for the electron beam apparatus. 
These operations and requirements do not exist in the electromechanical 
recording in a metal master method. 
The success of electromechanical recording short wavelength modulation 
(e.g., 0.6 to 1.6 micrometers) in a metal master depends upon a 
combination of several factors; among these are (1) quiescent groove depth 
(i.e., without modulation) of less than one micrometer, and (2) 
peak-to-peak groove modulation of less than the groove depth. The groove 
depth and the peak-to-peak groove modulation dimensions are of importance 
since (1) the force (or power) required in cutting a groove is 
proportional to the amount of material cut (removed) from the groove, (2) 
the amount of the material removed from the groove is proportional to the 
cross-sectional area of the groove, and (3) the cross-sectional sectional 
area of the groove is proportional to the square of the depth of the 
groove for a given included angle at the apex of the groove. In other 
words, the force required in cutting a groove is proportional to at least 
the second power of the groove depth. Because of the very small size of 
the groove and modulation necessary for proper recording characteristics, 
the force required in cutting a groove in a metal master is limited to a 
relatively small magnitude. The reduction in the force reduces the stress 
on the cutting edges of the stylus, whereby the metal cutting technique is 
made feasible and practical since the life of the cutting stylus is 
considerably extended. The recording of short wavelength modulation in 
very shallow grooves in a metal master gives excellent recording 
characteristics. 
It will be understood that the above description of the preferred 
embodiment is susceptible to various modifications, changes, and 
adaptations; and the same are intended to be comprehended within the 
meaning and range of equivalents of the appended claims.