Adobe Photoshop PDF...with VTA determination: One is a high frequency 1M lone consisting of 1.85 kHz...
Transcript of Adobe Photoshop PDF...with VTA determination: One is a high frequency 1M lone consisting of 1.85 kHz...
MORE THAN ONEERTICAL
_ANGLEJON M. RISCHand BRUCE R. MAIER'
Discwasher Laboratories has recently been investigating highperformance phonograph play
back, and as a result of our investigations, we have become aware of severalunusual aspects of the vinyl playbacksystem (VPS). In one of our studies, wewere concerned with the microscopicallysmall dimensions involved where therecord groove is being dragged past theplayback stylus, as well as more visibleaspects such as vertical tracking angle(VTA). In our tests, we wished 10 retrieve
as much undistorted information as possible, and among the VPS parameterswe felt should be optimized was VTA.Correct VTA for the VPS, in classical theory, involves having the playback anglematch the cutting angle (Fig. 1). Physically, VTA is the angle between the surface of the record and the line describedby the contact point of the stylus in thegroove and the pivot point of the cantilever. This relationship is analogous toand originates in the recording process.The theoretical importance of matching
VTAs between recording and playbackhas been widely expounded, and popular explanations of causes and effectsare generally available, but they shouldbe examined in some detail to fully understand their consequences. Mismatchof playback VTA to the recorded VTA issaid to cause one minor effect: Frequency modulation of the highs present inprogram material in response to any significant level of low-frequency verticalgroove motion. Referring to Fig. 1 again,notice that the vertical-modulation arc
Fig. 1 - The playback system,showing the relationships ofvertical tracking angle (VTA),stylus rake angle (SRA}, andvertical modulation arc; sideview through groove.
\VERTICAL
MODULATIONARC
IIIIIf.t--....StylusI RakeI Angle"
I
iActualptyotPoint
Verllcal Tracking Angle
Record Groove
•Members, Research Staff, DiscwasherLaboratories, Columbia, Missouri
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DiamondStylus ,"'------yERTICAl CONTACT SURFACE
Bottom of Groove
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Fig. 2 - Conical stylus tracingerror with vertical groove modulation,showing contact patch or footprinfshift from reference position;side view through groove.
Fig. 3 - Conicalstylus tracing errorwith lateral groovemodulation, showingcontact point(center of contactpatch) skew relativeto cutting stylus;fop view.
t
shown is characteristic of any vertical(out-of-phase) modulation in the groove.Because of this arc and the resultant forward and backward motion along thegroove, the momentary linear cuttingspeed would be varied proportionally tothe amount of horizontal displacementthis vertical motion causes. Any time thisoccurs, any frequencies higher than thedominant vertical-displacement low frequency will be modulated by that lowfrequency displacement. Of course, allthis is not a problem if the recorded VTAis replicated in playback, whereby thefrequency modulation (FM) is cancelleddue to the vertical-modulation arc beingthe same. The sonic effects of a VTAerror are similar to flutter, heard aswavering of the high frequencies. Thereis an important difference between typi.cal flutter and frequency modulation dueto VTA error, and this is that the FMcauses wavering in lelation to the lowfrequency content of the program material whereas flutter is generally a repetitive, continuing variance affecting theentire range of program material. It is thistime-dispersive FM distortion that arisesfrom VTA error and causes the theoreti·cal furor of concern over cartridges withhigher than optimal VTAs.
The literature indicates that if one hada cartridge with an in~erent VTA higherthan recorded angle, it could be tiltedback (cartridge body allowing, somewon't) and minimum distortion would beobtained. This isn't quite true, sincestylus shape, in addition to VTA, also influences FM distortion.
Conical (or spherical) styli have a certain built-in amount of FM distortion witheither a lateral or vertical modulation
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(Figs. 2 and 3). As shown in Fig. 2. dueto the ultimate rounded tip shape of theconical stylus, a significant vertical modulation will cause the contact patch toshift. From the center or reference position, a stylus drift or lateral thrust aheadand then behind will cause exactly thesame type 01 distortion as VTA error.Both are slope related and can augmentor tend to cancel depending on the sign(polarity) of the VTA error. The amount ofFM due to the stylus shape will vary inrelation to the stylus radius and the amplitude of the vertical modulation.
W ith lateral modulation (Fig. 3),the FM inherent to conicalstyli occurs on both channels
in opposite directions, so the mono(sum) signal would show little or no FMeffect (depending on the accuracy of lateral tracking angle). In stereo playback,however, the two channels illustratedwould have high frequencies recordedalong with the lows. The low-frequencymodulations cause the stylus contactareas to skew or twist, thus causing adegradation 01 high-frequency phasingand stereo imaging. The net result ofthese and other factors is that correctVTA for conical styli will only minimize,not eliminate, the FM distortions.
Other StylUS shapes reduce theseproblems because of their smaller contact scanning radii. Typically, today'sgenre of elliptical or modified-Shibatastyli have a scanning radius no smallerthan 0.2 mil, while conical styli rangefrom 0.5 mil to 0.8 mil. This smaller radius allows a more consistently definedcontact with the groove, reducing thesetypes of FM distortion.
Our tests indicated elliptical and Shibata-type styli have another parameter ofdramatic importance not present withconical styli. This overlooked factor iscalled stylus rake angle or SRA, which isthe angle that the vertical center line ofthe stylus contact patches make with thegroove modulation ridges. Stylus contactareas for different types of styli areshown as black vertical patches at thebottom of Fig. 4. As seen in this figure,the Shibata-type stylus shows the longest, narrowest contact. while the conicalstylus predictably has a circular contactpatch or footprint. The elliptical andmodified elliptical stylus shapes fall inbetween these extremes. Due to its long,narrow footprint, the Shibata-type stylusis theoretically very sensitive to positionalchanges in SRA. Any misalignment ofthe footprint relative to the groove modulation ridges will cause its vertical footprint span to increase, resulting in possible losses of very high frequency modulation (scanning loss).
Our detailed models showed that thisincrease in the effective scanning radiusdoes not result in a simple, smooth, effective broadening of the stylus footprint.Due to the nature of the tilted, narrowcontact edge, the manner in which thisedge contacts the groove modulations issomewhat nonlinear and more complexas compared to a simple conical stylusshape of comparable radii. Physicalmodeling showed the tracing errorswhich arise with a misaligned Shibatatype stylus are similar to, but greater indistortion level than those of a conicalstylus, and similarly result in some lowfrequency-dependent FM of the decoded high frequencies.
-50
dB->0
..., ..... , , ..........'u>o'T~••......,....
....HI .......,~-
............010"...
Bottom: Side viewthrough groovewith black patchesshowing styluS footprint or contactarea.
__=Iow frequency bands
___=high frequency bands
~O""""....,.............. Iw"'""'...
dBFig.5-AnIMdistortion vs. VTA variation comparison,showing null pointsin the high and lowfrequency sections.
•Top: End-on viewor profile as stylusrests in groove withno sink-in shown.
Fig. 4 - Groovecontact geometryof four commonstylus shapes.
We therefore hypothesized seriousgeometric potential for improper SRA ofShibata-type styli. The groove modulations can grab at the scanning edges ofthe stylus, torquing or attempting io twistthe stylus which can send vibrationalshocks up the cantilever.
From the modeling of these factors, itwould seem both VTA and SRA shouldbe corrected for optimal playback. Yetthe physical connection of cantilever andstylus causes both of these parametersto vary simultaneously. There is a fixedrelationship between the inherent VTA ofthe cartridge, when set up as recommended, and the SRA which resultsfrom the fixed stylus chip vs. cantileverattachment.
W hen considering the propervertical alignment of our testsystem, we had to decide
whether we should align for correct VTA,proper SRA, or some optimal compromise position so as to maximize undistorted information retrieval. Before wecould make that decision, we decided toassess the effect of varying VTA. Ourtest system consists of a Denon directdrive DP-80 turntable with a modifiedDA-401 tonearm with the ability of inplay rear-pivot-height adjustment via aprecision micrometer. Several differentmoving-coir cartridges were used in testswith an HA-1 000 pre-preamp runninginto a modified lab reference preamp.The output of this preamp was observedvia a Tektroni~ 466A Storage Oscilloscope and/or with a GenRad 2512Spectrum Analyzer. Test data could beplotted on an Easterline-Angus 575 X-Vplotter for reference plots, etc.
Some initial tests were conductedconsisting of spectrum analysis of vertical modulation 1M bands (CBS STR-11 2test record) where the vertical angle wasvaried using a moving-coil cartridge having a modified elliptical stylus. The results of these were somewhat inconclusive as there seemed to be no clear-cutindication of a minimum level of distortion at the various angles of playback.There were some subtle and generallyinconsistent shifts in the distortion spectra with changes in VTA, although theirsignificance was not determined untilsome time later.
In order to control the variables in ourtest system, cartridge tests were performed using the DIN 45-542 VTA testrecord [1], which has bands with varying
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V \J \t! \lJ
ponent; the horizontal ticks are 2 kHzapart on a linear frequency scale fromd.c, to 20 kHz. The dark lines shown arethe original distortion components (plussome noise components as the frequency goes up) at a VTA of 1617 degrees,which is the angle cut into the record [2].The dotted lines rising up at some pointsrepresent the increase of those distortioncomponents with an increase in the VTAby 4 degrees to 20Y;> degrees. Noticethe increases in the second-order components of about 5 dB, consistent withdata reported by others [31 who haveperformed 'tests with conical styli. Thereare a few other locations where the levels come up a bit, but no major trends
_ = Correct VTA____ =Angle Increased 4'
are indicated. The major increases thatresult from the 4-degree VTA error areroughly those predicted by theory andpast experimentation. Since the conicalstylus used has essentially no SRA, thedifferences in distortion spectra are dueentirely to playback angle not matchingthe recorded angle.
This experiment points out very clearly that if SRA is not a playback variable,proper matching of record and playbackVTA results in lowest playback distortion. It must be kept in mind that the increase in the second harmonic of 400Hz is due to slope-related waveform distortion, while the increase of the 4-kHzcomponent sideband is due primarily to
, ,
I:III!'~II,:' I, , ,
i I Ii ~, ,I
I '"
IiiiI
__= Correct SRA
____=Correct VTA
':
,, ,, :
,,,
I iIII
:,,
Fig. 7-SRA vs.VTA (or recordlplayback anglematCh), showingthe increase indistortion with misaligned SRA, eventhough the record!playback VTA ismatched.
Fig. 6 - Disfortionvariation with VTAchange, showingthe increase of distortion componentswith a 4-degreemisalignment inVTA.
VTA. Two groups of bands are involvedwith VTA determination: One is a highfrequency 1M lone consisting of 1.85kHz and 3.15 kHz with a high-side 1Mproduct of 5 kHz, whereas the other seclion is a low-frequency 1M tone consisting of 370 Hz and 630 Hz giving a high1M product of 1 kHz. Neither 1M productis harmonically related to the base frequencies, and thus no masking confusion occurs.
Figure 5 is the data plot from this DINrecord using the cartridge with the modified elliptical stylus. Notice the shallowness of the high·frequency bands null(point of minimum distortion) comparedto the low-frequency bands null. There isalso a difference in the angle at whichthe null occurs, which tends to hold truefor any stylus shape with an SRA potential. If VTA were the only effect beingmeasured, the 1M distortion nulls for thetwo bands should be very close in slope,shape, and location, We theorized thatthe observed difference in the nulls (Fig.5) was due to SRA interaction with theshorter wavelengths involved in the highfrequency section of the tests. The shallowness of the high-frequency null ismost likely a result of the different SRAto-VTA relationships between the cuttingsystem and playback cartridge (about25 degrees difference in this cartridge).We feel it was a similar effect in the initialVTA tests that caused the spectral plotof distortion products to show little overall change on the CBS test record.
Tests with a cartridge having a conical stylus always gave a much closercorrelation between the nulls for the twodifferent frequency sections and tendedto give a deeper null for the high-frequency bands than for other (elliptical,Shibata) types of styli. The reason thatnull points for the two frequency sectionsdo not give exactly the same angle anddepth was hypothesized to be due tosome of the tracing distortion mechanisms inherent in the conical stylusshape, as discussed earlier.
One means of isolating VTA parameters is to use a cartridge with a conicalstylus, which has no BRA because of itscircular contact footprint. Figure 6 is aspectral plot of a vertically modulated 1Mtest band (400 Hz and 4 kHz) made using a moving-coil cartridge with a conicalstylus. The vertical tick marks are 10 dBapart, with the top of the graph startingat -25 dB down from the 400-Hz com-
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\ (
V \J \t! \1I
ICBS STR 1t 2 test record. Band 7. Side B (400 Hz and 4 kHz).
Table I - Frequency modulation data for different conditions.
References1. Available from Gorham Audio Corp., 741 Washington 51., New YoriI, N.Y. 10014.2. White, James V. and Arthur J. Gusr, "Three FMMethods fOf Measuring Tracking Angles ot PhonoPickups." jour, ollhe Audio Eng. Soc" Vol. 27. No.4, April. 1979. p, 242.3. Halter, Jefome B. and J.G. Woodward. "VerticalTracking Angle Errors in Srereodlsk Systems, ,. Joor.01 the Audio Eng. Soc., Vol. 12, No.1, Jan.. 1964,p.8.
12.0%t 1.1%12.8%t6.4%
ConicalConicalConicalConicalShibataShibataConical
Stylus Type Sum of 2nd-Order Sideband
- (Maior Distor-tion Level)
standard for these lengths, and cartridges we have examined show grossdifferences in cantilever length and donot correlate to cutting systems.
Thus, our investigations clearly showSRA to be a more important controllablevariable than VTA. Our dialogue withcutting engineers indicates that VTA currently varies between 16 and 22 degrees, depending on the lathe system.SRA, however, is generally 91 to 95 degrees relative to the record surface in order to facilitate lacquer "chip" (cutawaystrand) removal.
Proper hi-fi set-up should thereforeconcentrate on cartridge adjustment thathas the tip of the stylus pointed "back"toward the tonearm pivot, and the top ofthe stylus tipped "forward" so that thecontact SRA face is 92 degrees between the styl.us and the record surface.Such alignment will at least approximatecorrect SRA. (One cautionary note: TrueShibata styli do not have their stylus contact area or footprint lined up with thebulk of the stylus chip, and this shouldbe taken into account when adjusting forproper SRA.) The effects are clearly audible on a fine audio system. AI
VerticalVerticalVerticalVerticalVerticaPLateralLateral
Modulation
Freq. Deviation at 4 kHz
21%1.8%22%2.5%t.4%
L 1%, R 0.6%L 1.3%, R 0.8%Mono 0.0% for
both
14 "16l-'i°19"21 y.! 0
tl14°14 "16Y.!°
These conclusions were further confirmed by some tests utilizing a laterallymodulated 50Q-Hz asymmetrical squarewave cut from Denon test record XG7003. This recorded signal has a seriesof finely detailed harmonics extendingabove 40 kHz. We postulated that amisalignment of the stylus would alter orlose the harmonics. When a 4-degree tiltto the optimal SRA was introduced, alteration of the harmonics as low as 5kHz and 7 kHz occurred and tosses ofharmonics above 30 kHz were evidentl
These changes are subtle, but at thesame time consistent and repeatable.
We studied the frequency deviationfor the 4-kHz component of the 1M toneused throughout these tests, An experimental comparator based on a PLL ICwas used for these tests, While absoluteaccuracy may not hold, the relative rankings remain accurate. Table I lists the results of these measurements taken under various conditions of VTA. It can beseen from these figures that there is analarming amount of distortion present,although in practice vertical modulationtends to be rare in recording. In fact, thethickness of the recording lacquer, commercial considerations, and engineeringexpertise generally keep vertical cuttinglow, and thus phase informationcoherent
Our calculations indicate that maximum cantilever vertical-arc travel is typically 1 degree due to these limitations.Another theoretical aspect of VTA matchoften overlooked is cantilever length,which should be matched between cutting and playback systems. There is no
increased frequency modulation of 4kHz - a more objectionable form of distortion than harmonic distortion.
The next step was to test for distortiondifferences between the optimization atVTA for proper SRA alignment or for vertical-modulation arc matching. At thisstage of our experimentation, we attempted some tests using a Shibata-typestylus with a bent cantilever tube to giveodd combinations of SRA-to-verticalmodulation arc alignment. We were never able to make a satisfactorily "clean"bend due to the Shibata-type configuration and its need for critical vertical(head-on) alignment, but the data produced were intriguing.
We were, however, fortunate 10 havein our stock of cartridges a unit deemeddefective due to a stylus misalignment.This cartridge had a modified-Shibatastylus which was slanted more than atypical unit. When properly aligned forSRA, this cartridge was slightly morethan 4 degrees "low" in proper VTAmatch. Figure 7 illustrates the results ofthe correct SRA versus correct VTA experiment, with the same basic data displayas Fig, 6. A distinct distortion increase is shown when SRA is misalignedana the correct VTA match is also made.Compare Figs. 6 and 7, and it will beseen that for an equal degree ofmisatignment, the SRA parameter ismost significant in causing a rise in distortion, especially higher order distortionproducts. Notice, too, that when themodified Shibata stylus is correctlyaligned for SRA, distortion products areat lower levels than when VTA is correctly aligned for the conical stylus. Thesedata are fairly conclusive regardingwhich parameter is of importance for dif-ferent styli. •
Reported listening tests concerningVTA alignment have said that as little as1/30 of a degree can make an audibledifference in the clarity of the music, witha higher than optimal misalignmentcausing excess brightness. These reports typically do not distinguish between VTA and SRA even when the report mentions the existence of SRA. Theresults of our tests indicate that theparameter being optimized in these reports was almost undoubtedly SRA. Ourown informal listening tests bear this outas well. When SRA is correctly alignedthe sound quality "locks-in" and the retrieval of minute details is enhanced.
AUDIO/MARCH 1981 25