Fishtails wrote:Hi people
What are the benefits of linear tracking if any?
Look forward to your opinions!!
It's time for me to weigh in on this, and I realize I'm poking a hornets nest with a stick. I've modified and built several arms since I was a teenager, and I've learned a benefit of linear tracking arms that I have not seen much discussed before. Paradoxically, it is related to offset angle geometry but has nothing to do with tracking error! IMO, the main and not-at-all obvious advantage of linear trackers (LT's) is that the LT produces an order of magnitude lower frequency modulation distortion [FMD] during mass/compliance resonance dynamics. Let me see if I can explain this without hijacking your thread.
Frequency modulation distortion in this case is essentially the same phenomenon as flutter in a tape transport or motion picture projector. Extensive research was performed on the topic in the 1950's and the bottom line is that perceptive sensitivity to flutter is a bell shaped curve with maximum sensitivity in the region of 6Hz. With tonearms, it occurs because of scrubbing of the stylus fore & aft along the groove during resonance conditions. In the vertical direction this is primarily governed by stylus cantilever length and the angle of the cantilever relative to the record surface. In the horizontal direction it is likewise governed by the cantilever length, and in this case by the angle of the cantilever with the TANGENT of the groove, but now the offset angle and overhang become major players in the equation. Of course, damping, compliance, etc., are factors. The basic concepts describing this were very well explained in Shure's literature discussing the dynamics of their viscous damping brush, and readers are encouraged to review that before I proceed further. The document is available on Shure's website here: http://shure.custhelp.com/app/answers/d ... b0ZiWWs%3D
OK, now that you're back, let's look at the vertical case (From Shure):
Vertical displacement of the head causes scrubbing along the groove. All arms are susceptible to this. Mass, compliance, damping, and cantilever angle are factors. You can't do anything about the tracking angle - that's fixed by the cartridge design. Shure's viscous damped brush is VERY effective on mitigating scrubbing induced flutter in the vertical mode.
For the horizontal case (From Shure):
Horizontal displacement of the head also causes scrubbing along the groove, and all arms are susceptible to this, too. HOWEVER, notice in this case that the angle is the offset angle. You CAN do something about the offset angle. In a linear tracker, the offset angle is zero. Vsin(zero) = zero! Eliminating the offset angle eliminates the scrubbing, which means no FM distortion (no flutter). 12 inch arms with a smaller offset angle do better than 9 inch arms, but not by much. Here's the horizontal case redrawn to the same scale except with the offset angle changed to zero:
Well, no, its not THAT simple. It doesn't totally eliminate the flutter, but linear tracking arms have an order of magnitude less horizontal mode scrubbing and hence, less FM distortion, than do conventional arms with an offset angle. Bruel & Kjaer noted this in their investigations of mechanical resonances in turntables and printed an application note on the subject in 1977. This is available in the VE library:viewtopic.php?f=46&t=1281#p3544
A detailed mathematical accounting of the correlation of tonearm geometry to the FM distortion phenomenon was published in 1982 in the Journal of the Audio Engineering Society:http://www.aes.org/e-lib/browse.cfm?elib=3825
To demonstrate the audible difference in FM distortion, or flutter sensitivity, between conventional pivoted arms and LT's you need a test record that has a horizontally modulated low frequency sweep combined with a mid-frequency pilot tone. Unfortunately, the Shure Audio Obstacle Course era 4 & 5 disks use a vertically modulated low frequency sweep, and most other test records that have a horizontally modulated sweep don't include a tell-tale mid-frequency pilot tone. The best test records I have for demonstrating this effect are the Ortofon #0001 and #0002, track #8, which has a 2349Hz and 2960Hz pilot tone superimposed on a low frequency horizontally modulated sweep. Ortofon chose those frequencies because we are very sensitive to pitch variations and flutter effects in that range. Another is Hi-Fi News, Side 2, track 2 which has a 1000Hz pilot tone superimposed on a low frequency lateral sweep. On these tests you hear a warbling or flutter effect on the pilot tone as the arm/cartridge encounters the mass/compliance resonant frequency. The flutter effect in the horizontal mode is much less audible with [servo driven] linear tracking arms than with conventional pivoted arms that have an offset angle and overhang. High compliance, low VTF cartridges are the most susceptible. I suspect that many of the posts I've seen lately on "Jico wobble" are related to this problem, and those cartridges would benefit greatly from a low mass linear tracking arm.
On real records with music the FM effect can be triggered by warps, as well as certain transients that cause a sudden change in stylus drag. Note that preamp subsonic filters can somewhat mitigate amplitude related problems associated with low frequency arm wobble, such as woofer pumping. But, once the scrubbing induced FM distortion is there from the head bobbing around and you hear the flutter effects, you can't filter that out.
I have been using a Rabco SL8E for which I retrofitted a low mass viscous damped unipivot arm and an opto-electronic continuous variable speed servo control of my own design. It is currently fitted with a Shure type 4, and this combination has been able to play anything I throw at it without a squawk. The damping brush pretty much eliminates vertical mode FM flutter, and the linear tracking geometry (no offset angle) pretty much eliminates horizontal mode flutter. Judicious damping at the unipivot stabilizes everything.
Another inherent advantage of this type of LT design is that the stylus drag forces are in the same plane as the cantilever and the horizontal motion pivot of the arm. Forces on the cantilever resulting from variations in stylus drag in arms with offset angle have a tendency to resolve themselves into both vertical and horizontal components. Tracking a warp typically also results in some horizontal movement. With LT's, what happens in vertical, stays in vertical, and vice versa.
I am a big advocate of linear tracking arms. Sorry, I'm not a fan of air bearing LT's.