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One of the issues with phonograph records is that the edge
of
the record necessarily moves faster than the middle, if the
turntable speed is constant. This inevitably means that
sound
quality differs between the first (outermost) and last
(innermost) parts of the recording. Fortunately,
a moment's
thought and a couple of well-aimed G&Ts have provided the
answer.
Suppose the outermost part of the groove is a conventional
circle (technically one turn of a spiral, but you understand).
And suppose also that the innermost part of the groove is
not a
circle, but a sine wave wrapped into a circle - a sort of
cogwheel shape but smoothly sinuous. There will be, say,
ten
sine-wave oscillations for one revolution of the record (so, a
sinuous ten-pointed cogwheel). If the amplitude of the sine
wave is chosen correctly, the total length of the innermost
track will be the same as that of the outermost (circular)
track.
Between these two extremes (that is, in the middle tracks),
the track will gradually become more and more sinusoidal,
so
that *all* parts of the track will move past the stylus at the
same speed. If you watched the tone arm as it played a
complete record, it would start out as usual by just
travelling
slowly inwards as the record played. As you approached the
later parts of the record, the tone arm would start to swing
back and forth, superimposed on a gradual inward motion.
A tiny fraction of extra space will be needed between the
tracks, to accommodate the gradual increase in
sinusoidality,
but this will be quite small and will be largely offset by the
greater overall length of each of the inner tracks.
YouTube: Techmoan: Tefifon player and Tefi cartridges
https://www.youtube...watch?v=nBNTAmLRmUg
Mentioned in my anno [notexactly, Sep 28 2019]
Wikipedia: Tefifon § History § 1930s
https://en.wikipedi.../wiki/Tefifon#1930s
Mentioned in my anno. Says: "Both [the Tefiphon and Teficord] use loose tape, unlike the cartridge-loaded tape of the Tefifon." [notexactly, Sep 28 2019]
Wikipedia: Helical scan
https://en.wikipedi...g/wiki/Helical_scan
Mentioned in my anno [notexactly, Sep 29 2019]
[link]
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Why not just move all the tracks further away from the hub ? |
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Because that would be pointless. |
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This is way to accommodate the same total amount of track,
but ensure that the linear velocity is the same throughout the
playback, whilst using a constant turntable speed. |
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Then why not make the recording truly linear, and avoid the problem of fixed angular velocity/variable linear velocity ? |
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A cylindrical form is not without possibilities ... oh, wait ... |
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This will cause the azimuth angle between the track and the needle to vary
several times over the course of a turn, more severely toward the center of the
disc, which will cause cross-mixing between the left and right channels if it's a
stereo record. |
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It should be possible to unmix the channels with analog electronics, I think,
maybe, but it would require knowledge of the current angle, which will vary
several times over the course of a turn. Maybe the turntable and discs should
have an index mark that you align when you start a record, and the player just
compensates for the mixing based on its knowledge of the current azimuth of the
platter. |
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Another solution could be to mount the needle in the tone arm in such a way
that it can rotate about its vertical axis, with the tip not quite on the axis of
rotation, so that it can find its own direction like a swivel caster. |
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[Ian], like a Tefi cartridge [link] but not continuous-loop? The Wikipedia article
[link] says the Tefi company first developed tape machines using grooved tape
that was loose rather than in a cartridge, but it doesn't say whether that was
reel-to-reel or not (probably). |
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Just use long, stiff, rectangular .2cm wide records and play them linearly. |
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Sorry, [Max], you've got a violation of Kepler's second law going on here. |
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//a violation of Kepler's second law// How so? |
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//cause the azimuth angle between the track and the needle
to vary// But if the record is cut using the same geometry...? |
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Well, all record players violate the "equal areas in equal times" thing, just
because the radius of the point-of-play changes throughout the course of
the recording. But, the diagram you'd draw to demo this looks almost
exactly like the Kepler's 2nd Law picture. |
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You're thinking along lines of "if an inner track has the same length as an
outer track, and the table speed is constant, the groove speed will remain
the same." Yes, it will, however... |
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If you look at discrete points along the curves, you'll note that the groove
speed is equal to the rotational velocity multiplied by the radius. A point on
an outer groove has a longer radius than that on an inner groove. That
means a groove cut tangentially to the radius of play will always have a
groove speed directly related to the length of the radius. |
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Your gimmick here is to add grove length radially to force the needle to
cover more distance during the period of rotation. However, being a sine
wave, there will be an inner point that's tangent to rotation, perpendicular
to the radius, and a similar outer point. It is trivial to show that the groove
speed will be higher at the outer radius than at the inner one; you won't
get a constant groove speed. The speed at the curve's innermost point will
definitely be an absolute minima; at the outermost point will likely be an
inflection point, but not necessarily a maxima. Depending on the amount
of radial distance added, the outer point could well be a local minima.
However, given the theorem of the mean, your groove speed will be a
curve which runs through at least those two points, all points in between,
and possibly points above (actually, *necessarily* points above if you're
going to get the *average* of the curve to be equal to the value of the
speed in the outermost groove). |
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Try an epicyclic needle mount next. It'll still fail, but will have really
interesting failure modes. |
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Hmmm. I have a horrible feeling that you may be right.
Damn. |
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What's needed is a square record with the track laid out in a back-and-forth pattern, using a fixed stylus and a supporting table with the ability to rotate. Then there's no need for a central hole so that gives a higher density because the record is held at the edges. |
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Why not just do what CDs do, and vary the rotational speed
as a function of playing-radius? It would need to be precisely
controlled at both recording and playing (and the 2 need to
match...) but it would be reasonably easy to do. |
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// It wouldn’t surprise me to discover that there was once a stylus or magnetic based
transduction system which involved keeping the tape still, laid out in a strip, and the head
races along it. // |
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Helical scan [link] (as used in VHS and other formats) is kinda like that. The tape and the head
both move, but the head moves much faster. |
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// But if the record is cut using the same geometry...? // |
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Then I feel that you might lose range or resolution in frequency, amplitude, or both, but
cannot say exactly why at the moment. |
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// you won't get a constant groove speed // |
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Yes; I realized that when I read the idea yesterday. But if the record is cut using the same
geometry…? |
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It's probably easier just to download the MP3 |
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Could the record be made as a series of annular rings, with internal gearing mechanisms, so that each ring rotated with the same linear velocity as the next? Adjacent rings could have their gearing synchronised so that the groove can pass from one ring to the next when they are lined up with the needle. |
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It would be a system of tiny nested epicyclic gear rings ... cool ! |
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Nail down the details and post it. |
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Nested annular rings could be challenging to keep
the entry and exit point of each track aligned at the
proper time. And the artwork overlayed would take
on the form of vizualized whirrled peas. |
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Alternatively, everyone could just adjust their singing
pitch and speed to compensate. Imagine the
challenge to the percussionist.
I'll see if Ross Bagdasarian is available. |
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