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ARSCLIST  February 2014

ARSCLIST February 2014

Subject:

Re: Squeaking cassettes Tape Tension and Dual Capstans

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Thu, 6 Feb 2014 10:29:41 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (160 lines)

Hi, Shai,

This is a very interesting data point!

Does the RX-202 rely on the built-in pressure pad to maintain good 
tape-to-head contact?

If we visualize a Venn diagram and we have an overall circle called 
"Soft Binder Syndrome" and completely enclosed within that circle is 
another circle called "Sticky Shed Syndrome" your tapes would fall in my 
taxonomy outside Sticky Shed Syndrome but within Soft Binder Syndrome.

It is, in fact, exactly this case (for reels and cassettes) which caused 
me to develop the concept of a taxonomy for these failure mechanisms 
that is, at least in part, based on effective restoration techniques.

In general, the type of squealing that you describe is not improved by 
baking, which is why it is not Sticky Shed Syndrome.

Now, why your data point is very interesting is that no one I have 
discussed this with has tried a tape which squeals on a higher-end Nak 
deck on a simple two-head transport as you just did.

My first reaction to this discovery is that it fully reinforces the Soft 
Binder Syndrome theory because the tape is, perhaps, not pressed against 
the heads with as much force as in the dual-capstan design. 
Additionally, it is possible that since the dual capstans are NOT 
running at the same surface speed (the feeding capstan is running 
slightly slower than the takeup capstan, though which is on speed may 
vary--see note below).

So, with less force and potentially less tension-related deformation 
(which may or may not aid in depositing the hooks onto the head that 
ultimately cause the squealing) it is easy to comprehend why the RX-202 
can play the tape.

The squeal is stick-slip between the tape and the head and often does 
not start immediately (though you have not said whether yours does). If 
it does not start immediately, then it is often caused by little pieces 
of binder being deposited on the head and then acting as the hook on the 
head side of the interface that actually connects with high points 
(asperities) in the tape and causes the sticking which then a few 
microseconds later slips.

The entire construct is not much different in concept (but, of course, 
in scale) to the slippage of tectonic plates past each other. The squeal 
is, of course, a high-frequency variation in the speed of the tape past 
the heads and why it prints to the transfer in most cases. If the squeal 
were not frequency modulating the audio, and was just annoying to the 
transfer technician, then it wouldn't print (or print as much) to the 
digital copy.

So, your experiment may be another option in treating squealing 
cassettes. I had a similar experience with squealing reels and found 
playing them faster and on a different machine helped.
http://richardhess.com/notes/2007/11/08/success-with-squealing-shamrock-031-tape/

As an aside, I have been led to believe over the last decade that this 
squealing cassette tape that was bulk-loaded into cassettes for 
duplication purposes (i.e. duplicated from pancakes and then loaded) is 
an Agfa product, but I have no hard proof.

NOTE on dual capstan configurations: I was recently thinking about 
dual-capstan configurations and did a little digging.

There appear to be two major modes of operation, but one is quite 
interestingly subdivided. Note that the concept of dual capstans also 
applies to single capstan machines with a "U" tape path where the 
capstan contacts the tape both going in and coming out.

----------
Dual capstans that run at different surface speeds:

This group is more widespread, I believe. There are many topologies that 
cause this to happen.

(1) The 3M reel-to-reel machines used a single capstan with two 
different diameters and the stepped pinch rollers pinched into the 
smaller diameter on the input side and the larger diameter on the output 
side.

(2) Two physical Capstans
(2a) Unidirectional transports often use two capstans driven by a belt 
where either the capstan diameter or the pulley diameter is different, 
creating the differential. The Nakamichi MR-1 is an example of this.
(2b) Bidirectional transports often use two separate capstan motors 
where the speed control of the individual motors governs the tension. 
Where this gets complicated is I had always believed that the output 
(takeup) side capstan pulled the tape at the proper speed while the 
supply (input) side capstan provided drag. I recently found out that the 
Studer A80QC reel-to-reel cassette tape quality control player sets the 
supply capstan for the proper speed and the takeup capstan runs a bit 
fast to set the tension. This leaves me totally confused. I have not yet 
seen a definitive writeup (leave it to Studer to fully explain something 
in their manual) on the Dragon as to which runs on speed.

Further to this, Studer switches which motor is connected to which servo 
card on the A80QC while I think the Dragon just sends a different 
control signal and the servos are mated to their specific motor. The 
Studer approach, while relay-intensive (these are AC servo motors), 
means that if the capstans are truly the same diameter (this is a Swiss 
product, remember), then the forward and reverse differentials are the 
same and there is no ambiguity as the actual supply and takeup servos 
are always in that position.


----------
Dual capstans that run at the same surface speed:

While the topologies vary, the concept is the same as I understand it. 
The simple thing is that while the capstans filter the reel tensions 
from the head loop, ultimately, the incoming and outgoing reel tensions 
also translate to the tensions within the loop because the tape deforms 
while passing the capstans, especially with pinch rollers.

The topologies are:

(1) Single capstan with two pinch rollers like the Technics RS-1500 (a 
U-shaped tape loop)

(2) Dual capstan with pinch rollers (I don't know for sure of any as 
it's too easy for two separate capstans to run at different speeds

(3) Dual capstans with three-point tape wrap, but no pinch rollers. It 
seems that pinch rollers were removed more from instrumentation 
recorders than from audio recorders. My 1970s Honeywell 101 machines 
have two capstans that must rotate at the same speed. The tape comes 
over a roller, a partial wrap around one side of the lower capstan, up 
past the record head, a 180 degree wrap around the upper capstan, past 
the repro head, and a partial wrap around the lower capstan and over a 
roller out to the takeup reel (on the same axis as the supply reel!).

These two capstans are tied together with a non-elastic belt (stainless 
steel) with one motor driving the upper capstan (if I recall correctly) 
and the lower following. The input and output rollers are there to make 
certain there is wrap around a portion of the lower capstan. The capstan 
ramps up (quickly) for play or wind (there are no tape lifters). The 
transport is effective.

Anyway, that's what I think I know at the moment for the concept of tape 
squeal and the generation of tape tension using two capstans.

Cheers,

Richard



On 2014-02-06 5:55 AM, Shai Drori wrote:
> I am transferring a whole bunch of tapes for a client and found
> something interesting. Tapes made by SKC from the 90's will squeak in
> all my decks (Naks included) except for the RX-202. It's a 2 head design
> with one capstan. The closed loop Naks squeak like a cat in heat. I
> think it has to do with tension and head geometry. Any thoughts?
>
-- Richard L. Hess                   email: [log in to unmask] 
Aurora, Ontario, Canada                             647 479 2800 
http://www.richardhess.com/tape/contact.htm Quality tape transfers -- 
even from hard-to-play tapes.

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