Setting repro head azimuth

May 30, 2010
13,900
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38
Portugal
#21
(...) If by this you meant without alignment tones, then that's another FAQ, and an excellent one that I believe the OP here asked me sometime in the past year. Let's leave it hanging for now.
Perhaps we could consider iterative methods reversing the tape, but it should be a nightmare!
 
Feb 20, 2015
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Cheltenham, UK
#22
Now this is starting to be a great thread :)
 

jdcolombo

New Member
Oct 29, 2017
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#23
You first want to consider that tapes will travel on different paths through different machines. From this we go into forced-guidance tape paths versus precision guidance ones.



This is a fun FAQ that people ask MRL.

The answer is given in a technical paper by J. G. McKnight that explains the method. If you're a physicist, you could find the method rather elegant, as it reverts to first principles.

I glean from your question that you might want a way to set the repro azimuth (ORA) just once, and then leave it alone?

You cannot do that, because each tape will most probably have its own ORA in any machine.

(No one has yet brought up the matter of accurate tape width, and of tape slitting. The variable that this introduces into the subject of guidance is huge.)

A question that I like to ask of training class participants (here I mean training tailored for tape professionals from the recording industry) is to ask them to explain the difference between forced-guidance and precision (non-forced) guidance transports and headblocks.

Or, ask them what is the major difference between the Ampex ATR-102 and the Studer A80 headblocks?

Very, very few will correctly answer this, before sitting through the day's training and actually observing what is going on with tape guidance and azimuth.



If by this you meant without alignment tones, then that's another FAQ, and an excellent one that I believe the OP here asked me sometime in the past year. Let's leave it hanging for now.
The Nakamichi Dragon cassette deck, first manufactured in 1982, recognized that correct azimuth would vary for each tape played, and employed a motorized azimuth correction system that automatically set the correct azimuth for the tape being played. It did so by splitting the playback head gaps for the right channel; the playback signals from these electrically-isolated gaps were sent to electronics that could detect whether there was the slightest time misalignment in the signals coming from the two gaps. If a misalignment was detected, a motorized servo circuit adjusted the playback head azimuth until no time alignment error could be detected. This was all done automatically every time a tape was played (and obviously was done for different sides of the same cassette). I always wondered why Studer or Ampex did not incorporate such a system in their TOL mastering decks. Perhaps one consideration was that the Nakamichi system was complex and required regular service; having the reproduce head attached to a motorized platform also may not have been acceptable for RTR mastering machines seeing heavy use in a studio environment - you would presumably want those heads "locked down" on as stable a platform as possible. And perhaps there was an understanding that in the studio world, tapes would all have alignment tones recorded, which could then be used to adjust azimuth for that tape on the reproducer - so no need for an automated system when the studio tech would take care of it manufally. But the Nak's system did not rely on alignment tones, and therefore could squeeze out the very best performance from any recorded cassette, including commercial dupes of dubious quality. That all this was done in the cassette format 40 years ago is pretty amazing, IMHO.

John C.
 
Feb 20, 2015
53
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6
Cheltenham, UK
#24
In an ideal world, it would be identical.

In the real world, the peak amplitudes for either channel could differ, as could their ORA.

Gap scatter is real.

So is the bias level dependent positioning of the write head's trapping plane.

Bottom line? You want alignment tones on your tapes.

That is, if you care about what's going on.
OK, I always check alignment tones on the distribution and "safety" (I won't get into an argument today about this, save to say that the provenance of all my tapes varies, but they almost all sound great) tapes, which I buy, and always put alignment tones on my own tape dupes.
 

Fred Thal

[Industry Expert]
Jul 15, 2016
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#26
. . . You are reliant on a company like MRL to get it right. . . .
Jonathan,

The MRL paper you want to read is titled "Azimuth in a Magnetic Tape Recorder"

I see that it's not available on the MRL website this morning. I can ask about that.

Meanwhile, the mechanical azimuth tolerance for MRL cal tapes is shown in section 5.2.3 of the MRL publication "Choosing and Using"

http://www.mrltapes.com/choo&u.pdf
 

Fred Thal

[Industry Expert]
Jul 15, 2016
147
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#27
Perhaps we could consider iterative methods reversing the tape, but it should be a nightmare!
Ah ha. You seem to have thought it through.

Essential to anyone understanding this thread, is first understanding the azimuth loss formula.

https://link.springer.com/chapter/10.1007/978-1-4615-2027-6_164

I think it can be easier to grasp this, if it's presented with some illustrations. (I like drawing the head gap and tape on a whiteboard.)

Can anyone cite an accessible reference where the azimuth loss formula is accompanied by illustrations?
 
May 30, 2010
13,900
3
38
Portugal
#28
Ah ha. You seem to have thought it through.

Essential to anyone understanding this thread, is first understanding the azimuth loss formula.

https://link.springer.com/chapter/10.1007/978-1-4615-2027-6_164

I think it can be easier to grasp this, if it's presented with some illustrations. (I like drawing the head gap and tape on a whiteboard.)

Can anyone cite an accessible reference where the azimuth loss formula is accompanied by illustrations?
Thanks for the reference. I found googlebooks fortunately previews this section of the book. https://books.google.pt/books?id=6UzaBwAAQBAJ&pg=PA344&lpg=PA344&dq=%22The+nature+of+azimuth+loss+is+shown+in+Figure%22&source=bl&ots=s35gJ2PEKZ&sig=fCM-veGDHXSdIkgHHlA6qs9-DKI&hl=en&sa=X&ved=2ahUKEwiU_qTqh5rdAhVIzRoKHX1ZCLEQ6AEwAnoECAEQAQ#v=onepage&q=%22The%20nature%20of%20azimuth%20loss%20is%20shown%20in%20Figure%22&f=false

People should look at the figure 164 of page 345 - it is copyrighted material I do not feel allowed to copy it to WBF.
 
Apr 10, 2014
80
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Fairfax, VA
#29
is it a mistake if Channel 1's VU meter is at its peak, but Channel 2's VU is no longer at its peak?
.
In that case, you adjust channel 1 for peak and mark the mechanical position of the adjustment screw. Then peak channel 2 and mark it's mechanical position. Then set the azimuth screw mid way between the two marks.
 

15IPS

New Member
Sep 15, 2018
9
1
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#30
Playback a 16K test tone for maximum output. (Coarse)
Fine tune for minimum spread using X-Y.
Watch your hearing. Use protection if possible.
 

15IPS

New Member
Sep 15, 2018
9
1
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#33
No, that's not really correct.

Absolute amplitude measurement accuracy plays no role at all in adjusting for ORA.

All you need to observe, is the meter needle peaking on the azimuth adjustment tones.

That said, your observation that VU meter circuits can be their own problem is noted and I agree.

But using VU meters for adjusting repro response is not what's under discussion here.
Well, sure they are. You would not align a machine for broadcast or duplication with a calibration tape without regard to output level and say that is has been lined up. Otherwise, your levels could be all over the place. I think what Bruce B. is saying is that if you are taking the time to perform a full alignment, you would first naturally calibrate the deck's meters externally to ensure that the levels indicated on the deck with the alignment tape reflected the correct head/eq/level alignment, and not other variables such as VU or line amp sensitivity.
 
Likes: Bruce B
Oct 11, 2018
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#34
I have to disagree on the correct method for azimuth. In my opinion there is only one reliable method for any recorder with more than 1 track. That is to use a dual trace oscilloscope in the normal mode, and display two channels, one on each trace. Then adjust azimuth so that the two waveforms line up. Using level alone only works on a mono machine. The level method can result in the phase performance being compromised. Note that in record mode, the record head azimuth if done in selsync, and the azimuth if recording through the machine and replaying, can be different, as the level of bias on each channel can affect where on the recording head the recording process takes place. Also note that reproduce eq will affect azimuth setting. Thats why the process of alignment should be repeated until all factors line up correctly, azimuth, 1 K level, and high frequency level. On a multitrack machine, there can be errors in the way the head is manufactured, so azimuth should be checked on multiple tracks. Just using track 1 and track 24 on a 24 track is not sufficient.