Setting repro head azimuth

topoxforddoc

Well-Known Member
Feb 20, 2015
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Cheltenham, UK
I was taught (as a beginner) that getting the repro head azimuth correct is the first part of lining up a tape deck. Incorrect azimuth reduces high freqs and leads to muffled sound quality. So how best to do this? Is it

a) using a scope on X-Y and a 10k tone on a test tape

b) estimating phase using mono/sum function on a stereo/2 channel PPM meter with a 10k test tone on a calibration tape

c) measuring frequency response and dynamic range on a spectrograph with a a frequency sweep

d) AN Other method

Answers on a (small) postcard please
 

squasher

Well-Known Member
Apr 7, 2016
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Do you take the trouble to use a special screwdriver (i.e. de-magnetized) when adjusting the azimuth on the head?

I'm assuming that if you're stuck without a special screwdriver, one can de-magnetize it with the same procedure one uses to de-magnetize the repro head (and capstan, rollers, etc)....
 

jdcolombo

Well-Known Member
Oct 29, 2017
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Method A

Note that MRL test tapes have two frequencies for azimuth adjustment: 8khz for "coarse" adjustment and 16khz for "fine" adjustment.

John C.
 

Bruce B

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Do you take the trouble to use a special screwdriver (i.e. de-magnetized) when adjusting the azimuth on the head?

I'm assuming that if you're stuck without a special screwdriver, one can de-magnetize it with the same procedure one uses to de-magnetize the repro head (and capstan, rollers, etc)....


I demag the heads a few times a year.... besides, the azimuth adjustment is actually a hex wrench that Studer issued with the machines.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
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138
Cheltenham, UK
I do A & B.

A) i) when I am setting up the repro head on a calibration tape and sometimes ii) when I am comparing the alignment tones on a new pre-recorded tape

B) when I am comparing the alignment tones on a new pre-recorded tape (if I am feeling lazy and don't have time to hook up my scope)

Charlie
 

Fred Thal

[Industry Expert]
Jul 15, 2016
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Answers on a (small) postcard please

My bad. The following might read more like an intro to a book chapter.

To better understand why the correct answer is D, it's helpful to keep in mind what it is that you're trying to accomplish.

Start by considering the definition of optimum read head azimuth (ORA):

"That orientation of a read head gap that produces the peak short recorded wavelength response from the tape"

To fully appreciate the above definition, make sure you first understand the azimuth loss formula and the relations of its terms and variables.

Then keep in mind that ORA will be unique to any given tape.

It can be thought provoking (and fun) to next throw in some new, but related questions:

What does finding ORA have to do with inter-channel phase matching?

(Inter-channel phase matching is of course what you're doing when you use a two-channel oscilloscope in X - Y display mode to adjust ORA.)

So what about stereo (two-channel) heads (both write and read) having gap scatter?

(Remember, almost all two and greater track heads have some degree of gap scatter.)

What is different in the case of a full track monaural recorded tape (reproduce alignment calibration tapes are in this class), versus tapes that are two-track recorded?

The above questions help illuminate some of the introductory topics found in the study of head azimuth and ORA.

The good news (and the take-away for any audiophiles reading this) is that you do not need to own, or borrow, or hook up an oscilloscope to find the ORA for any tape.

It is actually much simpler to do.

And your result will be correct.
 

squasher

Well-Known Member
Apr 7, 2016
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Hi Fred,

For those of us who are on the edge of our seat (like me), can you give us a hint as to what your procedure for "D" is?

This topic is very germane for me at the moment as I tried adjusting the Azimuth on the repro head of my Studer A807 while at a friend's house just LAST NIGHT (he's borrowing my A807).

Needless to say, I'm a newbie to this procedure but "thought" I knew what I was doing. I quickly ran into issues that I didn't understand and hoped to find out more info on the web before I continued. As an aside, I own an oscilloscope, a full-featured DMM, and the appropriate MRL tape for NAB (21J205, if anyone cares).

My procedure was as follows:

1) Hook up the outputs of my repro preamp to the 2 channels of my oscilloscope.
2) Put the scope in X-Y mode and dial the voltage sensitivity on the scope to be equal (and at suitable voltages)
3) Play the MRL tape (1 kHz tone) and adjust the azimuth on the repro head so that the lissajous pattern on the scope is a 45 degree line (and is not oval in shape)
4) "Fine tune" the azimuth so that the subsequent frequencies on the MRL are still displayed with a 45 degree line. My MRL tape contains the following frequencies, in order: (1k, 500, 8k, 16k, and a bunch of others not important to this discussion).

The first "problem" arose when I tried adjusting azimuth for the 8k tone. In this case, I could tweak azimuth so that the lissajous pattern displayed a straight line but the X-Y amplitudes were different (the 45 degree line was now a 30 degree line). Since this suggests that the X and Y voltages are different, I wasn't sure if this was acceptable (or is a give away that the High Frequency adjustments on the repro preamp are off).

The second problem was that when going back to the 1k tone, the lissajous pattern was an oval again....

These problems repeated when I tried adjusting to the 16k tone on the MRL tape.


Where did I go wrong?

Where can I get answers to the questions you pose that will lead me to your answer to "D".
 
Last edited:

Fred Thal

[Industry Expert]
Jul 15, 2016
161
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Hi Fred, can you give us a hint as to what your procedure for "D" is?

. . . Where did I go wrong?

. . . Where can I get answers to the questions you pose that will lead me to your answer to "D".

> can you give us a hint

Yes.

First, look again at the definition of ORA: "the peak short recorded wavelength response"

Next, remember that "peak response" implies an amplitude measurement (not phase).

The VU meters on the tape machine (when switched to read the output of the playback amplifier) are perfect for this purpose.

Owing to the VU's (or PPM's) analog meter movements, you can easily discern the amplitude peaking as you adjust the read head (aka the play head) azimuth.

So put away the oscilloscope and look at the VU meter(s).

Use the (coarse azimuth alignment) 8kHz tone first, and peak that by adjusting the read head azimuth.

Then, still looking at the VU meter, try to again peak the finer resolution, 16 kHz azimuth alignment tone.

This is the very definition of finding the ORA (optimum read head azimuth).

> Where did I go wrong?

Unable to see what you're doing, I'm unsure.

It's possible that by using the oscilloscope (with adjustable gain in its independent vertical deflection amplifiers) you're only making the task more difficult.

If for some reason you absolutely need to use an oscilloscope for your amplitude measurements (maybe you don't have VU meters?) then first try switching the scope out of its X - Y display mode to read only the amplitude of the displayed waveforms.

> Where can I get answers to the questions you pose that will lead me to your answer to "D".

Any good university's electrical engineering library would be a starting point. Or come take an ATAE course where you'll receive a suggested bibliography, in case you want to get into the weeds of analog magnetic tape technology.

But as I'm saying here, adjusting ORA is actually very easy. Just look at the VU meters.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
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138
Cheltenham, UK
Fred,

OK, does that mean that using the PPM meters on mono-sum is ok - i.e. peaking one meter whilst aiming to reduce the other meter to near zero is correct, as well?

Thanks

Charlie
 

jtinn

Industry Expert
Apr 20, 2010
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Here is where I have a problem with the calibration tape method. You are reliant on a company like MRL to get it right. After going through numerous tapes of theirs, I have very little faith in their consistency.

Chicken or the egg? How are they calibrating their machines to make the tapes? Are they using a calibration tape that they made from a machine that they calibrated.. how? Are they calibrating the machines using some other method? There has to be another way of doing this besides using a tape which you really have no idea if it is accurate.

How can you adjust azimuth without a tape?
 

squasher

Well-Known Member
Apr 7, 2016
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138
Hi Fred, Thanks for the lengthy response...

One key missing ingredient in your description is that azimuth should be adjusted until the peaks at all VU meters are be reached at the same time, right? (2 VU meters for 2-track, 4 VU meters for 4-track, 1 VU for mono, etc.)

i.e. 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?

Since traditional VU meters are needle based, how do you know that the current reading is at its peak? Seems like a harder thing to do visually than adjusting a lissajous pattern so that it looks like a line instead of an oval.


Both of my decks are wired to external repro amps and no longer have functioning VU meters.
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
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Fred,

OK, does that mean that using the PPM meters on mono-sum is ok . . . ?


Again, you want to first determine exactly what it is that you're trying to accomplish.

Go back and read the definition of ORA and see that adjusting for optimum read head azimuth means adjusting for peak short wavelength response for a recorded tape in question.

On the other hand, mono-summation optimization stems from concerns about minimizing comb-filtering when two (or more) channels are added. (A common historical example of the problem was the monaural broadcast of natively stereo programs.)

Mono-summation optimization is a perfectly legitimate concern, but it's a phase-related phenomenon, not an amplitude one.

That said, some mastering engineers do worry a great deal about inter-channel phase matching.

But if they are correcting for this on an analog tape playback machine by tweaking the read head azimuth, they might in fact be moving away from ORA. Do you see the danger here?

We shouldn't say that someone is wrong if they happen to be primarily concerned with inter-channel phase matching.

But we should understand how that is technically different than finding ORA.
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Here is where I have a problem with the calibration tape method. You are reliant on a company like MRL to get it right. After going through numerous tapes of theirs, I have very little faith in their consistency.

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.

Chicken or the egg? How are they calibrating their machines to make the tapes? Are they using a calibration tape that they made from a machine that they calibrated.. how? Are they calibrating the machines using some other method?

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.

How can you adjust azimuth without a tape?

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.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK
Go back and read the definition of ORA and see that adjusting for optimum read head azimuth means adjusting for peak short wavelength response for a recorded tape in question.


.

Thanks Fred, so does the peak amplitude on both channels of a stereo play head have to be the same, or just the peak (even if the amplitude is not identical on the meters)?
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Hi Fred, . . . 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?

If reproducing (that's tech lingo for playing) a full-width recorded (aka full-track monaural) reproducer alignment calibration tape, that could be an indication that you have gap scatter in your two-track read head.

(Are we deep enough into the weeds yet? Why don't you take a class?)
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Thanks Fred, so does the peak amplitude on both channels of a stereo play head have to be the same, or just the peak (even if the amplitude is not identical on the meters)?

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.
 

Bruce B

WBF Founding Member, Pro Audio Production Member
Apr 25, 2010
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The VU meters on the tape machine (when switched to read the output of the playback amplifier) are perfect for this purpose.

Owing to the VU's (or PPM's) analog meter movements, you can easily discern the amplitude peaking as you adjust the read head (aka the play head) azimuth.

I have never seen VU meters that are perfectly correct. You need to get the meters correct before you start any of this. This is where you need to feed a 1kHz signal at 1.23v into the meters to calibrate them. THEN you can do your other adjustments!
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
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123
I have never seen VU meters that are perfectly correct. You need to get the meters correct before you start any of this. This is where you need to feed a 1kHz signal at 1.23v into the meters to calibrate them. THEN you can do your other adjustments!

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.
 

microstrip

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Here is where I have a problem with the calibration tape method. You are reliant on a company like MRL to get it right. After going through numerous tapes of theirs, I have very little faith in their consistency.



IMHO this is a very strong claim. What were the values of the errors in degrees you found in their tapes? How many tapes did you evaluate and what was their reaction when faced with such problem?

I am an user of two MLR calibration tapes, that were not inexpensive, and feel worried when reading your post.
 

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