Creating your own Azimuth Tape?

Creating an Azimuth Tape
from a 
Full Track Master Azimuth Tape

Before I begin, I'll declare straight away that the processes and results of making your own azimuth tape from a 3-Head Nakamichi deck, is potentially flawed from the start. Although I cannot offer 100% guarantee of my claim, my findings suggest inherent problems in the creation of a precise azimuth tape of your own. So if you see one of these cheaply reproduced azimuth tapes on ebay, my advice is - think again!

My experiment with this idea began with checking the azimuth of my Nakamichi DR10 with an ant-audio.co.uk full-track reference tape. As it turned out, my Nakamichi DR10 and the said reference tape were identical (to within 0 - 22.5 degrees at 10Khz) as I checked the phase difference between Left and Right channels from the full track azimuth signal. 

Congratulations to both Nakamichi and ant-audio.co.uk! - I don't think their matching was down to pure chance, both parties know what they were and are doing!

The DR10 and other Nakamichi machines employ a sophisticated and highly engineered solution to aligning head tilt, track height, and azimuth for both record (RH), and playback (PH) heads independently. 

On the far right is a plastic (nylon?) slotted screw adjustment for PH azimuth, and to the far left - RH azimuth adjustment. Conventionally, on most other machines (inc the Revox machines), turning slotted screw heads allows alteration of all three alignment parameters. Of note, some force (although small) is needed to achieve these changes - not so with the DR10. The changes in azimuth require little force, and are indeed stepped in discrete increments so the engineer/technician can 'count' (if need be) how many discrete (clockwise/anti-clockwise) angle movements have been made - clever stuff!

 

Now that I knew that my Nakamichi DR10 playback head (PH) was (very probably) correctly azimuth aligned, the next step (as indicated in the service manuals) was to azimuth align the record head (RH) to the same 90 degree angle. This is achieved by injecting a 10Khz, 12Khz, or 15Khz  sinewave signal at any reasonable modulation (-20dB to -5dB, ref "0") into the record head, and observe the output (as seen by the PH head) on the oscilloscope. The theory is: observe the traces for maximum output, at which point both L/R channels should be in phase.

In the Nakamichi service manual, they do not mention phase alignment of both channels in this instance, but do instruct the engineer/technician to observe for maximum output. 

Why not mention phase alignment? The answer to this question came to me after several attempts to align the RH with the PH head. 

Sure, I could align these heads together perfectly - no problem, but, it wasn't until after checking the results with several different tapes that I realized that - every tape (Maxell, TDK, Sony, BASF/EMTEC etc) will probably yield a quantitatively different recording azimuth result!

Maxell XLS-1, or TDK CDing I

When I fed 12.5Khz into the RH for Maxell XLS-1, or TDK CDing I, I moved the azimuth for the RH for max output and perfect phase alignment - no problem!

L/R traces in phase (12.5Khz shown)

Then I experimented.....

SONY UX-Pro, TDK D (1988)

I put in these tapes, applied another 12.5Khz signal to the RH (record head).

The result - both the Sony UX-Pro and TDK D L/R output traces (based on the above (Maxell XLS-1/TDK CDing-I) RH alignment  settings) were not in phase! Wow, so what's going on?

L/R out-of-phase traces, 12.5Khz

Of course this can be corrected, if I altered the RH azimuth again to suit the Sony UX-Pro or the TDK D tapes.

Summary/Conclusion (so far)

Playback Head (PH) alignment is easy (using a full track professional alignment tape).

Record Head (RH) alignment may not yield a consistent result when mixing tapes - it seems to depend on the tape, and shell? The oscilloscope trace differences between RH and PH are small (45-90 degrees at 12,500Hz), but nevertheless measurable. I sense that Nakamichi and other manufacturers knew this too? 

No need to worry though, you and I won't hear these small high frequency phase shifts in music, but nevertheless I was surprised by the results.

What Causes this Azimuth Variability?

 

Well, I can only guess - the tape flow from Record Head (RH) to Playback Head (PH) is not perfect, there must be tiny stresses in the tape flow causing one channel to take longer to travel across than the other? 

 

31/3/2022: Also, there will be minute alignment
differences between left and right channels head gaps in both heads?

Homemade Nakamichi Azimuth Tape 

Playback on Other 3-Head Cassette Decks

Now here is another oddity that I cannot explain -

(1) The Nakamichi DR10 Playback Head (PH) is aligned to that of the full track ant-audio.co.uk Reference Tape. Result: stable, in phase, and maximum output achieve - great!

(2) Then I record a 10Khz test tone at any reasonable level (-20dB, -10dB .. -5dB) and then align the Record Head (RH) so that Playback Head (PH) plays back the 10Khz signal in phase and maximum level - great, no problem there!

The theory - the DR10's RH is now aligned to both the PH of the Nak DR10, and the azimuth of the full track Reference Tape.

This means I can effectively use this new tape 'copy' as an azimuth tape for my Nakamichi DR10, BUT not for other machines!? - their playback traces have shown phase shifts in the region of  60-90 degrees!? Why ... I do not know!? Despite these other machines having already been PH-azimuth-aligned via my ant-audio.co.uk Full Track Reference Tape.
___________________________________________________

I've been thinking about this - again, it must be uneven micro stresses between L/R channels from the record head (RH) on their way to the playback head (PH) is going to produce this variability. 

The tape may be stressed more on one side than the other, thus taking a longer time to travel.  Surely then, we can never be certain that the RH azimuth is ever aligned to the PH azimuth?  Small discrepancies of around 45/60/90 degrees of L/R phase shift are inevitable?

THE ONLY FAIL-SAFE WAY TO PRODUCE A TRUE AND RELIABLE AZIMUTH TAPE OF YOUR OWN IS TO SET UP AND ALIGNED AND STABLE TWO-HEADED CASSETTE DECK OR A STABLE FULL TRACK CASSETTE DECK?

 

Calculating phase delay distances:

 
The phase delay physical distance between the two peaks or indeed any similar part of the two sine waves can be calculated from using the following expression: 

v=f*λ

or

velocity=frequency*wavelength

This is a standard physics equation relating velocity, frequency, and wavelength. 

What we require is to first find the effective wavelength of the modulated sine wave on tape.  Transposition of formula gives ...

 

 λ=v/f

or

wavelength = velocity/frequency

The velocity of the tape is 0.0475 metres/second. 
The frequency of my test sine wave is 12,500 cycles/second.

Then the sine wave wavelength is ...

λ= 0.0475/12500 = 0.0000038 metres 
or
3.8μm 
(3.8 millionths of a metre)

However, the worst 'delay' between left and right channels was approximately 90 degrees or 90/360*3.8μm.

 

Therefore, the physical delay (or distance) 

between channels = 0.95μm

or for 45 degree phase shift = 0.475μm 

In plain speak, the delayed wave is 0.95/1000 or 0.475/1000 of a millimetre respectively.


(3/10/2019: This article is subject to the correction of mistakes, additions, or subtractions)
Latest changes/amendments: 12/10/2019 
Latest changes/amendments: 4/11/2019.
Small changes - 25/04/2020. 
Changes - 18th November 2020.

Minor correction: 24/01/2021.