Three Head Cassette Deck Phase Shifts

Three Head Cassette Deck Phase Shifts

Analysis
As my previous article suggested, there is a likeliness of phase shift between Left and Right channels from where the Record Head (RH) writes to the tape, to where the Playback Head (PH) reads the signal. 

At 10Khz, on my Nakamichi DR10 discrete 3-head deck, this was variable depending on the cassette tape used - on some tapes the phase shift was approximately zero, and on others approximately 90º. I could reduce this L/R phase shift to (effectively) zero even at 15,000Hz for a particular brand of tape, call this - Brand A set of tapes. But then would have to re-calibrate the record head RH azimuth for Brand B, Brand C etc . There were minute discrepancies at work here that I could not pinpoint, nor control.

I decided to perform a limited experiment with the Aiwa AD-F770 3-head deck, where the RH/PH heads are 'sandwiched' together. 

There is no independent control over either RH or PH heads here. Indeed, if there is some difference between the angle, or differential (L-to-L, R-to-R) distance of the two head gaps - that's it, I cannot change this!

Again, I decided to apply a test sine wave of 12,500Hz with a Maxell XLS-I tape inside.

As the trace shows, again there is a delay between left and right channels, perhaps some 60º?

Purely to satisfy my curiosity, I even altered the azimuth of the RH/PH during this experiment, just to confirm that no change should occur. Indeed there was no change in the delay angle.

According to my calculations, the physical delay between channels will have been in the region of 60/360*0.0475/12500 or 0.63um, ie just over 1/2 of a micron.


All very small indeed, and of course will not be noticed in speech or music recordings.

Whether these delays can be attributed to the tape warping/stretching between RH/PH heads, or a slight misalignment between the vertical left/right channel head gaps (effectively distances) is arguable - perhaps both?

 

At some point I realised that if this micro L/R delay distance 'D' is fixed by physical constraints, then ...

 

from basic physics we have,

 

v=f⨉λ   (velocity=frequency ⨉ wavelength)

 

also, the delay distance D can be written ...

 

D= [φ/360]⨉λ

 

and so substituting into v=f⨉λ, and rearranging we obtain ...

D = [v/360]⨉[φ/f]

 

where ...

  
v=0.0475m/s default tape speed, 
φ=phase shift, (degrees, not radians)

f = frequency of test signal.

If the above is true, then the ratio of φ/f will remain approximately constant, ie if you increase frequency f, then phase shift φ will increase also to match the constant ratio.

 

If that's a confusing concept for the reader (understandable), then I'll rearrange the formula and write ...

 

φ = D*f*360/v

So for my Aiwa AD-F770, we have ...

D=0.00000063 metres (from above calculation), 
f=12,500 Hz (test signal)
v = 0.0475 m/s (for all cassette decks)

φ =  0.00000063 * 12500 * 360 /0.0475 = 60º. (as before!)

Predicting then, the phase shift for ...

10,000Hz .... 48º,

5,000Hz ... 24º,

1,000Hz ... 5º,

400Hz ...  2º.

Indeed, at 1000Hz, and below, the visible phase shift between left and right channels was difficult to discern on the oscilloscope. I've not bothered to photographed this.

Conclusion
So it appears that we cannot guarantee to make a precise (very narrow phase shift) azimuth tape from (any) three headed cassette deck? Simply because - we cannot trust the RH (Record Head) to effectively follow the azimuth line of the PH, even when we know the PH (playback head) is precise.

Perhaps the only precise and persistent (ie accurate) way to create azimuth tapes of our own - is to first correctly set up a two headed machine (Erase + Record Head) with one single full-width track record head, which is also the playback head. Of course such a machine must have excellent transport properties.

Exception: Today, I dug out the Revox B215. By chance, I put the machine through a similar limited experiment. At 6,300Hz input, the result was ~ 0º (zero!) L/R phase shift on play back, with just an occasional and slight differential wobble of about 10º. Wow, this machine is in a different league! (12/10/2019)

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3/10/2019: Articles are subject to the correction of mistakes, typo errors, and additions. 
Small amendment made on 25/04/2020 

25/8/2020 - partially rewritten article for clarity.