Since many people kept mentioning "fixed azimuth", I just want to know clearly what does this exactly mean?

It means the unit has been designed and manufactured in such a way that there is no way to adjust the azimuth. Hence fixed azimuth.

Units that are not designed as fixed azimuth usually have screws that are spring loaded and by turning the screws alter the azimuth.

Fixed azimuth are cheaper to manufacture as the manufacturer makes an assumption that the azimuth will always be at a predetermined angle. However sometimes (more often usually) due to poor tolerances in manufacture the azimuth in a fixed azimuth unit can be badly out of spec and very difficult or next to impossible to correct.

On the other hand non-fixed azimuth units are more expensive to manufacture because they rely on a worker to calibrate and set each unit at the correct angle before they are shipped out. Over time the seal on the adjustment screw can loosen and the unit lose its calibration, but at least you can always recalibrate and reseal the adjustment screw again.

Here is an example for Fixed Azimuth-housed in a plastic mold:

Here is a non fixed Azimuth:

Nevertheless we must consider also another interesting aspect of the matter.

A system with a fixed azimuth have a unbeatable feature.

If you're lucky enough to have a walkman with a fixed azimuth, fully calibrated in both directions, or if you can find a technique to calibrate it, leaving it fixed, the head will not move never for ever, retaining his position indefinitely.

The same thing can not be said of the heads with azimuth adjustment, especially on the system used in Aiwa walkman, always subject to shifting and wear of components. In addition, the plastic structure, which supports the head with azimuth fixed, has the characteristic of better isolate the head by the vibrations, having a resonant frequency different from the metal. 

You're right about that Dottor Walkaman!

Fixed azimuth is like winning the casino...if you're lucky enough, but chances are very slim it will be calibrated to the desired point especially in auto reverse.

IMHO it's sounds unreal to have a $30~$250 Walkman be precisely fixed like that. However, for an expensive walkman that could be more realistic, to have it fully calibrated in both directions. Maybe in today's manufacturing standards and technologies which everything is electronically controlled, but back in 1980~1990s just like Plop said: you have to "rely on a worker to calibrate and set each unit at the correct angle before they are shipped out", and that what makes it an expensive Walkman.

Taking as an example the DD9, we can better understand the issue.

The DD9 Walkman have the most expensive and carefully chosen materials and construction. Yet Sony has chosen to adopt the system with a fixed azimuth. I do not think the choice has been to save money by having the excess quality in all other parts of the DD9. Obviously, my belief is confirmed by many tests and comparisons.

The Sony has chosen to adopt the system at a fixed azimuth with plastic support on his walkman better and more expensive.

Among the many DD9 who have passed through my hands, some had the perfect azimuth in both directions. Those DD9 were the best ever, the azimuth was always perfect. Unfortunately, because of the inevitable tolerances in the manufacturing process, the majority of DD9 not have an azimuth perfect, but with a modification careful, one can obtain an azimuth perfect, which not longer moves. Also do not forget the function of the plastic backing, which insulates better than the head.
I checked many Aiwa walkman, with the oscilloscope. They have always in need of calibration azimuth. But even after it is fully calibrated, it is not as stable as you want, there are always small displacements and oscillations. These displacements and oscillations, are always in continuous increase, in the course of its use, until, after a certain period, again need to be recalibrated.

Well...you're talking about the DD9 - a $400~$500 Walkman with all the bells and whistles that are possible in a player, Sony put a great effort in it - no wonder!

Aiwa on the other hand with it's PX303 (Non Fixed - bi-azimuth head) is a $250 unit.

But the real benefit of non fixed azimuth is, it can be fixed even by a newbie with a plain screwdriver. while for a fixed head you need a professional to accomplished that.

I thought the USP of the DD9 was that it was void of bells and whistles and concentrated it's efforts on good old fashioned playback.

Exact Agentorange, Sony has concentrated all efforts in DD9, for maximum quality without frills or unnecessary things. I am convinced that Sony has considered a frill is also the possibility of setting azimuth, getting a head that does not change its position, even after many years of use.
The calibration of the azimuth of the PX-303, can be done by an amateur, but only if they possess an oscilloscope. The azimuth tuning by ear, especially on a head auto-reverse, is like playing roulette in a casino (in the words of TheLion), it might seem that you have reached the right position, but not absolutely so.

Fixed azimuth units although cheaper and quicker to manufacturer, do not necessarily equate to cheap to sell on. A manufacturer may be trying to reduce costs and maximise profits, and so any potential savings to the end consumer negated. For a manufacturer to justify that higher price tag at their premium end, they may select parts made to a tighter tolerance to guarantee a higher percentage of units with the correct azimuth.

Similarly for non-fixed azimuth you may find them in budget buy units too. For example old production lines where it is too costly to re-tool for fixed azimuth. It may still need a human to calibrate the alignment, but how much is the factory paying for their time and is it still cheaper than to re-tool for fixed azimuth?

The savings in either method of implementation (moreso in the fixed azimuth case) both in terms of money and continued correct tracking can only be realised by how good a job was orignally done in assembling the parts in the first place.

With 2 separate motors, Disc Drive, quartz lock, Dolby B-C etc. we can call it a premium player for maximum quality, don't get me wrong for using the term Bells and whistles (I meant in quality terms), DD9 have all the luxury of a full size cassette deck.  

Don't you think? 

I Totally agree, but we shouldn't underestimated the ear, some people with the "Golden ear" can hear with a great accuracy and hear some anomalies made using Oscilloscope.

But I liked the "roulette in a casino" part..ha.. ha LoL 

Surely there are the golden ears, but can hardly provide objective information. Each person with their own ears feels so subjective. The oscilloscope is objective, can do no wrong, shows you exactly when the waves are in phase, the human ear can not do it, even if it was of gold studded with diamonds.
Yes, I agree, the sentence of the casino is great fun, so I used it also gives a good idea to seek the correct azimuth with only ears, golden or not.

I Totally agree but I have to add that although I am aware of the technical side and the audiophile side, we shouldn't forget that the whole science of music is subjective including its technical side; from musical instruments through sound reproduction tools, is based solely on the ear. As less we use technical tools it is more acceptable to the ear ie natural. For example: Sometimes the extra ("accidental") note is what really makes the music piece sounds as a masterpiece.
Another example is: the debate between the analog and digital, while analog is warmer and easy on the listener, the digital is more colder and precise but more commonly acceptable.
I like your analogies, they are humerous...Studded with diamonds...lol..you're cool

An excellent technician and a cool member.

Thank you, you are very kind. I must add that you and I are agree on almost everything. Also I prefer the analog on digital, I try only to get the most from analogue, also using instruments not totally analog, if necessary.

Non-fixed azimuth are the types that I've seen in cassette decks. Some cassette decks use bi-directional azimuth adjustments.

If the head is 90 degree in either the FWD or REV side, it means that all the 4-channel gaps are perfectly aligned. You do not need to align separately for a stationary 4-track head separately.

What do you mean "especially in auto reverse"? If you are talking about the rotating head version, I can quite understand but aligning a 4-track version is the same as a 2-track head.