The topic of the thread is misleading: the "facts" I will state here may not be factually correct. Here is how the story goes.
I have tried to experience good quality cassette performance, I had a Nakamichi, which died, I bought another deck (an Aiwa), then got a Sony Professional Walkman, and have been playing, measuring, tweaking and reading up a lot. In the process, I began to become curious about this thing called azimuth alignment. I don't think I have my facts right.
Therefore, I thought the best way to get the facts right will be to list down what I've understood, in a thread here, and invite all of you to correct my understanding. Later, if this endeavour works out, I can maybe create a web page out of it.
So, I'll now list all the things I've "understood" (right or wrong) about azimuth alignment, with the aim of asking you all to correct me. Don't treat these points as facts till they are confirmed by those who know much more than I do.
This is what I have understood.
Corrections and comments eagerly invited.
I have tried to experience good quality cassette performance, I had a Nakamichi, which died, I bought another deck (an Aiwa), then got a Sony Professional Walkman, and have been playing, measuring, tweaking and reading up a lot. In the process, I began to become curious about this thing called azimuth alignment. I don't think I have my facts right.
Therefore, I thought the best way to get the facts right will be to list down what I've understood, in a thread here, and invite all of you to correct my understanding. Later, if this endeavour works out, I can maybe create a web page out of it.
So, I'll now list all the things I've "understood" (right or wrong) about azimuth alignment, with the aim of asking you all to correct me. Don't treat these points as facts till they are confirmed by those who know much more than I do.
- What is azimuth: Azimuth is the "angle" which the tape head makes to the tape. Ninety degrees is what is desired. It's almost never ninety degrees. (What is this "angle" exactly? Which part to which part?)
- Azimuth alignment impacts HF response: If the azimuth is slightly off, then HF response suffers. As per this review of the Nak Dragon, an error of 0.25 degree (one quarter of one degree, or 15 minutes) results in a response loss of 14.6dB at 15KHz and 25.5dB at 17KHz. We may not believe the exact dB figures, but we can appreciate the seriousness of azimuth for HF response.
- Record and playback heads must have matching azimuth: With all this discussion of azimuth of 90 deg being desirable, there is a second, more confusing dimension. It seems that it's not good enough to have an absolute azimuth of 90 deg. It's important to have the record head azimuth match the playback head azimuth. The error between these two azimuths seems to be the thing which causes HF loss. (Does this mean that the absolute azimuth angle is, sort of, not relevant? Will I get CD-quality HF response if both record and playback heads are both exactly matching at, say, 88 or 89 deg?)
- Azimuth issue affects all tape recording: Azimuth is an issue for all types of tapes, be it analog or digital, cassette or R2R, audio or video. It's related to the basic physics of recording a signal on a moving magnetic strip. In this thread, we'll keep the discussion restricted to analog audio cassettes, but let's just put on record that the "scope of the problem" encompasses all types of tape recording and playback.
- Azimuth adjustment using a screwdriver: In more primitive tape machines, azimuth is adjusted by turning a screw (I think it's a bolt, not a screw, but whatever). This bolt probably drives a worm gear, thus allows a quarter turn of the bolt to make a fractional-degree change in the head's alignment. It seems that in all tape head assemblies, the azimuth adjustment bolt is on the other side of the head to that which has the tape guide (the Y-shaped plate through which the tape passes). So if the tape guide Y-shaped thing is on the right side of the head, the azimuth adjustment bolt is on the left side. Exactly how you know that the azimuth is correct varies from the simplest play-it-by-ear approach to the X-Y oscilloscope approach.
- Azimuth calibration by end-user: In more sophisticated tape machines, there are automatic or manual mechanisms to help the user get the "correct" azimuth without using a screwdriver.
- What you adjust against what, in order to get the "correct" azimuth, depends on what type of record and playback head arrangement you have.
- Single combined record and playback head: these are "two-head decks" where one big head is used for both sensing the magnetic field (during playback) and driving current through the coil and creating the right magnetic field (during recording). It's just one coil, used differently at different times. All cheaper decks and all recording Walkmans, including the top-end Sony Professional WM-D6 series, have heads of this type. In these mechanisms, the azimuth adjustment will impact both record and playback functions. So, all you can do here is use a carefully recorded externally prepared test tape and play back the HF signal on this tape, adjusting the azimuth to get maximum HF output, and thus align the playback head to this external standard. And then trust that this alignment will hold for recording too, because it's finally the same head. (They are called two-head decks because the erase head is always physically separate, in all decks, from the cheapest to the most expensive.)
- Three head decks, with record and playback heads in one assembly: these decks have separate heads with separate coils for record and playback, but both record head and playback head are encased in one enclosure, therefore azimuth alignment will impact both record and playback heads together. (A lot of Japanese 3-head decks from Aiwa, Akai, Sony, etc, are built this way.) In these decks, all you can do is use a carefully recorded externally prepared test tape and play back the HF signal on this tape, adjusting the azimuth to get maximum output, and thus align the playback head to this external standard. And then trust that this alignment will hold for the record head too, because both of them are in the same hard, fixed enclosure and you just have to trust the manufacturer that he's maintained perfect alignment between his record and playback heads when he put them inside the enclosure.
- Three head decks with physically separate record and playback heads: these decks need to have their record head azimuth and playback head azimuth aligned perfectly, and separately. So, in these cases, you may do it the following way, with, say the ZX-9, whose record head is adjusted by the "azimuth calibration" controls:
- Step 1: adjust the playback head's azimuth manually, with a screwdriver, by using an externally prepared test tape
- Step 2: record an HF signal on a blank cassette on this deck, so that the record head is used
- Step 3: play back this HF signal cassette, see if the azimuth is aligned as expected, and if not, then adjust the record head azimuth till it comes into perfect alignment with the playback head.
- End-user azimuth correction needs tone generator: Decks which provide azimuth calibration, whether manual or automatic, all have a tone generator which generates a test tone at high frequency and then either allows you to adjust the azimuth using a knob (as with the Nak ZX-7/ZX-9), or adjusts the azimuth itself, automatically, using some sort of servo motor (as in the Revox B215).
- One head fixed, the other adjusts: All decks which allow the end-user to "adjust azimuth" without using a screwdriver, using a "calibrate" button of some kind, allow him to adjust either the playback head azimuth or the record head azimuth, but never both. The logic is that one is fixed, and the "adjustment" or correction you are doing is to align the other head with this one.
- No front-panel azimuth correction in two-head decks: No two-head decks have any feature to let the end-user "calibrate" the azimuth, because it's not possible to do so -- the two heads are actually one, and they can't be operated in record and playback mode simultaneously. This simultaneous record and playback (called "off-tape monitoring") is needed to allow automatic or manually-assisted azimuth calibration.
- No front-panel azimuth correction in three-head decks with single enclosure: No three-head deck which houses the record and playback heads in a single enclosure provide any feature to let the end-user "calibrate" the azimuth, because such calibration requires the record and playback heads to move separately. If they are both in the same enclosure, they can't be adjusted separately.
- Why the Nakamichi Dragon is special: All these fancy azimuth calibration and tuning features in these high end decks, like the Nakamichi ZX-9/CR-7, are only useful to align its own record and playback heads with each other. If you, like me, have lots of cheap pre-recorded cassettes made by EMI, HMV, Tips, and what-not, their azimuths are all over the place, and your perfectly tuned ZX-9 will be as out of alignment with them as any cheap deck will be. The only answer to this is to align the playback head's alignment with each cassette's alignment without using any recording or any test signal. In other words, your deck must have the ability to align itself with each imperfect, flawed, cheap tape you play back on it, automatically, without recording any test signal or bringing the record head into the picture. This technology exists only in one deck in the world and that is the Nakamichi Dragon. The technology is called NAAC. That's why the Dragon is superior. Its auto-reverse feature is of very little importance to understand its greatness: its NAAC (Nakamichi Automatic Azimuth Correction) technology is what makes it great. And to do this, it needs a very special right-channel playback head which actually has two coils set a certain angle apart, thereby picking up signals with a certain phase difference from just the right-channel track of a stereo cassette. It measures the phase difference and automatically aligns the playback head continuously using this data, and therefore gives perfect playback azimuth alignment with any pre-recorded tape you can throw at it, without having to record any signal on the tape. There is no other deck with similar technology ever made, and probably will never be made.
- Azimuth alignment should ideally be done every time: In a perfect world, playback azimuth alignment would need to be re-done for every cassette, every time it's played back. The same cassette played back a hundred times on the same deck will have a different azimuth alignment the hundredth time compared to the first time, because of wear and tear of the spools, the tape, and so on. Even things like expansion and contraction of various parts of the deck mechanism will change the azimuth alignment over time.
- Pre-recorded cassettes will play imperfectly even on the best decks: Even the best tape decks (the Nak ZX-9, CR-7, the Tandberg 3014, the Revox B215), all aligned and calibrated as per factory spec, will be only average or mediocre when playing back a set of pre-recorded cassettes obtained from different publishers. Each of these decks will accidentally have their playback azimuth alignment very closely aligned with some of your cassettes, but will have poor alignment with other cassettes, and there will be nothing you can do about it. The manufacturers of pre-recorded cassettes, specially in India, on the whole, did not follow close tolerances when manufacturing their cassettes. They even followed mass manufacture processes which make such close tolerances impossible to obtain consistently. Therefore, even the world's best decks are going to be only average when retrieving the last bit of HF information from my bunch of pre-recorded cassettes. The only exception is a correctly working Dragon with a correctly aligned NAAC sub-system.
This is what I have understood.
Corrections and comments eagerly invited.
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