|8 Track Technical Aspects - Solving the Mystery
|In addition to its growing cult following, the 8 track format in particular has a special fascination due to its dramatic
differences from other tape formats. On this page I will show you exactly how the 8 track tape format works, while
at the same time explaining why proper alignment is so critical to prevent the all-too-common problem of crosstalk.
|What is a "track", and why are there 8 of them ?
|This is a good time to point out the difference between a "track" and a "program". Quite often, the term "track" is used
in place of the more accurate "program" - such as "My favorite song is on Track 3". To get a bit nit-picky and describe
it as accurately as possible, this is actually PROGRAM 3 which contains the material recorded on TRACK 3 and TRACK 7.
|Huh ? You kinda lost me there !
|Not to worry, we're going to take care of this right now ! Below is how the tracks are arranged into programs. Keep
studying this page, because very soon you will be able to see the playback head actually moving through the tracks.
|In the above illustration, roughly one inch of tape length is represented. The gray area is the actual 1/4 inch width of tape
material, while the different colored bands represent the individual TRACKS on which music is recorded. As can be seen
from the illustration, PROGRAM 1 contains TRACKS 1 and 5 - that is, TRACK 1 contains information for PROGRAM 1 (left
channel), and TRACK 5 contains information for PROGRAM 1 (right channel). Only 2 tracks are played at any one time.
|Now that we have that figured out, here is how the playback head moves through the tape to play PROGRAMS 1 through 4:
|This is a standard STEREO tape, played on a standard stereo 8 track player. As the animation shows, the playback head
contains 2 "gaps" or pickup elements - one for the left channel, and one for the right channel. When one program is
finished playing, the foil splice on the tape closes a switch which then triggers the track change mechanism to advance
to the next program - see, even I'm interchanging the terms "track" and "program" by saying "track change mechanism".
Of course, all 8 track machines also have a manual "program change" button which the user can operate at any time. Of
particular interest is the Panasonic "Plunger" portable, which does not even have an automatic program change feature.
Soon, we'll look at how a QUAD tape works, as well as how a stereo tape is played on a Quadraphonic machine - but first,
let's take a little time to talk about dimensional data, and thus why proper alignment is so important on 8 track machines.
|Why am I getting crosstalk, hearing two tracks at one time ?
|Since the playback head moves mechanically to different tracks on the tape - and since this motion is generally very sudden
and jerky - the playback head will eventually fall out of proper alignment. This can be the result of wear on the various parts
which work together to move the head, slight bending of metal support brackets - and the very common cracking or even
breaking of the plastic head mount bracket on the hundreds of models which use the rather cheap Mitsumi tape transport.
The latter cause is so common that it should be expected, and in fact has probably already happened. The picture below is
of the playback head on a brand new, never-installed Mitsumi tape transport. As the arrows point out, the plastic head
mount bracket is cracked in three places - simply from age and the pressure from the screw on top which is tightened to
secure the head in place. Over the years, the plastic becomes brittle, and cracks or breaks even if the machine is not used:
|Naturally, if the plastic head mount is cracked, that means the head has moved somewhat and has now become loose in the
bracket. In the picture above, the cracks may seem too small to make much difference - until we take a look at some figures:
Referring to the track layout near the top of this page, we can see that the "guard bands" - that is, the blank areas between the
active tracks - are a mere .01075 which is approximately the width of TWO HUMAN HAIRS. Thus, in this case we can say quite
literally that the head alignment on an 8 track machine can't be more than a hair off, because two hairs off can make the head
gaps straddle two different tracks. If you're a guitarist, you know that your small E string is the same width as the guard band.
Thus, all 8 track machines have a head height adjustment, as well as an azimuth adjustment. The height adjustment, of course,
moves the head up and down. The azimuth adjustment, as you're looking at the head from the front, rotates the head CW and
CCW. If the azimuth is very far out of adjustment, it will cause the head gaps to tilt and straddle more than one track, making
it impossible to eliminate all crosstalk regardless of the head height adjustment setting. Because the clearances involved are
so critical, a cracked head mount bracket must be repaired before it is possible to perform accurate and lasting adjustments.
Interestingly, all AKAI machines except the CR-83(D) have a third adjustment which tilts the head backward and forward. This
keeps the moving tape perfectly square in relation to the head and reduces its tendency to wander across tracks in operation.
|What about 4 channel (Quadraphonic) tapes ?
|Since Quad requires 4 tracks, a Quad tape only has 2 programs - and thus, the playback head only needs to have 2 positions:
|Actually, some Quad machines DO use 4 positions for the playback head, using only the first and third head gaps for all tracks
as it runs its course through the 4 programs. But this comes with a price, in the form of 3 successive rapid-fire track changes as
the machine quickly skips over programs 3 and 4 while in Quad mode. In the case of the Akai CR-80D-SS, this might have
you running for cover as it sounds like a machine gun spraying a quick burst of 3 bullets. Nope, it's not broken - just LOUD.
Some other manufacturers depended on a "nervous chatter" type action to skip over the unused programs while in Quad mode.
This method is not especially consistent or reliable, and can be very annoying when it sometimes chatters for several seconds.
Thus, in most cases the manufacturers stayed with a consistent 2 head positions, and electronically switched the appropriate
head gaps in and out of the circuit as required by the current position of the head. There are different ways to switch the head
gaps. For instance, on some Craig/Pioneer units, there is an actual mechanical switch with a linkage to the track change cam -
whereas on the Ford Quadraphonic car units, electronic logic gates (integrated circuits) handle the head gap switching function.
Obviously, the two-position method creates less noise and saves a little wear on the track change mechanism. However, trying
to watch it in action can be extremely confusing. Thus, the animation below was created to run more slowly for increased clarity:
|That head sure gets jerked around a lot - should I purchase an alignment tape ?
|Call 928-533-9666 (Paying Repair Jobs ONLY)
|DO NOT CALL FOR BELTS, PARTS OR FREE REPAIR ADVICE!
|Note that the head only moves in two
positions, while the active gaps are
moving through all four positions
For programs 1 and 2, the first and third head gaps are active
For programs 3 and 4, the second and fourth gaps are active
( Note that the active gaps are green, inactive gaps are shown by the lightened
areas, and each pair of gaps is switched "in" or "out" of the circuit as needed )
|But If the Head Moves in Only 2 Positions, How Does it Cover All 4 Programs on a Regular Stereo Tape
|Lastly, the four-position method used in the Akai CR-80D-SS (which already has the loudest track change mechanism in history)
|It depends on what kind of alignment tapes you can find. I have found that alignment tapes that were sold to the general public
such as the GC and Nortronics tapes contained test frequencies that are inaccurate enough to make me skeptical of their ability
to properly align the head. I think it's much better to find a tape that was specifically made for service shops. I don't see any
shame whatsoever in using a known good music tape for alignment, for reasons I attempt to explain in the following paragraphs:
It's fairly well known that the duplicators were not always in precise alignment, especially near the end of the 8 track craze when
sales were down, per-tape prices had dropped significantly and measures were needed cut production costs. This explains
why some later 8 track machines featured a front panel head height adjustment, so that the listener could compensate for this.
In the record industry, no supervisor walked around to the press operators saying "Hey man, this is Peter Frampton - make sure
it sounds good" because all the operator did was place a lump of vinylite (not vinyl as is widely believed) on the platter, pull on
a lever, trim the flash from the pressed album and place it on a stack. The operator had no control over the sound quality. By
the same token, no one walked around saying "These are alignment tapes - shut down production so we can make adjustments".
Alignment tapes were another gimmick, similar to "head cleaning" cartridges which do, literally, absolutely nothing useful. A
VCR head cleaning tape at least removed enough residue to get you through a movie - but this is only because the video heads
on a VCR spin on a drum at 1200 RPM, the equivalent of running the tape at an almost supersonic speed. But at 3.75 inches per
second, the only thing an 8 track cleaner did was move residue from the head and deposit it on the capstan - making it WORSE.
Think about it and you'll agree: The cleaning material rubbed against the head, "possibly" removing a tiny amount of residue.
However, the capstan does not "rub" the tape, it drives it along with no rubbing or abrasive action. This means that any type
of material removed from the head - that is, slippery graphite - would immediately end up on the capstan which could not be
cleaned without the abrasive action. So now you still have a dirty head AND a slippery capstan causing speed variations !
Another factor concerning alignment tapes is that they all used continuous tones which very quickly caused ear fatigue unless
you could view them on a scope and SEE the results of adjustments. The standard 1 kHz tone used for HEIGHT adjustment
was somewhat helpful, but it's impossible to tell the difference between a louder and softer 8 kHz tone after just a few seconds
when you're adjusting the azimuth for maximum treble response. AHH, there's another point: The optimum adjustment is not
always maximum volume on both parameters, but rather a happy medium between overall volume, crisp, clear highs and
hopefully a complete absence of crosstalk between tracks. On some machines, a tiny amount of crosstalk is tolerated in
order to make it sound best. By "tiny amount", I'm referring to a small but audible amount of bass heard only between songs.
Thus, the only "alignment tapes" I've had any success with are the ones I've made myself on a machine I've adjusted using
several different tapes. But as effective as my tapes are, the bottom line is that we use these machines to listen to MUSIC -
not test tones, white noise, pink noise - or even my favorite, violet noise which doesn't contain as much distracting bass.
Take it from the world's most recommended 8 track technician, and tell all your friends to align it using only MUSIC tapes.
To adjust height and azimuth, I listen to the "tick tick" of the drummer's closed hi-hat during the verses. Since this is usually
buried in the mix somewhat, once you can bring it out and hear it clearly, that's it - all finished. Just keep in mind that you're
trying to bring out as much treble as possible, and you'll have your machine tweaked almost to perfection. Other useful high
frequency sounds include applause, although its typically short duration doesn't allow much time for proper adjustment. A
distorted electric guitar which contains high frequency harmonics is effective, as well as the "s" and "t" sounds of lead vocals.
Also, remember the concept of ear fatigue, and always recheck your adjustments later with a fresh ear before closing the unit.
Make sure you check all 4 programs on the tape, as it is possible to get it sounding great - but an entire track "off". If this should
happen, you will only hear one side on either Program 1 or program 4 since one of the head gaps is completely off the tape.
When time permits, I will post a video on YouTube which proves that my adjustment methods surpass those using alignment tapes.
To view my collection of videos - most of which are taken using actual customers' units as models, search "Barry's 8 Track Repair".
I use either an RCA alignment tape, or ones I make myself, using spectrum analyzers for best results - check out my video HERE
|Note that since this head moves in 4 positions,
gap switching is not necessary - just nerve pills !
|Typical Capstan Torque
Ranges from approximately
2 to 3.5 inch-pounds.