The Bell Labs/Leopold Stokowski Stereo Experiments of 1933 used a Lateral-Vertical system, with the Left stereo channel attached to the Lateral cutting pole and the Right stereo channel attached to the Vertical pole. Unfortunately, most rumble was in the Lateral channel, and most high frequency distortion was in the Vertical channel. The reason this was abandoned is discussed below in the Cook Binaural System.
We are estimating that sometime after that, the following machine was invented. We have only seen a picture of it in a customer's coffee-table book once.
The machine was basically two gramophones, folded up side by side, facing each other. The two "speakers" faced the listener directly and the two gramophone heads faced each other as well---one playing the left side of the disc outside-in and the other playing the right side of the disc also outside-in. The small article under the picture describes the operation thusly:
The pedal is pressed into the base of the machine, separating the two gramophone heads and releasing the two halves of the turntable from one another. The stereo disc is then placed on the spindle (similar to what was later to become a Seeburg jukebox mechanism) and the pedal is released. A small caption under the picture cautions the operator to release the pedal with care as doing so rapidly will cause the two spring-loaded gramophone heads to crash into the disc, causing breakage. The gramophone heads are then manually guided into the first groove and the electric motor switch is activated. In a second or two, pure two channel stereo is achieved.
Apparently to our best guess, as limited research that is available suggests, the discs were manufactured on a pair of early electric Scully lathes operating simultaneously. Since these lathes featured selsyn motors, the same kind to sync the lathe with a 35MM movie camera or projector, the two lathes in question would lock perfectly, the left lathe taking the feed from the left half of the orchestra and the right lathe would receive the input from the right side of the orchestra cutting in sync with one another. The left-side lathe cut in a standard fashion, while the right side-lathe was swapped around, cutting in a counterclockwise fashion with its' cutting stylus installed in reverse. Educated guesses say that the resulting disc sides were then processed independently, and the resulting stampers aligned in the press to coincide with one another, similar to those of a movie soundtrack disc in the 20's.
On a slightly different subject, various early disc-based dictation machines such as the Grey Autograph are briefly discussed elsewhere on this site, and viewers may be interested in the similar operation of the various factions of other types of embossing recorders.
The Sound Scriber machine mentioned featured a square hole and cut vertically at 33RPM into a fixed-pitch, constant-angular-velocity neon-green 6-inch resin based disc (although eight-inch versions on blue resin and four inch versions on red resin have been seen) the same as the Grey Autograph. The playback stylus was similar in fashion to a 2-minute cylinder stylus---i.e. sort of doorknob-shaped vs. spherical.
The Reference Recorder used a 12-inch clear resin disc similar to the Grey Autograph in style. The discs had a "drive hole" in addition to the standard quarter inch spindle hole, similar to a transparent version of the standard steel based home recording disc of the period. Again, similar to the Grey Autograph, the disc sat on a small three-inch turntable with the drive hole mounted on it and was driven in a similar fashion of two rubber drive tires engaging the disc from above and below. The recording (i.e. embossing) styli were stationary, similar to the Autograph, beginning in a center-start fashion near the label and terminating near the edge. The machine featured a 30 Minute speed for a 12 inch disc and a 60-minute speed as well, however at the 60-minute speed recordings were not able to be made at the same depth or volume level as 30 Minute recordings.
Commercial constant linear velocity disc recordings were attempted by several manufacturers in the early part of the 20th Century. The most well known of these was a system whereby a vertically-mounted drive tire engaged the turntable from the underside. This machine also featured a tracer stylus, similar in fashion to the many home-disc recorders of the period. Rather than engaging a lead screw it engaged a spiral trace on the underside of the platter, guiding the cutting stylus from the center to the outside of the disc utilizing a fixed pitch. Other platters were available utilizing different fixed-pitches, enabling the user to increase the time on the disc by using a tighter pitch, or by increasing the fidelity of the disc by utilizing a looser pitch.
This machine is also similar to the Lenco turntable popular among audiophiles in the 50's. However, rather than feature constant linear velocity, the Lenco Turntable used its' vertical drive tire to allow a continuous variable speed from three RPM to 100 RPM.
Other similarly operated machines used the same embossing technology and/or the constant linear disc speed format. These include the Edison Voicewriter and or Dict-A-Belt recorders as well as the Reference Recorder, mentioned above.
The standard Edison Voicewriter used a slightly less-than-7-inch 1.5 inch hole red in color resin based disc into which the recording was then embossed. This is similar to the Autograph in operation, save the constant angular velocity format. The machine operated at a constant slightly-less-than-33.3 RPM speed and the disc was held in place by a weight over the label similar to the type used in vertical linear tracking turntables that became conversation pieces in the 70's.
The Edison Dict-A-Belt recorder offered a variation on the Edison Cylinder theme, utilizing a resin based "belt" or cylinder (usually red the same as the disc versions) into which the recording was then embossed, similar to the other office dicatation machines in place at the time.
The Edison Magn-A-Belt recorder was of similar design, utilizing a thin single channel head and a short lenth of four-inch magnetic tape spliced onto itself in a continuous loop. Tracks on the Magn-A-Belt were spiral and similar in nature to the Dict-A-Belt recorder mentioned above, but as the recording material was magnetic, it was possible to erase and re-use recording material.
Another dictation device that used an Edison Magn-A-Belt type head as well as a tracer used in the constant-linear-velocity disc formats, was the Dict-O-Graph. This machine used a 6-inch paper based re-useable disc upon which magnetic particles had been impressed. The magnetic head was installed into a tonearm much the same as a phono cartridge and offered the user up to four and a half minutes of time on a disc that ran roughly 12 RPM. Tracking the evenly-spaced spiral was accomplished by a pin located on the inner portion of the tonearm containing the magnetic head, tracing a spiral impressed upon a three-inch bakelite disc placed on top of the magnetic paper disc. This disc served another purpose, to anchor the disc via a secondary drive hole to its' turntable to prevent slippage.
This development is important in the history of recorded sound because this head technology gave rise some years later to the Home Music Library System. The HMLS was mounted permanently into a case, inaccessible to the operator. Removing the faceplate exposes a permanent, interior-mounted two-hour non-changeable reel of the same 4-inch magnetic tape used in the Magn-A-Belt. An interior-mounted head of a similar design to the Magn-A-Belt was suspended by two bars over a short section of exposed tape, similar to looking at a B-Wind reel to reel from in front of the head area. An exterior dial on the faceplate allowed the operator to access a total of seven available programs featuring nine individual monaural tracks apiece, traversing the tape from top to bottom.
Of course, to make recordings, an operator would have had to stack their records up on their changers and just let fly, flipping the stack over halfway through. The few units I have seen were used by the families that owned them to record their extensive collections of 78RPM classical and dramatic works before disposing of the original discs. Magn-A-Belt heads work perfectly in reproducing the sound should you perhaps be the lucky owner of the HMLS or it's country cousin the Commercial MLS.
The first known commercial releases of this unit were monaural, one directional, and featured manual track select for recording and playback, the same as the Home units, requiring the operator to rewind at the end of each two-hour program and manually select the next track to be recorded. Recently discovered notations indicate that Auto Rewind and Auto Select mechanisms for this unit were at least manufactured as prototypes if not for commercial release. The automatic sequential track-select mechanism is said to be extremely similar to those later developed for 4-track Muntz stereo cartridges and 8-track tapes. The mechanism was apparently featured in order for the operator to achieve several hours of uninterrupted recording and/or playback save the two-to-three minute rewind time.
The same notations also indicate the addition of an Auto Reverse/Reverse Record function. Diagrams of this feature suggest that it featured four heads---one each for erase and record/play for each direction about an inch apart, suspended on the same two poles and set up for the track immediately succeeding the track the operator was on. It appears therefore that odd tracks in the program were recorded left-to-right and even tracks were recorded right-to-left. Upon reaching a strip of foil placed at each end of the reel, it was said to be possible to record an entire work week's worth of music onto one tape (126 consecutive hours) after which the unit is said to disengage and power down.
Further notation describes the addition of Stereo to the unit, reducing it once again to a uni-directional play-and-rewind affair. Diagrams exist however to show the arrangement of a Stereo Auto Reverse and Reverse Record version of this unit by simply having two sets of monaural Edison Magn-A-Belt heads on the same set of rods.
It appears then that Channels 1 and 3 were used for Left and Right in the Top to Bottom direction and Channels 2 and 4 for Left and Right for the Bottom-to-Top direction, similar to the then-developing 4-track open reel format. Apparently this was done so that the Stereo players (if they were ever built) could accommodate the previously recorded monaural single direction versions, monaural auto reverse versions, or the stereo one-directional versions. However this seems puzzling because the reel of tape installed in these units was not consumer-serviceable. If the tape broke, or any other malfunction occurred, the unit needed to be dismounted from its console and taken in for service. It is noted that the unit donated to Inventors' Magnet School was not working properly due to missing heads.
It is discovered that Library for the Blind style 4-track cassette heads work reasonably well enough to hear what is on any given reel. These type heads unlike conventional cassette heads have either two or four evenly-spaced tracks rather than one track (for monaural cassettes) or two, a wide center gurardband and one or two more tracks. The same head configuration was later used for piped-in music systems when material was programmed onto special cassettes. These cassettes ran at 1+13/32 IPS from a 22.5 IPS master rather than the standard 1+7/8 IPS cassette which from a 30 IPS master.
Earlier background music formats included reel to reel players featuring half-inch and quarter-inch two-track (and later four track) reels as large as 19 inches across. These massive machines stored as much as roughly two miles of tape (ten thousand feet) running nine hours per track in each direction at 3+3/4 IPS. This was done to offset individual numbers from being heard at the same time in any given shift.
Nineteen-inch reels of half-inch tape were also used in early Ampex videotape experiments. The stationary head design of these machines required the tape to be run at a staggering 20 feet per second in order to register the fuzzy black and white low-band images, and offering only eight minutes of recording time per 19-inch 10K-foot reel. The fragile acetate-based tape was prone to breakage, and at such a fast speed was prone to shredding if broken by the machine. As such, only a few of these recordings survive.
In the early 50's, before cassettes were perfected, AA-size endless-loop cartridges were perfected for conveniently playing commercials and short news pieces in radio stations. At the time of its' development, this format offered a single program track with a cue-tone track (for stopping the tape at the start mark) and up to eleven and a half minutes of programming at 7.5 IPS in a cartridge approximately 4 inches long by 3 inches wide, similar to an 8-track.
Several larger versions of the endless-loop tape cartridge were developed as well as one smaller version. After this format was introduced to the broadcast world, this format was adopted by record companies as the predecessor to the standard 8-track-size stereo cart , the speed was reduced to 3+3/4 IPS. This size cart is referred to as "AA-size" holding the equivalent of a three-inch reel, allowing up to 22-and-a-half minutes of programming per track on one-mil graphite-treated tape. Multiplied by four, the resulting complete set of four monaural programs was 90 minutes in length. Introducing two-third mill tape (described in greater detail below) as a compromise between the half-mil tape judged too thin for the mechanism and the aforementioned one-mil tape extended the programs to 30 minutes apiece, resulting in a complete two hours of uninterrupted music.
At the same time the AA-size carts were developed which carried the equivalent of a four-inch reel, there were many other size quarter-inch cart sizes developed for other emerging markets.
AAA size carts, one size smaller than the 8-track size AA, were effectively the first "Cassette" Single having a four-minute capacity at 3+3/4 IPS. Approximately two inches wide by three inches long, these small carts offered one song on program 1 and one song on program 2 in true full fidelity stereo. After being deleted from the consumer market, remaining units were then reprogrammed and offered as samplers of the forthcoming background music systems described below.
A size carts, one size bigger than the standard AA size carts, were developed which carried the equivalent of a 4-inch reel, easily allowing 30 minute programs due to the extra space without the need for thin tape.
B size carts were developed for airline music programming services, which carried the equivalent of a 5-inch reel. This size offered each track 45 minutes long and space for two standard 60-second commercials. This offered the listener an uninterrupted three-hour playing time, more than enough for a vast majority of flights. For a short while, B-size carts were recorded at a speed of 1+7/8 IPS the same as today's modern cassettes, allowing four 90-minute programs for coast-to-coast airline travel. However that format was abandoned due to the loss of fidelity at such a low speed on standard-grade tape. Higher-grade chromium dioxide graphite-backed tape was simply too expensive and too abrasive on the heads of the period for such extended use
C-size carts were developed around the same time for pretty much the same uses. Although the 6-inch reel (or 7-inch with a NAB hub) offered up to 36 minutes of programming the larger size became simply too bulky for use on a majority of transportation carriers with their tight spaces. However, because of the flexibility of being able to have a 30 minute music program and enough space for a few short promotional pieces, this format and the next became perfect for supermarkets and shopping malls.
Also around the same time, the D-size carts containing the equivalent of a 7-inch reel were being deployed. The larger cartridge coupled with new 2/3rds-mil graphite lubricated tape increased the capacity of the cartridge to 2250 feet, allowing one tape to run all day, featuring 4 programs of 2 hours each for a total of 8 hours. The magnetic coating added another third of a mil to the new 2/3rds mil base making the resulting tape truly a mil thick. Standard one-mil tapes are generally slightly over 1.25 mils thick allowing for 1800 feet on a 7-inch reel.
Once the 4-track endless loop cartridge formats found their way into supermarkets and malls, sooner or later their speed was halved to 1+7/8 IPS doubling the time. The program fidelity was not as important in this application compared to airline applications for in-flight music programming.
After a few years, half-speed 8-track commercial cartridge tapes made an extremely brief foray into the limelight. Since now there were eight monaural programs instead of the usual four, and the tape speed was now 1+7/8 IPS instead of the original 3+3/4 four times the program material could now be recorded. Utilizing the new two-third mil tape described elsewhere in this piece, over an hour per track of programming could now be had in a small 8-track cartridge. This allowed the nine hour program previously available on one of four tracks of a 19-inch reel available in a small, pocket sized package easier to make and cheaper to ship. The improvement of having the drive roller installed into each tape instead of remaining in the player made player maintenance much easier.
Just about the same time, the La Belle Media Company adapted the format by returning to the broadcast format of the quarter-inch endless-loop cartridge (single track and cue tone track) and effectively made 16MM self enclosed filmstrip/tape presentations. This was done by assembling the lower half of an 8-track cart and the upper half of a 16MM endless loop film cartridge (then used in stores as promotional films for a product). The tape ran at 3+3/4 IPS and also used the cue tone track for tones to advance the 16MM filmstrip.
Somewhere in here should go a short discussion of the various filmosound cartridges used in schools and stores as promotional material. These machines were placed as aisle endcaps and resembled a small television in appearance. Each cartridge carried a short message, up to 30 minutes in length about a product or service offered by the retailer. These films would play continuously from open to close to increase sales.
Most 35MM filmstrip versions featured an endless loop filmstrip and an automatic-replay turntable built for either 7-inch 45 or 16RPM or 12-inch 33RPM. The filmstrip cartridge was simply loaded into the mechanism, and the record placed on the turntable and engaged. Endless-loop sync tape accompaniments were later developed similar to the LaBelle cart described elsewhere before the filmstrip-tape system we know today came into existence. 16-MM versions of these originally featured magnetic sound, but as the magnetic soundtracks tended to get erased, optical soundtrack versions were later used.
A format of 16MM filmstrip was used in the 60's and 70's by Borg-Warner in its' Teaching Machine. The filmstrip was imbedded in a large caddy with various holes in it and accompanied a 15-RPM vertically-cut disc (rather than 16 RPM). Four choices were offered to the student for each question asked. Starting in the center of the disc and the center of the filmstrip, a right answer would advance the filmstrip one frame and the tonearm on the disc approximately a quarter inch. A wrong answer, up to 25 tries, would cause the filmstrip slide to keep its' place. The last frame of film would have an "END" slide and the audio would then say "That is all. Now take the record and filmslide out please".
You can always tell engineers or students with extensive experience with this format when you find them in the retail world. When asked if there's anything else they require, they will say the first half of the Teaching Machine End Slide line, "That is all." and see if the other person responds with "Now take the record and filmslide out please" and then laugh.
When the 16MM endless loop film cartridges were replaced with 8MM, it was necessary to go back to magnetic sound, as facilities for creating optical sound on 8MM film did not exist. Some other formats merely included a flexible plastic phonograph disc attached to a spindle on the bottom or top of the cartridge in order to offer sound, similar to the early Talking View Master format
Various flavors of 8MM silent endless loop formats found their way into educational and home uses from the late 50's through the mid 70's until home videotape recorders became available.
A smaller version of the LaBelle cartridge mentioned above was also developed, assembling the bottom half of a PlayTape format described below with the top half of an 8MM-endless-loop film cartridge to achieve this result
PlayTape, a miniature version of the endless-loop cartridge had it's own brief foray into the limelight in the mid 60's. This utilized true eighth-inch tape (0.125-inch), slightly narrower than the 3/20-inch tape (0.15) used in today's cassettes. These commonly held the same material as their 7-inch 45-or-33 RPM EP counterpart, roughly six minutes of monaural programming (top track then bottom track) or three minutes of stereo (utilizing one pass of both tracks simultaneously). This format was later adopted for telephone answering machines until digital tapeless versions appeared some years later.
16-inch Electrical Transcriptions (and their Acoustical Transcription counterpart for movie soundtracks), are briefly mentioned elsewhere in this site. Viewers may be interested to know that although most transcriptions were recorded in the standard lateral-cut outside-in format, several if not all of the transcriptions from the World Broadcasting Service arm of what was then known as The Decca Record Company were cut inside-out and in a vertical i.e hill-and-dale fashion. This was done for the same fidelity-matching reason that many transcriptions employed when they altered inside-out to outside-in as the fidelity on the edge of the disc was much better than that near the label.
Simply put, puck-drive characteristics on turntables of the day are partly to blame for the fact that turntable rumble was mostly found in the lateral plane. The vertical modulations of the day ensured that minimal turntable rumble would be transmitted over the air and/or interfere with the carrier wave. Several of these 16 and 17.5 inch discs were later used in commercial background music applications, the vertical method combined with the size of the disc to ensure non-rights holders would not be able to play the disc.
One of the first commercially available high fidelity---and later Stereo disc players, came to be simply because the rumble in most phonographs of the early-to-mid 20th century was most noticeable in the lateral plane. (See Bell Labs/Stokowski discussion above). Using a 16-inch disc, a two-head system was developed. The treble portion of the program was cut laterally starting at the edge of the disc where greater linear velocity was available, and the bass portion of the program was cut vertically starting halfway across the disc ending at the center. Then a high pass filter was employed to eliminate whatever remaining rumble their was in the treble portion of the program, and a low pass filter was employed on the bass portion of the program to eliminate problems from the treble portion.
This system was later refined and adapted by The Cook Company for its' series of binaural records in the middle 50's. The same two-head system was used, however in this case, as playback systems had become much quieter, both channels were recorded laterally. The left channel was recorded with no equalization beginning on the outer edge of the disc and the right channel was recorded beginning halfway through the disc with a 500cps crossover and offered about ten minutes of discrete stereo music per side.
In the mid 50's Mintner developed a way to put Compatible Stereo onto an LP disc by utilizing a 30-to-45 cps carrier wave in order to modulate a difference or left-minus-right signal in addition to the original monaural laterally cut signal. This was done because to ensure compatibility with the many millions of monaural turntables in place at the time. The user merely had to buy a high quality monaural—i.e. laterally compliant cartridge, a demodulator and another amplifier in order to play these discs. There was said to be no danger of destroying the "stereo" portion of the disc by playing it on a monaural player, like the warnings on the back of stereo LP jackets of the same period. The records were cut at half speed, i.e. 16RPM and made to play at 33RPM. This was done due to the fact that cutter heads that could steadily cut a 30-to-45 CPS tone had not yet been perfected. The system was later doubled and used by JVC in the 70's for CD-4 quadraphonic disc.
Also, several independent transcription producers were the first to use the Mid-Side Lateral-Vertical Cutter style used in The Bell Telephone Stereophonic Experiments of 1933. Several examples of these deep green acetate pressings are reported to survive in the archives of The Telephone Pioneers of America.
A different version of this idea, called a matrix, was used in the 70's as Matrix Quadraphonic and is still in use today as the various kinds of Dolby Surround and Dolby Pro Logic. The rear channels of the SQ matrix superimpose a 90-degree phase difference between the left front and left rear, and the opposite 90-degree phase difference between right front and right rear to create decodable rear channels. The descendants of SQ include Stadium Matrix, Arena Matrix, and other forms of large venue auditoriums.
QS matrix (the original Dolby 4.0 from 1976) on the other hand has a monaural rear channel 180-degrees out of phase from the remaining program material and a monaural front center channel at 0-degrees. This was done to ensure the dialog channel for the film in question was not lost in noise in a large auditorium. Descendants of QS include Hall, Club, Theatre, Living Room and other mid-size rooms.
As the new millennium approaches it appears that some proponents are attempting to overlay a descendant of the SQ matrix with a descendant of the QS matrix. It is uncertain at this point whether what will more than likely come to be known as Dolby 7.1 will catch on with producers and consumers or not.
The number 16 of course also brings up the 16-RPM speed as well as the 16-inch transcriptions mentioned above. Apart from the standard talking book for the blind uses known by everyone familiar with records, the 16-RPM speed has been used as early as the `50's for a series of background music in retail shops.
These background music discs have been recorded onto discs as large as 22 inches in diameter and pressed onto every size disc imaginable. The 14-inch size was manufactured to ensure the consumer would not be able to play it on a conventional turntable, and discs 11, 10 and 9 inches across were also made. Twelve-and Nine inch discs with their have been noted to have been used in background music systems, replacing the 14-inch 2.7 mill groove predecessor. These 12-inch discs with their 2-inch holes (half an inch larger than 45 RPM) offered 90 minutes of uninterrupted music per side and 65 minutes of uninterrupted music for the 9-inch variety. This was accomplished by the 0.5 mil playback stylus, which was exactly half the width of the conventional 1-mil monaural LP stylus of the day.
16-RPM Eight-inch discs have been used in a variety of commercial applications such as inside of childrens' manual-spin talking books. These books had a dozen or so pages and were maybe an inch thick, 99 percent of which was the plastic soundbox containing a quarter-inch thick 16-RPM eight inch styrene disc. The soundboxes had their own sapphire tipped steel shank needle attached to a styrene reproducer and acoustically to a styrene speaker. The spindle was attached to a large flat dial on the top of the soundbox for children to dial to hear the story being read to them. Discs in these types of applications were recorded at a pitch sometimes as wide as 70 lines per inch with a groove size as wide as three mils similar to a 78-RPM record until the late 50's when the 7-inch 1.5-inch large-hole disc was adopted. This was done as 45RPM discs were gaining in popularity, as the stampers of these types of discs would be able to be used in standard 45-RPM presses ensuring the ease and simplicity of manufacture.
Apart from the Talking Book Society of St Joseph Michigan, 7-inch 16-RPM standard-hole discs were adopted briefly for the 1956 model car by The Chrysler Corporation for use in its' Highway Hi Fi series of in-dash phonographs. These discs were extremely thick, utilizing twice the vinyl compared to a standard seven-inch disc. This was necessary to ensure against warpage in the tight spaces in the car at the time.
At a quarter-mil, the playback stylus was one-fourth the size of the standard 1-mil monaural LP styluses in place at the time. This is one of several reasons that when Highway Hi Fi discs are located, they are damaged usually beyond use. Ignoring the warnings on the discs and the sleeves and playing the discs with a standard stylus is tantamount to playing a 33.3 LP disc with a 78 stylus. The most valuable of these discs is Elvis Presley's entire first album with the picture sleeve stating "For Auto Use Only" which has been known to command several thousand dollars at auctions.
16-RPM six-inch, five-inch 4-inch and discs as small as three inches in diameter used in a small variety of acoustically reproduced manual talking toys in the mid part of the 20th century when high fidelity was not an issue. Discs in these types of applications were recorded in the same manner listed under Eight Inch Discs above.
16RPM discs in a similar fashion were briefly prototyped by background music companies resulting in a similar product to the Seeburg BMS system. This prototype was different from the production model due to the fact that the discs were recorded in the same fashion as described in the previous paragraph, but owe a nod to the 78 RPM Stereo Discs mentioned at the top of this piece. This is due to the fact that the BMS Prototype needed as few moving parts as possible.
The disc would use 10-inch discs instead of the nine-inch discs used in the production model and be stacked 25 high as in the production model. The prototype however would set the disc in space supported only by a small catch mechanism and start with a 2-headed cartridge on the BOTTOM side of the first disc in the center. The mechanism would turn the disc clockwise at all times, the stylus would travel from the center to the outside of the disc, and trip a change sides mechanism. The tonearm would then "fall up" so to speak past the edge of the disc and the disc would drop approximately a half inch so as to be ready to play the top side of the disc in the conventional fashion. Upon reaching the center of the disc on the top side, the tonearm would then "fall up" again engaging the lead-out-used-as-lead-in groove of the bottom side of the next disc and dropping the current disc down to a platter out of the way. This process would be repeated until the end of the top side of the 25th disc was reached. Then the mechanism would reach down to the storage area below the playback mechanism and reset the 25 discs for another run through.
And finally we come to the use of 78RPM counterclockwise discs popular in the mid 20th century in several other applications other than the right side of the aforementioned Stereo Discs described above. These discs were mainly used for toys and other noncommercial use in specially designed phonographs. It was therefore possible to merely wind the recording master tape to the right-side spool, flip the tape over so that is was upside down and tails-out, and simply cut a standard record from outside in using a master that was being played backwards.
The playback arm in such a machine was set up NOT to start all the way into the lock groove at the end of a disc. Such a player would pick up what amounted to the lead-out groove, turn counterclockwise and use it for a lead-in groove, similar to the prototype of the Automatic Background Music System described below.
We hope this primer on long forgotten alternative recording formats and their Six Degrees of Separation has been informative and enjoying.