Electronic organ

Yamaha GX1, a synthesizer organ in 1970s

WERSI Scala, an open architecture workstation/software organ platform in 2002

A custom three manual Rodgers Trillium organ console installed in a church. Note the sound module (the Rodgers MX-200 on top of console, right) for additional pipe and orchestral sounds and the laptop (top of console, left) used for sequencing the organ and organ sound module in worship and musical performances.

An electronic organ is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally, it was designed to imitate the sound of a pipe organ, a theatre organ, band sounds, or orchestral sounds. Today, it has developed into three or more types of instrument; the Hammond organ-style instrument used in popular music genres; the digital church organ, which imitates a pipe organ and is used primarily in churches; and various other types of organs which include combo organs, home organs, and software organs, etc.

History

Predecessors

Harmonium

The immediate predecessor of the electronic organ was the harmonium, or reed organ, an instrument that was very popular in homes and small churches in the late nineteenth and early twentieth centuries. In a fashion not totally unlike that of pipe organs, reed organs generated sound by forcing air over a set of reeds by means of a bellows, usually operated by constantly pumping a set of pedals. While reed organs had limited tonal quality, they were small, inexpensive, self-powered, and self-contained. The reed organ was thus able to bring an organlike sound to venues that were incapable of housing or affording pipe organs. This concept was to play an important role in the development of the electric organ.

Pipe organ

In 1930s, several manufacturers developed their electronic organ based on functionality or quality of pipe organs. From then, some manufacturers think that emulation of pipe organ is only and most valuable aim of electronic organs.

However, even at the time, it was not the only aim of all manufacturers. Various type of electronic organs have been released on market, and some of these have established reputations and its own unique market.

Telharmonium console by Thaddeus Cahill 1897.

Early electric organs (1897-1930s)

Electricity arrived on the organ scene in the first decades of the 1900s, but it was slow to have a major impact. Thaddeus Cahill's gargantuan and controversial instrument, the Telharmonium, which began piping music to New York City establishments over the telephone system in 1897, predated the advent of electronics, yet was the first instrument to demonstrate the use of the combination of many different pure electrical waveforms to synthesize real-world instrument sounds. Cahill's techniques were later used by Laurens Hammond in his organ design, and the 200-ton instrument served as the world's first demonstration of electrically-produced music on a grand scale. Electrically powered reed organs appeared during the first decades of electricity, but their tonal qualities remained much the same as the older, foot-pumped models. Meanwhile, some further experimentation with producing sound by electric impulses was taking place, especially in France.^{[citation needed]}

A Typical Tonewheel organ, Hammond B3.

Tonewheel (right) rotates beneath electro-magnetic pickup (left)

Drawbars

Tonewheel organs (1930s-)

See also: Hammond organ

The first widespread success in this field, however, was a product of the Hammond Corporation in the mid-1930s. The Hammond Organ quickly became the successor of the reed organ, displacing it completely.

From the start, electronic organs operated on a radically different principle from all previous organs. In the place of reeds and pipes, Hammond introduced a set of rapidly spinning magnetic wheels, called tonewheels, which excited transducers that generated electrical signals of various frequencies that were mixed and fed through an amplifier to a loudspeaker. The organ was electrically powered, replacing the reed organ's twin bellows pedals with a single swell (or "expression") pedal more like that of a pipe organ. Instead of having to pump at a constant rate, as had been the case with the reed organ, the organist simply varied pressure on this pedal at will to change the volume as desired. Unlike reed organs, this gave great control over the music's dynamic range, while at the same time freeing one or both of the player's feet to play on a pedalboard, which, unlike nearly all reed organs, electronic organs incorporated. From the beginning, the electronic organ also had a second manual, also very rare among reed organs. While this meant that the electronic organ required greater musical skills of the organist than the reed organ had, the second manual and the pedalboard along with the expression pedal greatly enhanced playing, far surpassing the reed organ's limited capabilities.

The most revolutionary difference in the Hammond, however, was its huge number of tonewheel settings, achieved by manipulating a system of drawbars located near the manuals. By using the drawbars, the organist could combine a variety of electrical tones and harmonics in varying proportions, thus giving the Hammond vast "registration." In all, the Hammond was capable of producing more than 250 million tones. This feature, combined with the three-keyboard layout (i.e., manuals and pedalboard), the freedom of electrical power, and a wide, easily controllable range of volume made the first electronic organs far more flexible than any reed organ, or indeed any other musical instrument in history except, perhaps, for the pipe organ itself.

Hammond Novachord (1939)

Electronic organs (1930s-)

In the wake of the Hammond's invention of Tonewheel Organ (1934), other models — including Hammond Novachord (1939) and other competitors — used various combinations of oscillators and filters to produce electric tones.^{[note 1]} The heat generated by early models with vacuum tube tone generators and/or amplifiers led to the somewhat derogatory nickname "toaster".^{[note 2]}

Electronic organs were once popular home instruments, comparable in price to pianos and frequently sold in department stores. After their début in the 1930s, they captured the public imagination, largely through the film performances of Hammond organist Ethel Smith. Nevertheless, they initially suffered in sales during the Great Depression and World War II. After the war they became more widespread; for example, the Baldwin Piano Company introduced its first (with 37 vacuum tubes!) in 1946.^[1] They peaked in popularity in the mid 1970s, by which time sales began being undercut by the rapid growth of television and high fidelity audio systems as home entertainment alternatives. Home electronic organ models usually attempted to imitate the sounds of theatre organs and/or Hammonds, rather than classical organs.

Schematics of Frequency divider organ using transformer-divider (In French)

Frequency divider organs (1930s,1958-)

Baldwin Electronic Organ.
Originally designed by Winston E. Kock^[1]

Early electric organ products released in 1930s/1940s were already implemented on frequency divider technology with vacuum tubes.

With the development of the transistor, electronic organs that used no mechanical parts to generate the waveforms became practical. The first of these was the frequency divider organ, the first of which used twelve oscillators to produce one octave of chromatic scale, and frequency dividers to produce other notes. These were even cheaper and more portable than the Hammond. Later developments made it possible to run an organ from a single radio frequency oscillator. Frequency divider organs were built by many companies, and were offered in kit form to be built by hobbyists. A few of these have seen notable use, such as the Lowrey played by Garth Hudson. Its electronic design made the Lowrey easily equipped with a pitch bend feature that is unavailable for the Hammond, and Hudson built a style around its use.

Console organs (1930s-,1950s-)

A typical console organ (Johannus Sweelinck 35)

Console organs, large and expensive electronic organ model, resembled pipe organ console. These instruments had a more traditional configuration, including full-range manuals, a wider variety of stops, and a two-octave (or occasionally even a full thirty-two note) pedalboard easily playable by both feet in standard toe-and-heel fashion. (Console organs having thirty-two note boards were sometimes known as "concert organs.") Console models, like spinet and chord organs, had their speakers mounted above the pedals, though the classic Hammond design of the 1930s and 1940s made use of free-standing loudspeakers called "tone cabinets" and sometimes enhanced by rotating speaker units, usually manufactured by Leslie, that produced a higher-quality sound than a spinet organ's small built-in speakers. With their more traditional configuration, greater capabilities, and better performance compared to spinets, console organs were especially suitable for use in small churches, public performance, and even organ instruction. The home musician or young student who first learned to play on a console model often found that [s]he could later make the transition to a pipe organ in a church setting with relative ease. College music departments made such available as practice instruments for students, and church musicians would not uncommonly have them at home.

Home organs (1940s,1950s-)

A typical home organ (Kimball Broadway)

During the period from the 1940s through approximately the 1970s, a variety of more modest self-contained electronic home organs from a variety of manufacturers were popular forms of home entertainment.^[2] These instruments were much influenced from the theatre organ in its style, and often these stops contains imitative voicings such as "trumpet" and "marimba". In the 1950s, as technology progressed, they increasingly included automated features such as one-touch chords, electronic rhythm and accompaniment devices, and even built-in tape players. These were intended to make playing complete, layered "one-man band" arrangements easy, especially for people who had not trained as organists. While a few such instruments are still sold today, their popularity has waned greatly, and many of their functions have been incorporated into more modern and inexpensive portable keyboards. The Lowrey line of home organs is the epitome of this type of instrument.

Spinet organs (1949-)

A Typical Spinet organ (Hammond TR-200)
has two short manuals arranged with offset.

Spinet organ's pedalboard spanned only a single octave.

Following World War II, most electronic home organs were built in a configuration usually called a spinet organ, which first appeared in 1949. These compact and relatively inexpensive instruments became the natural successors to the reed organs. They were marketed as competitors of home pianos and often aimed at would-be home organists who were already pianists (hence the name "spinet", in the sense of a small upright piano). The instrument's design reflected this concept: the spinet organ physically resembled a piano, and it presented simplified controls and functions that were both less expensive to produce and less intimidating to learn. One feature of the spinet was automatic chord generation; with many models, the organist could produce an entire chord to accompany the melody merely by playing the tonic note, i.e., a single key, on a special section of the manual.

On spinet organs the keyboards were typically at least an octave shorter than is normal for organs, with the upper manual missing the bass (typically 44 notes, from F3-C7 in Scientific Pitch Notation), and the lower manual missing the treble (typically F2-C6). The manuals were usually offset, inviting (although not requiring) the new organist to dedicate the right hand to the upper manual and the left to the lower, rather than using both hands on a single manual. This seemed designed in part to encourage the pianist, who was accustomed to a single keyboard, to make use of both manuals. Stops on such instruments, relatively limited in number, were frequently named after orchestral instruments that they could, at best, only roughly approximate, and were often brightly colored (even more so than those of theatre organs). The spinet organ's loudspeaker, unlike the original Hammond models of the 1930s and 1940s, was housed within the main instrument (behind the kickboard), which saved even more space, although it produced a sound inferior to that of free-standing speakers.

The spinet organ's pedalboard normally spanned only a single octave, was often incapable of playing more than one note at a time, and was effectively playable only with the left foot (and on some models only with the left toes). This limitation, combined with the shortened manuals, made the spinet organ all but useless for performing or practicing classical organ music, but at the same time it allowed the novice home organist to explore the challenge and flexibility of simultaneously playing three keyboards (two hands and one foot). The expression pedal was located to the right and either partly or fully recessed within the kickboard, thus conveniently reachable only with the right foot. This arrangement spawned a style of casual organist who would naturally rest the right foot on the expression pedal the entire time, unlike classically-trained organists or performers on the earlier Hammonds. This position, in turn, instinctively encouraged pumping of the pedal while playing, especially if already accustomed to using a piano's sustain pedal to shape the music. Expressive pumping added a strong dynamic element to home organ music that much classical literature and hymnody lacked, and would help influence a new generation of popular keyboard artists.

Chord organs (1950-)

A chord organ (Hammond S-6). Array of chord buttons are placed on the left side.

Main article: Chord organ

Shortly after the debut of the spinet the "chord organ" appeared.^[3] This was an even simpler instrument designed for those who wanted to produce an organlike sound in the home without having to learn much organ (or even piano) playing technique. The chord organ had only a single manual that was usually an octave shorter than its already-abbreviated spinet counterpart. It relied more heavily on automatic chord generation than other models; it also possessed scaled-down registration and no pedalboard or expression pedal (volume being determined by a knob near the manual instead, an inefficient arrangement that effectively eliminated the dynamic playing that an expression pedal allowed). The left hand operated not a keyboard but an array of chord buttons adapted from those of an accordion. Early models had two pedals sounding the root and fifth of the selected chord; later models incorporated a drum machine and auto-accompaniment unit. As was the case with the spinet, the loudspeaker was housed within the kickboard.

A combo organ (Vox Continental) using transistor. It's light, compact and portable.

Combo organs (1960s-)

Main article: Combo organ

In the '60s and '70s, a type of simple, portable electronic organ called the combo organ was popular, especially with pop and rock bands, and was a signature sound in the pop music of the period (e.g. The Doors, Iron Butterfly). The most popular combo organs were manufactured by Farfisa and Vox.

By the 1960s, electronic organs were ubiquitous in all genres of popular music, from Lawrence Welk to acid rock to the Bob Dylan album Blonde on Blonde. In some cases, Hammonds were used, while in others, very small all-electronic instruments, only slightly larger than a modern digital keyboard, called combo organs, were used. (Various organs made by Farfisa were especially popular, and remain so among retro-minded rock combos.) The 1970s 1980s and the 1990s saw increasing specialization: both the gospel and jazz scene continued to make heavy use of Hammonds, while various styles of rock began to take advantage of more and more complex electronic keyboard instruments as Large-scale integration and then digital technology began to enter the mainstream. Although the original Hammond tonewheel design was phased out in the mid-1970s, it is still very much in demand by professional organists, and the industry continues to see a lively trade in refurbished instruments even as technological advance allows new organs to perform at levels unimaginable only two or three decades ago.

History of church organs

Pipe/electronic hybrid organs (1934-)

The combinations of electronic tone generators and pipe organs had been developed in the 1930s.^[4] Custom electronic organ consoles occasionally replace aging pipe consoles, thereby updating the electrical control system for the pipes as well as adding electronic voices to the organ.

For combination organs, which join pipes to electronic stops, an important issue is that pipes change pitch with environmental changes, but electronic organ do not follow by default. The frequency of sound produced by an organ pipe is determined by its geometry and by the speed of sound in the air within it. The speed of sound changes with the temperature and humidity of the air; therefore the pitch of a pipe organ will change as the environmental changes, so the pitch of the electronic side in a hybrid instrument must be re-tuned as needed. The simplest way this can be done is with a manual control that the organist can adjust, but some recent digital models can make such adjustments automatically.

Electronic church organs (1939-)

The first full electronic church organ was built in 1939 by Jerome Markowitz, founder of the Allen Organ Company, who had worked for years to perfect the replication of pipe organ sound through the use of radio tube based oscillator circuitry. In 1958, Rodgers Organ Company built the first solid-state (transistor) organ.

Separate oscillators

In contrast to the frequency divider circuitry with only a few independent pitch sources, quality church organs have always have had at least one oscillator per note and often additional sets to creature superior ensemble effect. For instance, Rodgers Opus 1, a three manual Model 38 the world's first solid-state, transistorized church organ in 1958, featured eight sets of transistorized pitch generators. Even today, digital organs use software-based digital oscillators to create large numbers of independent pitch and tone sources to better produce the effect of a large pipe organ.

Digital church organs (1971-)

Allen introduced the world's first digital organs (and first digital musical instrument product) in 1971 – the Allen Digital Computer Organ.^[5][6] This new technology was developed for use in home organs by North American Rockwell (project leader Ralph Deutsch) and licensed to Allen, who ended up putting it in their church organs. Allen later sued Rockwell and Deutsch and gained sole rights to the digital computer organ technology.

Microprocessor (1980s-) and DSP (1990s) in church organs

In 1980, Rodgers introduced the first church organs controlled by microprocessors partially based on research at the University of Bradford. The university's "Bradford Computing Organ" has technological descendants in some European digital organs using synthesis technology today.

In 1990, Rodgers introduced the first software-based digital church organs with its patented Parallel Digital Imaging© technology which paralleled many Digital Signal Processor (DSP) to generate stereo imaged pipe organ sound.

Modern digital organ technologies (1980s-)

A modern digital combo organ using DSP technology.

Electronic organs are still made for the home market, but they have been largely replaced by the digital keyboard or synthesizer, which is not only smaller and cheaper than typical electronic organs or traditional pianos, but also far more capable than the most advanced electronic organs of earlier years. Modern digital organs, by the same token, are far more advanced in design and capabilities than their ancestors.

Today's instruments incorporate real time tone generation based on sampling or synthesis technologies, MIDI, and Internet connectivity for downloading of music data and instructional materials, as well as making use of USB flash drive or media card storage. While electronically they are radically different from their predecessors, their basic appearance makes them instantly identifiable as the latest generation in a long line of electronic organs that now reaches back more than seventy years.

DSP

See also: Digital signal processing and Physical modelling synthesis

The best digital organs of the 2000s have a number of features which distinguish their sound from earlier, simpler instruments. Digital organs may use multiple Digital signal processor (DSP) chips paralleled for extremely fast processing and generation of the organ sound. Organ sound in most current digital organs is derived from digital signal processors in either a sampled or synthesis type generation system. Sampled technologies use sound files recorded from various ranks of organ and then processed to be the root basis for generation of the organ sound. In synthesis systems, the wave shape is created without using an actual sample as a guide. Both systems actually generate organ tone, sometimes in stereo in better systems, rather than simply playing back recordings as a simple digital keyboard sampler might do. It has been claimed by its manufacturers that an organ voices might have slightly more control in some synthesis systems. Marketed as a "Real Time System" by Eminent and also sold by Wyvern, Copeman Hart, Cantor, and Van der Pole in Europe, synthesis organs may use circuitry purchased from Musicom, an English supply company. On the digital pipe organ category, synthesis-based systems are rarely seen outside of Europe.

Sampling

See also: Sampler (musical instrument)

Sampled systems may have individual samples of actual organ pipes for each note, or may use only one or a few samples which are then frequency shifted to generate the equivalent of a 61 note pipe rank. Some digital organs like Walker Technical and the extremely costly Marshall & Ogletree organs claim to use longer samples for additional realism, rather than having to repeat shorter samples in their generation of sound. Most sampling systems are typically done with 24-bit or 32-bit resolution instead of CD-quality 16-bit resolution. Sampling in 2000s-era organs is done at a higher frequency than the 44,100 samples per second of CD-quality audio.

Pipe organ simulations

Modern digital church organs have reached a degree of sophistication, complexity, and expense surpassed only by the pipe organ itself. The consoles of some of these instruments, at first glance, may be almost indistinguishable from those of large pipe organs.

Drop of windchest pressure

To better imitate pipe organs, some digital organs simulate changes of windchest pressure (in a pipe organ, the air pressure may drop slightly when many notes are sounding at once, which changes the sound of all the pipes).

Swell box effect

Digital organs may also have simulated models of Swell boxes which mimic the environmental effects on pipes, pipe chest valve release and other pipe organ characteristics. These effects are often added to the computer sound generation in modern electronic organs to create more realistic pipe organ tone.

Room acoustic

Pipe sound can be generated to include sampled or modeled room acoustic. Rodgers patented "RSS" technology which uses biaural and transaural processing to create real time acoustic models and Allen's "Acoustic Portrait", purchased from Lake Industries in 2004, are examples of better quality acoustic systems where room acoustic is part of the pipe sound generation.

Surround sound

On most typical organs, several audio channels are used to create a more spacious sound. Higher quality custom digital organ builders use custom audio and speaker systems and may provide from 8 to 32 or more independent channels of audio, depending on the size of the organ and the overall budget for the instrument. With dedicated high power subwoofers for the low frequencies of the sound, the best digital organs can thus approach the physical feeling of a pipe organ if a sufficient number of subwoofers and sufficiently powerful amplifiers are used.

A software pipe organ system

Software organs (1990s-)

See also: Software synthesizer

Software pipe organs

The processing power of today's personal computers have brought the world of organs closer than ever before. Software applications are available that store digital pipe samples and sum them in real time in response to input from one or more MIDI sources. Some of these are Hauptwerk, MyOrgan, GrandOrgue, jOrgan, Aeolus, SCPOP, and Miditzer which emulates theatre organ of Wurlitzer. Many hobbyists have used these tools to assemble home-built organs that can rival the sound quality of commercially built digital organs at a relatively lower cost.^[7]

In church

Digital organs in church

2006 Johannus 'Rembrandt' digital organ - an example of a large digital organ

These are instruments designed as pipe organ replacements or as digital consoles to play existing pipes. They have developed greatly over the last two decades, and are now a common alternative to the pipe organ, particularly in churches. These are often referred to as digital organs. The technology has advanced to such a level that there is very little difference in sound timbre between piped and pipeless instruments, although this is still debated by some organists, who may argue that there is no substitute for a real pipe organ. However, many churches that are unable to afford costly pipe organs have turned to less-expensive electronic organs as a viable alternative; even a congregation that could afford a modest pipe organ may instead opt for a digital organ that simulates a pipe organ that would be larger than they could afford.

Digital organs by custom builders have also become a viable alternative for churches who may have had a pipe organ and can no longer afford to maintain it, or for those situations where a pipe organ is not financially possible. Some proponents of pipe organs claim that digital organs should be regarded as no more than multi-note hi-fi systems of durability no more than standard electronic equipment, in contrast to pipe organs that might still be playing without major rebuilding for many years. However, the high initial cost of pipe organs has limited their production, and all-digital and pipe/digital combination organs now significantly outsell pipe organs.

Most of the current digital church organs produce sounds based on recorded pipe samples, while others may model the pipe sound by digital synthesis. Custom digital organs can require large and expensive computer systems and an organ "voicer" may finish the organ, much like the process of regulating and voicing a pipe organ. These organs typically use very high quality custom-designed audio systems. The builders of both custom and factory organs include the firms of Allen, Ahlborn-Galanti, Johannus, Viscount, Makin, Rodgers and Wyvern.

This style of instrument is also popular with popular concert organists, such as Carlo Curley, who tours with a substantial Allen Organ in the USA and with an Allen in the UK, which means he does not need to spend time getting used to a new pipe organ for every concert he performs. These instruments will often contain effects that are not seen on pipe organs, and there may be additional features, such as orchestral and percussion sounds, and console aids. The most advanced digital organs also offer some capabilities and features not found in pipe organs, such as changing historical pitch standards and temperaments.

A modern electronic organ. Though it resembles a 1950s spinet organ in appearance, its digital tone generators and synthesis modules can imitate hundreds of instruments.

In popular music

The Hammond-style organs used in more popular genres such as Jazz, Gospel and Pop music.

In 2006, Yamaha launched its latest model of Electone, the Stagea.

See also

• Organ
• List of electronic organ makers
• Farfisa
• Hammond organ
• Content Organs
• Johannus Orgelbouw
• Kawai
• Maestrovox
• Rodgers Instruments
• Roland Corporation
• Wersi
• Yamaha Corporation

References

1. ^ ^{a b} Hans-Joachim Braun. "Music Engineers. The Remarkable Career of Winston E. Knock, Electronic Organ Designer and NASA Chief of Electronics". 2004 IEEE Conference on the History of Electronics. IEEE. http://www.ieeeghn.org/wiki/images/8/8e/Braun.pdf. 
2. ^ (in Japanese) All About Electronic & Electric Musical Instruments. Seibundo ShinkoSha. ASIN B000JAAXH6, 電子楽器と電気楽器のすべて. http://www.denhaku.com/mukasi/chomks/allthe.htm.  – A guide book for various electronic organs manufactured or imported in the mid-60s Japan.
3. ^ Laurens Hammond, Encyclopædia Britannica Online, 2009
   His later inventions included the chord organ (1950), on which chords are produced simple by touching a chord button.
4. ^ Synthetic Radio Organ Church Diagram French Print 1934, The ILlustration Newspaper of 1934, Paris
5. ^ Peter Manning (1993). Computer and Electronic Music. Oxford Univ. Press. ISBN 0193119188. 
   The first software digital instrument, MUSIC was developed by Max Mathews in 1957 at Bell Labs, although it was non-realtime system.
6. ^ Graham Hinton (2002). "EMS: The Inside Story - The Non-Products". http://www.ems-synthi.demon.co.uk/emsstory.html#nonproducts. 
   The first digital sampler may be EMS MUSYS system ca. 1969, or EMS DOB (Digital Oscillator Bank) ca. 1972.
7. ^ Images of Hauptwerk consoles, PCorgan.com; Hauptwerk's customer set-ups
   For example, Canadian organ builder Artisan Classic Organ has a division called Classic Organ Works for supplying their parts to other builders and hobbyists. And already many hobbyists built their own organs using PC software and additional hardware parts (i.e. manuals, pedalboard, touchscreen for stop control, studio quality monitors and subwoofer, etc ).

Footnotes

1. ^ Today's modern electronic organs use high-quality digital samples to produce as accurate a sound as possible.
2. ^ Today's solid-state instruments do not suffer from this problem nor do they require the several minutes the vacuum tube organs needed to bring the filament heaters up to temperature.

External links

• "Pipeless Organ Turns Electricity Into Music", September 1931, Popular Mechanics 1931 article on early all electric organ
• "Electric Pipeless Organ Has Millions of Tones" Popular Mechanics, April 1936
• TheaterOrgans.com FAQ
• Schober From the 50s to the 70s, Schober produced a popular line of build-your-own organ kits. Models ranged from spinets up through AGO consoles.
• Aeolus: A synthesised (i.e. not sampled) pipe organ emulator.
• Download MP3 files of a Makin digital organ at Hammerwood Park in Sussex after serving a dozen years at Londonderry Cathedral, where visitors had said it was "remarkably effective". This has now been enlarged to 5 manuals using further electronic organ units known as "expanders", often used to enhance pipe organs, made by Content in Holland and Ahlborn in Italy.
• Electronic organ manufacturers at the Open Directory Project