Roland RA-50 (Realtime Arranger) & L.A. Synthesis

The RA-50 is essentially the module version of the E-20 keyboard. 
These date from 1988. The name "Arranger" is shared by the Roland TR-66 (1973)

The heart of it is the MT-32 based sound module which uses LA (linear arithmetic) Synthesis like in the Roland D-50. I far as I know, there are no dedicated drum machines that use LAS. The Roland R-8 & R-5 for example came out just one year later in 1989 and they still used ROM based samples. 
The RA-50 can however be used as a stand-alone drum.

LA synthesis combines subtractive synthesis with PCM-based samples. The synthesis is all digital (linear)
and the sounds are summed (arithmetic). This method should not be confused with additive synthesis.

Lots of music styles ... though you can program your own patterns. There are 30 variations of drum rhythms. There is also built-in digital reverb. 8 Types ( Hall 1 / 2, Plate 1 / 2, Room 1 / 2, Delay 1 / 2 )

At the time (1987), the DX7 FM synth of Yamaha produced excellent metallic, percussive sounds. This was something that Roland's synths, using subtractive synthesis, were not great at. Roland's answer was the D-50
which went on to become one of the best selling synths of all time. Thus, the RA-50 is a unique sounding & excellent drum machine.

Chord intelligence' function allows you to play full chords with just one or two fingers.

 'Chord hold' sustains a chord on the lower 

 The RA-50 (& D-50) use a combination of samples. These were divided into 2 groups.
1. Those for the attack transients.
2. Those intended for embellishing the body of the sound. These were mainly single-cycle sampled waveforms that could be continuously looped.

Melody intelligence: this function creates block chords for accompaniment based on the melody and bass note


 These two types of samples were "summed" and embellished the subtractive sounds that the synth itself produced. So LA systhesis was the best of both worlds ... it was both a sampler & synthesizer.
The synth part sounds very analog with low-pass-resonant filters.

'Sync start' lets you start the performance automatically when you press a note.

I haven't even started on about the rhythm arranging cababilities of this machine.
Though it's not the reason I bought this, it's still a very capable instrument for auto acompaniment.... band in the box duties.The  'intelligent' Arranger section automatically produces a accompaniment consisting of drums, bass, and other instruments ( such as piano, percussion,and horns ). This is based on the chord you play and the music style you choose. there are 32 preset music styles.

Rear Panel: Ext. Pedal jacks x 3, MIDI ( KBD ) In / Out for both keyboard & sequencer.
DC in jack, Power switch, 
Power Supply: 9V DC 800mA PSU required. Centre -ve.( when using supplied adaptor) 

Output Jacks x 2 ( L / Mono, R ), Headphones jack,
Card slot for expanding memory.

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For more info on the history of Roland Drum Machines click here