The Roland Rhythm 55 (sometimes known by its model number, the Roland TR-55), is a preset drum machine (or "beatbox"), produced by Roland in 1972.
It was one of a trio of drum machines released by Roland that year, offering features intermediate between those of Rhythm 33 (TR-33) and Rhythm 77 (TR-77).
"TR" stands for "transistor rhythm".
The Rhythm 55 had analogue voice circuits to produce its drum sounds, and offered approximately 20 permutations of preset rhythms.
Roland Rhythm 55 Wikipedia
The Rhythm 55's drum voices are listed below. (They are the same as for the Rhythm 33.) The sounds are generated by analogue synthesis techniques, and, as is typical with this technology, the sounds themselves, although possessing a certain musical charm, are not particularly realistic imitations of their acoustic namesakes.
The following voices are generated by "tuned resonance" type circuits:bass drum
The following voices are generated by shaped white noise:snare drum
The only control on the front panel for adjusting the sounds is a balance control, which alters the relative levels of the bass drum and the cymbal/hi hat/maracas sounds.
The Rhythm 55 offers the user around 20 preset rhythms. There is no way to adjust the preset patterns per se, or to program your own patterns. The only control available is a (uncalibrated) knob to adjust the tempo, and a push-button which doubles the tempo. However, it is possible to select more than one rhythm at once, and also to effectively "mute" drum sounds from a pattern using the balance knob (see above), thus providing more possibility for rhythmic variation than might initially seem feasible.
Main preset patterns:waltz
rock and roll
Additional "variation" patterns:"2 beat":
bass & snare drum
bass & snare drum
The Rhythm 55 can not interface with other instruments (e.g. to synchronize its tempo with another machine). The Rhythm 55 has a mono audio output, available on two unbalanced phone connector sockets—one socket is high impedance, the other low impedance.