Bontempi B20

Bontempi B20 (mini keyboard with gritty digital lo-fi sounds)

This keyboard sounds and behaves similar like Bontempi B30. The main difference is that it has fewer preset sounds and rhythms and (like B40) selects all functions through keyboard keys. The sound is less harsh without ear tormenting beep, and particularly percussion sounds cleaner.

different main features:

speaker without nasty tinny midrange resonance.
less harsh DAC or amplifier (no PWM beep)
15 preset sounds {piano, marimba, clarinet, accordion, flute, guitar, mandolin, organ, oboe, trumpet, synth drum, telephone, pearl drop, alarm, space runner}
7 preset rhythms {rock, 16 beat, rap, techno, latin, twist, waltz}
all functions selected through keyboard keys + select button
volume +/- control (16 steps, only affects main voice)
tuning +/- control (128 steps)
tempo +/- control (20 steps)
percussion is less distorted (low resolution samples). Drumpad effect sounds are made by main voice generator.
CPU= "Mitsubishi M38503M2-418FP, CD-27434240, 228B100" (42 pin SMD)

notes:

All functions and auto-power-off sound a low conga (tuning sounds 'a' note to indicate pitch), which disturbs live performance. In opposite to B30 the timbre is less harsh, a bit warmer and particularly lacks the intense ear tormenting beeping PWM overtone. Only the speaker in my exemplar is loose and so often makes a crunching noise. (Shake it or push down the speaker grill to stop it.) Interesting is that the B20 is the only model of the series with a more modern looking SMD CPU by Mitsubishi inside, which is clocked by an LC oscillator and also produces the percussion (in quite acceptable quality). The chassis still has the embossed button hole row of B30 and even 2 empty PCB hole rows for those ribbon cables, which makes me conclude that the B20 came out later. (I haven't examined the hardware closer yet.)

I identified the CPU by datasheet as a "Mitsubishi - 3850 Group (Spec. H)" 8 bit CMOS microcontroller with 512 byte RAM, 8 KB mask rom and software number 418. It has an 8 bit PWM DAC output and unused 5 channel 10 bit ADC. The "CD-27434240" likely corresponds to the COMUS number of earlier ICs, which is Bontempi's internal software and CPU numbering scheme. The use of a generic microcontroller proves that the B20 (likely the whole B-series) uses a softsynth on a chip.

The preset sound timbres slightly differ from B30. Particularly they have more mids, which may be result of different DAC. The 'guitar' decays slower. The 'trumpet' has faster delayed vibrato. Particularly effect sounds seem to be duller and may have less FM modulation depth. Low 'synth drum' notes are easier to hear (B30 has very poor bass response). 'telephone' sounds more melodic (rings in a different chord?). 'pearl drop' howls less resonant. The 'alarm' howls at half speed (pitch LFO 0.5 Hz, depth 1 note) with smoother (less knocking) waveform. 'space runner' has a different LFO waveform (falling instead of rising pitch).

Also preset rhythm patterns differ. '16 beat' and 'latin' were changed. 'techno' is new.

The sample percussion is pitched 5 semitones lower than in B30 and (despite low resolution) more recognizable (less truncated decay). And the drumpad effect sound mode here sounds exactly like 4 of the main voice presets {alarm, telephone, synth drum, space runner} because they are obviously played by the monophonic main voice sound generator (i.e. they can not sound simultaneously with keyboard notes, but don't mute running rhythm). I conclude that in B30 the percussion has less resolution to make the effect samples fit, since B20 has none and percussion sounds more realistic; but this may be also result of a better microcontroller.

The drum sequencer ("program style") is programmed by pressing "REC." =>a conga metronome sounds and you can now enter in realtime a monophonic drum pattern using the drumpads. Unlike B30 here the metronome does not mute automatically after entering sounds. But the general behaviour is similar. So the pattern keeps repeating in a loop while you can still add further percussion in realtime and even play on the keyboard during this. It can be stopped and restarted with the "STOP" and "PLAY" keys. The pattern length is always 4 beats (16 steps).

The mambo demo tune slightly differs from the B30 arrangement; it repeats loudly in a loop and disables all other controls except "demo off". The employed rhythm is the 'latin' pattern of B30 (differs from B20), but after stopping the demo, rhythm defaults to 'rock'.

*removal of these screws voids warranty...*

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