Yamaha PSS-20 (monophonic squarewave keyboard with nice electronic percussion & arpeggio)

This small white tablehooter from 1989 (embossed case date) has coarse electronic blip percussion that is interesting for tekkno.

This was the certainly trashiest keyboard ever made by Yamaha. The whole thing is only monophonic and everything is selected through 2 big silicone rubber buttons ("SELECT GREEN", "SELECT BLUE") pressed together with keyboard keys. The digital volume control badly steals bit resolution from the already grainy percussion. But despite crappy specs it is fun to play; so it has strange multipulse timbres, vibrato, sustain and even "arpeggio", which here however replaces the main voice with a kind of monophonic single finger accompaniment. Additionally there is a simple audiogame for pitch recognition.

main features:

modifications:

notes:

I like here how much play fun Yamaha created from an ultra-low spec chip. Many modern toy instruments are way less inspiring despite their polyphonic hifi samples. Annoying is that all decaying sounds (piano etc.) ignore key press duration. But keys are nicely responsive and also permit short blipping notes. The only additional control beside green & blue button is the big grey "stop" button - likely it was made so big to quickly shut up the annoying demos in shopping centers. But it also tends to accidentally stop rhythm when another function is selected with green or blue. The arpeggio accompaniment can NOT be used together with melody play, but replaces it and always starts rhythm. All 8 preset sounds can be used for it. The transpose function makes the lowest notes wrap around into high notes (also within arpeggio).

The main voices are nothing really great; they resemble Casio VL-1 but sound thinner and harsher. The 'violin' has mild vibrato. Most interesting are the following 3 multipulse timbres: These are the hollow and sonorous 'vibes' (vibrato can not be disabled), the 'banjo' (rings with 12 Hz?) and the 'marimba'. The latter automatically toggles between 2 octaves with 6 Hz, but unlike VL-1 the toggle tempo can not be adjusted. The optional sustain is 1s long, the "vibrato" is rather a 6Hz tremolo.

The rhythms of this small tablehooter are very impulsive and quite interesting for tekkno-like things (although there is no tekkno base drum included). Their tempo can be set quite slow but only medium fast (depending on selected rhythm).

The audiogame starts with a jingle and then plays a row of ascending and descending notes. The player has to listen an press the corresponding key as a pitch recognition training. Hitting the correct one sounds a signal (high note).

The 6 demos start a rhythm in the background, but belong to the most boring monophonic arrangements ever heard by Yamaha. They simply doodle in an endless loop and neither change sound nor rhythm.

  1. When the Saints Go Marching In
  2. Brother John
  3. Brahms' Lullaby
  4. Jingle Bells
  5. Camptown Races
  6. A Little Brown Jug
This song list likely even became the model for the even lousier tooting melody chips in transistor tooters like Golden Camel 7A, those include most of them.
 

hardware details

The Yamaha PSS-20 is based on the single-chip CPU "Yamaha YM7108" (crystal clocked @ 720 kHz). The power amp IC is a "NJM 386S, JRC 8603" (9 pin SIL).
preset sound multipulse pattern vibrato
sax 3:5  
violin 1:7
X
oboe 1:15  
clarinet 1:1  
piano 1:3  
vibes 1010101000000000
X
banjo 1010000000000000  
marimba = vibes  
The main voice is made from multipulse squarewave of 16 steps length. (For readability, simple patterns are written as ratio.) The 'vibes' and 'marimba' pattern has 4 spikes followed by a blank half. 'banjo' has only 2 such spikes and then 3/4 blank. These can be also interpreted as normal 1:1 or 1:4 squarewave with logical AND of a 3 octaves higher plain squarewave.

The logarithmic volume envelope is 31 unequal steps high. The 'banjo' and 'marimba' with held key retrigger their envelope (mandolin ring) when the level falls to volume step 16. The vibrato in 'vibes' can not be disabled.

(The ratios were estimated on an analogue oscilloscope, which isn't very accurate.)

keyboard matrix

Beside a power-off there are no eastereggs. But here is it for completion. The keyboard matrix is grouped by 6 (N#=notes, B#=groups. The official pin numbering is strange. I hope I haven't messed it up.)
 
13 N5
12 N4 
11 N3
10 N2
9 N1 
8 N0
 
CPU pin
out 1
out 2
out 3
out 4
out 5
out 6
out / in
 
power off
stop
select
blue
select
green
o
F1
o
F#1
in 1
7 B5
o
G1
o
G#1
o
A1
o
A#1
o
B1
o
C2
in 2
6 B4
o
C#2
o
D2
o
D#2
o
E2
o
F2
o
F#2
in 3
5 B3
o
G2
o
G#2
o
A2
o
A#2
o
B2
o
C3
in 4
4 B2
o
C#3
o
D3
o
D#3
o
E3
o
F3
o
F#3
in 5
3 B1
o
G3
o
G#3
o
A3
o
A#3
o
B3
o
C4
in 6
2 B0

The input lines are active-low, i.e. react on GND. Any functions can be triggered by a non- locking switch in series to a diode from one "in" to one "out" pin.
 

legend:
"o"
 = keyboard key
orange
background 		 = easteregg (unconnected feature)

pinout YM7108 (GE13)

The "YM7108" (18 pin DIL) is the CPU of Yamaha PSS-20; it seems to be the simplest single-chip keyboard CPU ever made by Yamaha. The multipulse squarewave main voice is only monophonic. The blip percussion has 4 sounds made from squarewave and a hissy semi-metallic waveform. Both are output multiplexed through a time slice DAC (3 internal channels) on a single pin. Unlike other Yamaha keyboards, the waveforms not even return to a center voltage, but are only pulled into one direction (up from GND, to save battery?). The only modern feature is that the volume envelopes are logarithmic; the nonlinear envelope DAC has 31 steps (5 bit) of varying height (upper end is higher).

This pinout is based on partial schematics photos of Yamaha PSS-20.
 
pin name purpose
1 VDD supply voltage +5V
2 B0 key matrix in
3 B1 key matrix in
4 B2 key matrix in
5 B3 key matrix in
6 B4 key matrix in
7 B5 key matrix in
8 N0 key matrix out
9 N1 key matrix out
10 N2 key matrix out
11 N3 key matrix out
12 N4 key matrix out
13 N5 key matrix out
14 XOUT crystal out
15 XIN crystal in (720 kHz)
16 IC reset
17 GND ground 0v
18 AOUT audio out
 

More advanced instruments with similar timbres and simple synthesizer are Yamaha PSS-80 and the great PSS-100.
 

 removal of these screws voids warranty...    
WarrantyVoid
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