HP Princeton HP-21 - Manual (Page 13)

Press the [DSP] button to turn on/off the DSP.
The status will appear on the LED.
5. Transpose
The Transpose function allows you to change
the pitch of the instrument in semi-tone steps,
up to a maximum setting of 12 semitones
above or below standard pitch.
To restore the normal Transpose position,
press both the +/- buttons simultaneously.
6. Tempo
You can speed up or slow down the tempo of
a song or metronome by pressing the
[TEMPO +/-] buttons.
To restore the default tempo, press both the
+/- buttons simultaneously. When you press
both the +/- buttons when the
song/metronome is stopping, the tempo will
return to 120.
When you are adjusting the tempo, the tempo
indicator will appear on the display.
7. Touch Response