Digitrax decoders with numbers ending in "3" have SuperSonic™.
Decoders control the locomotive's speed through Pulse Width Modulation.
When these pulses are within the human hearing range, it can cause some buzzing in some locomotives.
The SuperSonicTM feature allows you to adjust the PWM (pulse width modulation) frequency of your decoder to reduce the humming noise that is characteristic of the DCC signal.
Digitrax decoders that have this feature are shipped with the value set at 000/x00 or maximum PWM.
To change this value, you can set any value from 000/x00 (max) to 255/xFF (min).
Use of this feature will make the locomotive run more quietly but will reduce the torque generated by the motor.
Digitrax torque compensation can be used to compensate for this effect.