On FM two-way radios, tones are usually sent at a level that equals two-thirds of system deviation. For example, in a ±5 kHz deviation system, the tone encoder is set to produce 3.3 kHz of transmitter deviation, (modulation,) or less. Because the tones are audible, in systems with solid received signals, tone levels are sometimes set lower so that radio users are not forced to listen to them at a high level. Keeping the tone modulation below system maximum preserves the clean sine wave produced by the encoder. Sending digits at higher levels causes the transmitter's circuits that are designed to prevent over-modulation to distort or clip the waveform of the tones. Distorted wave forms may not decode properly or may include harmonics that cause falsing. Tones are usually sent at a minimum of 500 milliseconds (ms) to 3 seconds (3,000 ms) in length.