On FM two-way radios, digits are usually sent at a level that equals two-thirds, (66%,) of system deviation. For example, in a ±5 kHz deviation system, the DTMF encoder is set to produce 3.3 kHz of transmitter deviation (modulation), or less. In systems with solid received signals, tone levels are sometimes set very low so radio users are not forced to listen to them at a high level. Keeping the DTMF 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. Digits are usually sent at a minimum of 55 milliseconds (ms) in length with at least 55 ms of silence between each digit. Some decoders may require much longer-duration digits. DTMF digits consist of paired tones: a row tone and a column tone. The levels of row and column tones must be similar in order for a decoder to interpret them reliably.