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. Radios with DTMF decoders may monitor all system traffic or remain muted until called, depending on the system design. When the radio receives the correct digit string, it may momentarily buzz or sound a Sonalert. An indicator light may turn on and remain latched on. In most systems, the radio's receive audio would latch on after receiving a valid digit string if normally muted. Many companies have trademarked names for their DTMF features. For example, Motorola calls their DTMF options, Touch Call. Because DTMF is a standardized format, most of the features are interchangeable. Generally, any radio that is equipped to decode the digit string 0-1-2-3 would be compatible with any system using DTMF. Some systems use DTMF for push-to-talk unit ID. Each time the push-to-talk is pressed, the radio sends a string of DTMF digits. Each radio has a unique string of digits. This allows the base station to know who last called or who last pressed the push-to-talk. Two-tone sequential Two-tone sequential, also known as 1+1, is a selective calling method originally used in one-way, tone-and-voice paging receivers. Many companies have their own names for two-tone sequential options. General Electric Mobile Radio called it Type 99. Motorola called it Quik-Call II. For example, the encoder sends a single tone followed by 50 to 1,000 milliseconds of silence and then a second tone. Decoders look for a valid first tone followed by a valid second tone within a defined length of time, (a time window).