SITOR-A mode sounds very distinctive, with a chirping sound. In this mode, three characters are transmitted at a time. The receiving station then sends one character back, indicating whether or not the three characters were correctly received. If not, they are re-transmitted. Each cycle takes 450 milliseconds, so you hear two “chirps” per second.
You will notice two types of chirps are heard, one longer than the other. The longer chirp is the data, the shorter chirp is the acknowledgement from the receiving station. Often, you may only be able to hear one of the stations from your location. You must be able to hear the longer chirp to decode data.
Properly tune in the station, using the tuning indcator, as described in the Baudot RTTY section.
 
You'll notice that three colors are displayed in the signal window - black, white, and gray. The black and white colors represent valid mark/space tones being received, the gray means neither tone is present. Normally you'll see a burst of white/black bits, surrounded by gray. When you first tune in a SITOR-A station, it may take MultiMode a few seconds to properly synchronize on the signal.
 
The N button selects Normal shift, the I button selects Inverted shift. The ABC button acts as an LTRS shift, and will return the program to letters mode, from figures mode. Very useful if a burst of noise shifts the program into numbers mode.
The left and right arrows may be used to manually shift the sampling period of the signal, to help synchronize reception.
 
The BAD or OK lamp will flash as each packet is received. This can help in properly tuning in the station. When you can only hear the receiving station transmitting the ACK/NAK symbols (a shorter chirp sound), the OK lamp will flash BLUE, to indicate that something is being received, although it cannot be decoded.
