its are able to operate synchronously, for improved performance. Data is transmitted in groups consisting of four blocks, each containing a 16-bit information word and a 10-bit check word as shown in Figure 7.6. The large check word is required to ensure the system operates correctly even under conditions of low signal strength or high interference. Once Broadcasting 213 104 bits Block 1 Block 2 Block 3 Block 4 26 bits Information Check word 16 bits 10 bits Figure 7.6 RDS data structure detected it also allows the receiver to correct errors, thereby making the system far more resilient. Once generated the data is encoded onto the subcarrier in a differential format. When the data level is at a logical '0' the output remains unchanged. When a logical '1' appears the output changes state. The baseband signal has to have its spectrum carefully limited. This is done to ensure that no crosstalk between any of the decoders takes place. This is achieved by encoding each bit as a biphase signal and also by passing the coded signal through a filter. RDS provides a variety of facilities not previously available with VHF FM broadcasts. The most widely publicized is the travel service that is of particular use for car radios. There are two flags associated with travel messages. The first is the TP flag. This indicates that a station carries travel announcements. The second is the TA flag that indicates when a travel message is actually being transmitted. When the TA flag is asserted the receiver may pause the tape, turns up the volume or takes the receiver out of a muted condition SO that the message can be heard. After the message is complete the TA flag returns to normal and the receiver resumes its previous state. RDS also enables tuning facilities to be improved. Again this is very useful for car radios. As the car moves from the service area of one transmitter to the next it can automatically tune to the strongest transmitter. The set accomplishes this by looking at the alternative frequency (AF) list and checks that si