Interleaving
The parity system gives good protection against corruption of the data words for single-bit errors, but impulsive interference can take out com- plete data words, making necessary further protection. It is achieved by a simple interleaving system in which the data is written into memory and then read out non-sequentially according to an address sequencer held in ROM. The data readout order ensures that bits which were originally adja- cent become separated by at least 15 other bits. An error burst in the received data can corrupt several consecutive bits, but when the words are reassembled at the receiver the errors are distributed among several of them so that the damage to each is usually minor and capable of repair by parity correction and/or error concealment as necessary. The digital signal now has enough protection to enable it to pass unscathed through all but the worst propagation conditions.
Framing
Along with the audio data, it is necessary to send ‘housekeeping’ data to control the decoding process. Figure 17.8 shows the composition of a
single stereo broadcast data frame, each of which spans 1 ms and contains 728 bits. The frames are sent end-to-end with no gaps between them.
The block starts with a FAW which synchronises the decoder at the receiver. It is the same in every frame, and consists of the 8-bit sequence 01001110. Next comes five control bits, C, to define the type (stereo, bilin- gual, data) of signal being sent and thus to control the decoder’s operat- ing mode and the switching and routing of its output signals. The next data block AD is reserved for ‘additional data’ and can be used for various purposes.
The initial section of the frame has used 24 bits. The remaining 704 convey the stereo or dual-channel sound data in sixty-four 11-bit words, the A channel (stereo left) and B channel (stereo right) samples being sent alternately throughout the period, 32 of each. The two sound channels (plus the control/data preamble) are thus transmitted as a single serial data stream, and the bit rate of each is approximately doubled as a result. This is a form of time-division multiplex (TDM) and the required time com- pression is achieved by writing each set of data, A and B, simultaneously into memory then reading them out alternately at double speed. TDM is used in many data storage and transmission systems to match signal den- sity to channel bandwidth. The order of A and B samples shown in Figure
17.8 represents the sequence before bit interleaving takes place. Only the 704 bits of sound data are interleaved.