Timing Recovery and Scramblers in Data Transmission

Recovery and tracking of the symbol rate, or timing frequency, is one of the most critical functions performed by a synchronous modem. Most modems are "self-timed" in that they derive their timing frequency and phase directly from the information-bearing signal, instead of using a separate subchannel to send synchronization information. A technique that is commonly used to acquire the symbol rate* (which is the receiver's basic sampling rate) is to filter the envelope of the modulated data signal. Our investigation will consider several problems related to this method of timing recovery which arise in high-speed modems incorporating both an adaptive equalizer and a scrambler. * This technique is also used to provide the sampling epoch, or phase, within a symbol interval. 569 The envelope-derived timing recovery system is a well-studied topic. 1,2 However, as the degree of excess bandwidth decreases, the ease with which timing can be recovered using this approach rapidly diminishes. We focus our attention on periodic input sequences. These sequences are used to train (or adapt) the data receiver during start-up and during the idle period between blocks of random data. To provide a densely spaced line spectrum of uniform amplitude (which is necessary if the equalizer coefficients are to remain properly adjusted for random input data), high-speed modems use a scrambler to "randomize" the short periodic inputs commonly used during the idle period. We investigate the effect of the scrambler on both the line spectrum and the strength of the timing tone.