Scrambling of analogue electrical signals

DETAILED DESCRIPTION In a first aspect the invention provides a method of scrambling an analogue electrical input signal having a certain frequency spectrum.
The signal is divided into segments and said segments are re-ordered to form a re-order signal.
A defined binary sequence having a frequency spectrum substantially similar to the said spectrum of said analogue electrical input signal is added to the re-ordered signal in synchronism with the segments.
In a preferred embodiment of the invention a television sound signal is in analogue form and constitutes part of a conditional-access television signal.
The signal is formed into segments, each typically 80ms long, with each group of four segments constituting a block.
The signal is scrambled by re-ordering the blocks in an essentially random order as controlled by a pseudo-random sequence generator.
Preferably, the joins between segments are identified by superimposing on the re-ordered signal a low-amplitude binary sequence, with a spectrum that is roughly uniform between the lower and upper parts of the sound spectrum, say from 100 Hz up to 3 kHz.
That is to say the spectrum of the added binary sequence approximates (apart from overall amplitude) to that of the input audio signal.
Typically an essentially random signal will achieve this function with normal broadcast material.
In particular, a pseudo-random binary sequence can be used as the added signal.
One complete cycle of this sequence could, for example, correspond to the length of one segment.
At the descrambler a correlator checks for this sequence and having decoded the correct phase of the sequence, the position of the joins between segments can be found.
A signal having the waveform of a regenerated version of the same sequence is then subtracted from the scrambled signal so as to reduce the level of interference from the added sequence to the descrambled output signal.
There are many different ways in which a binary sequence with the desired spectral characteristics can be generated