Patent Publication Number: US-2004054881-A1

Title: Level allocation

Description:
[0001] The invention relates to signal processing. In particular, the invention relates to ascribing levels to bits of binary information.  
       [0002] According to one aspect, the invention provides a method of producing a communications signal from a stream of binary bits to be transmitted, comprising, for each bit, allocating a level to the bit by attaching to the bit a number of bits describing an assigned level.  
       [0003] According to a second, and related aspect, the invention also provides apparatus for producing a communications signal from a stream of binary bits to be transmitted, comprising allocating, to each bit, a number of bits describing a level assigned to the bit.  
       [0004] Advantageously, the invention allows levels to be assigned to the bits of a binary datastream just prior to transmission from a transmitter. This means that the assignment of levels to the bits can be deferred to a late stage in the digital processing of the binary datastream, thus facilitating the upstream manipulation of the bitstream.  
       [0005] The invention also extends to the transmission of a stream of binary bits, by producing a communications signal representing the stream in the manner described above, converting the communications signal to the analogue domain treating each bit from the binary stream as a sign bit and its attached bits as denoting a magnitude, and transmitting the resulting analogue signal. 
     
    
    
     [0006] By way of example only, an embodiment of the invention will now be described by reference to FIG. 1, which illustrates a transmitter.  
     [0007] The transmitter  10  of FIG. 1 comprises a digital processing section  12  which produces a stream of bits S 1  for transmission to a receiver. The stream S 1  comprises a series of datawords, each several bits in length. The datawords represent, for example, a digitised and encoded speech signal that has been provided with error correcting information, etc.  
     [0008] The signal S 1  is supplied to an allocator  14  which assigns a level to each of the bits in signal S 1 , as will now be described.  
     [0009] S 1  can be regarded as a stream of bits B 0 , B 1 , B 2 , B 3 , B 4  . . . Allocator  14  appends each of the bits of S 1  with a 3-bit (in this example) binary number A 1 , A 2 , A 3  which is assigned to the bit. Accordingly, allocator  14  outputs a signal which is a stream of 4-bit numbers, each 4-bit number comprising a bit from S 1  with an allocated level of 3-bits, viz: B 0  A 1  A 2  A 3 , B 1  Al A 2  A 3 , B 2  A 1  A 2  A 3 , B 3  A 1  A 2  A 3  . . . Essentially, S 1  has been converted into a “sign-magnitude” representation, wherein each bit of S 1  becomes the “sign” bit of a word in S 2 .  
     [0010] Signal S 2  is then passed to a digital to analogue converter (DAC)  16  which produces an analogue signal S 3  by operating on each 4-bit word of S 2  to determine the level of S 3 . The analogue signal S 3  is then amplified at  18  and transmitted from antenna  20 .  
     [0011] Within the allocator  14 , the level or magnitude word A 1  A 2  A 3  can be adjusted to change the amplitude or power present in signal S 3 .  
     [0012] To combine communications signals destined for transmission, other bit streams that have been converted into the sign-magnitude representation can be simply added into S 2  prior to it being acted on by DAC  16 .