ROM-based finite impulse response filter for use in mobile telephone

A finite impulse response (FIR) filter apparatus for use in filtering an input signal stream, said filter apparatus comprising:

• a receiver receiving an input signal stream having groups of consecutive non-null signals bracketed by groups of consecutive null signals;
• a memory unit containing FIR filter output values, wherein the memory unit stores only FIR filter output values for each of a predetermined set of unique permissible input stream patterns, with the set of permissible input stream patterns including only patterns containing either leading non-null signals followed by trailing null signals, leading null signals followed by trailing non-null signals, all non-null signals or all null signals;
• and
• an input control unit for successively applying sequences of consecutive signals from the input signal stream to memory unit to read out a sequence of output values corresponding to a filtered version of at least a portion of the input stream.

An apparatus for converting a digital signal into an analog signal, said apparatus comprising:

• means for receiving an input digital signal stream having groups of consecutive non-null signals bracketed by groups of consecutive null signals;
• means for storing digital FIR filter output values, wherein the means for storing stores only FIR filter output values for each of a predetermined set of unique permissible input stream patterns, with the set of permissible input stream patterns including only patterns containing either leading non-null signals followed by trailing null signals, leading null signals followed by trailing non-null signals, all non-null signals or all null signals;
• means for successively applying sequences of consecutive signals from the input digital signal stream to the means for storing to read out a sequence of digital FIR filter values corresponding to the input stream patterns;
• and
• digital to analog conversion means for converting the sequence of digital FIR filter values into an analog signal.

A method for converting a digital signal into an analog signal, said method comprising the steps of:

• receiving an input digital signal stream having groups of consecutive non-null signals bracketed by groups of consecutive null signals;
• successively applying sequences of consecutive signals from the input digital signal stream to a memory unit containing digital FIR filter output values to read out a sequence of digital FIR filter values corresponding to the input stream patterns, wherein the memory unit contains only FIR filter output values for each of a predetermined set of unique permissible input stream patterns, with the set of permissible input stream patterns including only patterns containing either leading non-null signals followed by trailing null signals, leading null signals followed by trailing non-null signals, all non-null signals or all null signals;
• and
• converting the sequence of digital values into an analog signal.

An apparatus for converting a digital signal into an analog signal, said apparatus comprising:

• a receiver unit connected to an input line providing an input digital signal stream having groups of consecutive non-null signals bracketed by groups of consecutive null signals;
• a memory unit storing a table containing digital FIR filter output values, wherein the memory unit stores only FIR filter output values for each of a predetermined set of unique permissible input stream patterns, with the set of permissible input stream patterns including only patterns containing either leading non-null signals followed by trailing null signals, leading null signals followed by trailing non-null signals, all non-null signals or all null signals;
• a shift register, connected between the receiver unit and the memory unit, said shift register successively applying sequences of consecutive signals from the input digital signal stream to the memory unit to cause the memory unit to read out a sequence of digital FIR filter values corresponding to the input stream patterns;
• and
• a digital to analog converter connected to the memory unit, said digital to analog converter operating to convert the sequence of digital FIR filter values into an analog signal.