Halfband filter,etc

GNU Radio
1

Hi people

Can I please ask a question or two about the HBF? So I know that it
is has 31 taps and what the coefficients look like from reading a bit.
Is it just a lowpass decimation filter from Goodman and Carey’s list?
How does it work? Is it just like a standard FIR filter where the
input gets convolved with the filter taps? Is the decimation achieved
because every second tap is 0? I know that we receive an input value
every 64e6/(decimation/2) from the CIC filter. I know for a
decimation of 16 the hb_strobe is supposed to be 4MHz. I just don’t
know where the last clocking down by 2 is done. I’ve seen this bit of
Verilog:

always @(posedge clock)
begin
start_d1 <= #1 start;
start_d2 <= #1 start_d1;
start_d3 <= #1 start_d2;
start_d4 <= #1 start_d3;
start_d5 <= #1 start_d4;
start_d6 <= #1 start_d5;
start_d7 <= #1 start_d6;
start_d8 <= #1 start_d7;
start_d9 <= #1 start_d8;
start_dA <= #1 start_d9;
start_dB <= #1 start_dA;
start_dC <= #1 start_dB;
start_dD <= #1 start_dC;
end // always @ (posedge clock)

reg mult_en, mult_en_pre;
always @(posedge clock)
begin
mult_en_pre <= #1 phase!=8;
mult_en <= #1 mult_en_pre;
end

assign clear = start_d4; // was dC
wire latch_result = start_d4; // was dC
assign strobe_out = start_d5; // was dD

Can anyone please help?

Thank you in advance.

Sebastiaan

–
Sebastiaan H.
Radar and Remote Sensing Group, University of Cape Town
Tel: +27 83 305 5667

2

On Wed, Oct 29, 2008 at 4:14 PM, Sebastiaan H. [email protected]
wrote:

know where the last clocking down by 2 is done. I’ve seen this bit of
start_d7 <= #1 start_d6;
begin
mult_en_pre <= #1 phase!=8;
mult_en <= #1 mult_en_pre;
end

assign clear = start_d4; // was dC
wire latch_result = start_d4; // was dC
assign strobe_out = start_d5; // was dD

Can anyone please help?

Taken from here:

http://www.dspguru.com/info/terms/filtterm/index2.htm

The excerpt:

“Half-band filter - a type of FIR filter where the transition region
is centered at one quarter of the sampling rate, or fs/4.
Specifically, the end of the passband and the beginning of the
stopband are equally spaced on either side of fs/4. Half-band filters
are often used in decimation filtering because (almost) half their
time domain coefficients are zero. This means, for example, you can
achieve the performance of an M-tap FIR filter while only paying the
computational price of (M+1)/2 + 1 multiplications per filter output
sample.”

The decimation can occur at either the input or the output. It’s
basically taking 1 out of every 2 output samples. Since you throw
away one entire output, you don’t have to perform that convolution.
This in conjunction with every other tap being 0, allows for a very
efficient filter + decimation scheme.

Hope this helps.

Brian