Support Forums › Forums › FIR Designer M Support › Linear-phase brick-wall crossovers › Reply To: Linear-phase brick-wall crossovers
In Second view/group delay of your filter example I see group delay is same in whole band for each band(HF, MF, LF), but on different levels. For HF it is less then 1ms, for MF it is around 3ms and for LF it is around 20-30ms. Can bands be adjusted to same group delay ?
Yes. The different group delays relate to the different “delay” parameters in each of the FIR filters. To align these, additional delay needs to be added to the MF and HF so that their group delay matches the LF. “FIR Designer M” automatically tells you what delay to add in the “FIR Alignment Delay” on the System “Export” tab, and this delay can be included in the delay blocks in the processor (and not in the FIR filters). This alignment delay is already included in the “Wavelet” plot when viewing the wavelet of the combined system response.
Another problem that I can’t solve is very steep crossover between LF and MF area(about 300Hz). Even Brickwall is chosen, in export system response graph it seems slope is not brick wall like at 2kHz(MF/HF point). At 300Hz it looks more like LR4 slope shape. I tried to get brick wall shape at 300Hz, but I couldn’t. How I can achieve brick wall shape at 300Hz crossover ?
On both the channel target and channel mag adjust tabs, either set the Brick-wall filter to be longer – by increasing the time length in “ms” – or use one or more higher order filter prototypes. Also, on the channel export tab you will need to set the filter to be longer, to ensure the filter can realise the steeper crossover shape. Remember, steeper crossover filter shapes, implemented in FIR filters, require longer FIR filters. It’s also worth considering the capabilities of the processor you will be using. For example, if the processor can only run 4096 length filters, then you need to adjust your crossover steepness to be realisable within 4096 taps.