Audio processing is likely my preferred application of dsp. Specially because it is very easy to detect a good or bad processing, only listening the sound you have processed. FPGA and MATLAB are excellent tools to experiment with different audio or songs. You can filter with a low pass filter a song and extract the sound produced by the bass or the bass drum. In the same way, you can filter with a high pass filter and extract the sound of the instruments like a violins. Separate high and low, or sometimes high, medium and low frequencies is the aim of the crossovers. These devices generally have 2 inputs, one for the left sound, and the other for the right sound. Then, using different filters, the crossover separates the different bands and apply a different gain to them. This task seems very easy, we can even make it in an analogical way, but, like all, it has some difficulties that we havBujLXBRWKetK9iKe to solve. The first one, the delay of each band. If we want that the input sound will be the same as the output, only with the differences in the gain of the different bands, we need to ensure that the three paths, in case of a three band crossover, produce the same delay. We already saw that in this post, and also the way to fix it using all-pass filters. Also, we can design a crossover based on FIR filters to avoid that effect. The other difficulty we will find when we design a crossover, is the gain of the system. A crossover is built using different filters in parallel. All the filters has the same input, and the output of them is added in order to join the different bands. The gain of this addition must be the unit, because in case that all the gains are the same, the output must be the same signal than the input. This effect is obviously granted in the extremes of the band, where the filter corresponding to that band has a gain of one, and the others must have a very little gain, but, what happens with the transition zones? In this post we will talk about these zones, and then we will HDL Coder and the Microchip’s Icicle kit to implement a dual band crossover using Linkwitz-Riley (L-R) filters.

"Crossover diagram"

"2nd order Butterworth"

"2nd order LR2"

"4th order LR4"