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(MOVED) - FIR discussion spinoff
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<blockquote data-quote="Michael John" data-source="post: 41593" data-attributes="member: 830"><p>Re: (MOVED) - FIR discussion spinoff</p><p></p><p>Just some more comments about FIR filters.....</p><p></p><p>Often when I see FIR filtering mentioned in the context of live sound, people frequently mean Linear Phase FIR filters. These filters have a symmetric impulse response and a fixed group delay (across frequency) which is half the impulse response length. FIR filters can be designed to have other phase characteristics, including minimum phase, with little to no latency.</p><p></p><p>Regardless of the FIR filter design, the difficulty is implementing the filtering with low latency. (This implementation latency is a separate issue to the latency/delay in the impulse response itself.) There are techniques whereby an impulse response can be chopped up into smaller pieces and the smaller pieces processed in a parallel fashion. Lake/Dolby has some IP in this area. The implementation latency is then reduced to the size of the smaller piece. The convolution result is identical to brute force convolution of the long impulse response. The overall latency is the sum of the latency of the filter design, the smallest block size of the filtering and finally the A/D and D/A converters. In the interest of full disclosure, I work for Dolby.</p><p></p><p>Someone earlier mentioned linear phase brick wall FIR filtering. I've successfully used these in biamping some (previously passive) EAW VR21 cabinets to essentially eliminate off-axis lobing in the crossover region. Below is a link to a photo of one system I designed using this. Here the VR21 is using linear phase brick wall FIR crossover with a group delay of 5ms. The "sub" is delayed 5ms to match but operated up to about 300Hz to help steer the low mid frequencies. It has a 24dB/oct LR low pass filter and the VR21 has an all-pass filter to match the phase change of the low pass on the "sub." The result was great - directed sound with no significant lobing issues below the cabinet and no feedback with mics 1.5m below.</p><p></p><p><a href="http://www.flickr.com/photos/michael_smithers/6310460687/" target="_blank">http://www.flickr.com/photos/michael_smithers/6310460687/</a></p><p><a href="http://www.flickr.com/photos/michael_smithers/6310460687/" target="_blank"></a></p><p>Best,</p><p>Michael</p></blockquote><p></p>
[QUOTE="Michael John, post: 41593, member: 830"] Re: (MOVED) - FIR discussion spinoff Just some more comments about FIR filters..... Often when I see FIR filtering mentioned in the context of live sound, people frequently mean Linear Phase FIR filters. These filters have a symmetric impulse response and a fixed group delay (across frequency) which is half the impulse response length. FIR filters can be designed to have other phase characteristics, including minimum phase, with little to no latency. Regardless of the FIR filter design, the difficulty is implementing the filtering with low latency. (This implementation latency is a separate issue to the latency/delay in the impulse response itself.) There are techniques whereby an impulse response can be chopped up into smaller pieces and the smaller pieces processed in a parallel fashion. Lake/Dolby has some IP in this area. The implementation latency is then reduced to the size of the smaller piece. The convolution result is identical to brute force convolution of the long impulse response. The overall latency is the sum of the latency of the filter design, the smallest block size of the filtering and finally the A/D and D/A converters. In the interest of full disclosure, I work for Dolby. Someone earlier mentioned linear phase brick wall FIR filtering. I've successfully used these in biamping some (previously passive) EAW VR21 cabinets to essentially eliminate off-axis lobing in the crossover region. Below is a link to a photo of one system I designed using this. Here the VR21 is using linear phase brick wall FIR crossover with a group delay of 5ms. The "sub" is delayed 5ms to match but operated up to about 300Hz to help steer the low mid frequencies. It has a 24dB/oct LR low pass filter and the VR21 has an all-pass filter to match the phase change of the low pass on the "sub." The result was great - directed sound with no significant lobing issues below the cabinet and no feedback with mics 1.5m below. [URL="http://www.flickr.com/photos/michael_smithers/6310460687/"]http://www.flickr.com/photos/michael_smithers/6310460687/ [/URL] Best, Michael [/QUOTE]
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