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Fulcrum Acoustic FA12, FA15 and FA28 (1 of 4)
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<blockquote data-quote="David Gunness" data-source="post: 49826" data-attributes="member: 1032"><p>Re: Fulcrum Acoustic FA12, FA15 and FA28 (1 of 4)</p><p></p><p>First off - kudos to Langston for his usual excellent analysis!</p><p></p><p>The latency associated with A/D and D/A is primarily due to the anti-aliasing filters. In today's high quality A/Ds, it is typical to oversample with a low-order analog anti-alias filter, then use an FIR-based brickwall, linear phase low-pass filter as the anti-alias filter before downsampling to 48 kHz. The linear phase part produces about 0.5 ms of latency on both the input and output, so between the A/D and D/A there is about 1 ms of latency. To that is added firmware latency (i.e., DSP calculations are often done in "frames" of 8 or more cycles, because the coefficients can be loaded once and applied multiple times instead of once). You could realize the same anti-alias filters with half the latency at 96 k Hz, but since the filters don't need to be as sharp, you can use shorter FIRs and reduce it even further.</p><p></p><p>We're currently in a (temporary, I'm sure) situation where the digital electronics industry hasn't kept up with what us speaker guys are doing with processing, and they're guessing to some extent. "Why are we putting this FIR in here?" "Umm. I think the speaker guys want brick wall crossovers." "OK - well then we should put in a compensation delay set to the latency of the lowest frequency brickwall filter they might use. That way, we can tie it to the latency of the filter they select so the latency doesn't change if they change the frequency." Thanks guys for thinking of us, but no thanks. They will eventually figure out the difference between loudspeaker designers creating presets for their products and users getting confused when the arrival time changes because they changed a crossover frequency. We'll get there, but in the mean time, we'll have to deal with things like an I-Tech amp that puts in 5 ms of unnecessary latency in my loudspeaker settings, supposedly to save you all from getting confused ;-)</p><p></p><p>Oh, and Bennett: You definitely need a different preset when running at 96 kHz, regardless of the high frequency response of IIR filters. FIR filters are essentially impulse responses, so if you change the time scale from 20.6 us to 41.2 us everything shifts up an octave in frequency. I can tell you from experience that that doesn't sound very good ;-)</p><p></p><p>Dave</p></blockquote><p></p>
[QUOTE="David Gunness, post: 49826, member: 1032"] Re: Fulcrum Acoustic FA12, FA15 and FA28 (1 of 4) First off - kudos to Langston for his usual excellent analysis! The latency associated with A/D and D/A is primarily due to the anti-aliasing filters. In today's high quality A/Ds, it is typical to oversample with a low-order analog anti-alias filter, then use an FIR-based brickwall, linear phase low-pass filter as the anti-alias filter before downsampling to 48 kHz. The linear phase part produces about 0.5 ms of latency on both the input and output, so between the A/D and D/A there is about 1 ms of latency. To that is added firmware latency (i.e., DSP calculations are often done in "frames" of 8 or more cycles, because the coefficients can be loaded once and applied multiple times instead of once). You could realize the same anti-alias filters with half the latency at 96 k Hz, but since the filters don't need to be as sharp, you can use shorter FIRs and reduce it even further. We're currently in a (temporary, I'm sure) situation where the digital electronics industry hasn't kept up with what us speaker guys are doing with processing, and they're guessing to some extent. "Why are we putting this FIR in here?" "Umm. I think the speaker guys want brick wall crossovers." "OK - well then we should put in a compensation delay set to the latency of the lowest frequency brickwall filter they might use. That way, we can tie it to the latency of the filter they select so the latency doesn't change if they change the frequency." Thanks guys for thinking of us, but no thanks. They will eventually figure out the difference between loudspeaker designers creating presets for their products and users getting confused when the arrival time changes because they changed a crossover frequency. We'll get there, but in the mean time, we'll have to deal with things like an I-Tech amp that puts in 5 ms of unnecessary latency in my loudspeaker settings, supposedly to save you all from getting confused ;-) Oh, and Bennett: You definitely need a different preset when running at 96 kHz, regardless of the high frequency response of IIR filters. FIR filters are essentially impulse responses, so if you change the time scale from 20.6 us to 41.2 us everything shifts up an octave in frequency. I can tell you from experience that that doesn't sound very good ;-) Dave [/QUOTE]
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