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(MOVED) - FIR discussion spinoff
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<blockquote data-quote="Kimo Lewis" data-source="post: 26540" data-attributes="member: 497"><p>DSP latency</p><p></p><p>NOTE:</p><p>What I'm about to say does not only apply to FIR calculations in a DSP, but applies to all DSP calculations such as crossovers, parametric EQ's, Graphic EQ's, time delay, limiting, and compression, etc.</p><p></p><p>GIVEN: </p><p>It is well established and accepted that DSP latency should be minimized as much as possible and any DSP developer is well aware of algorithms (FFT vs DFT for example) which affects the throughput of a DSP engine.</p><p></p><p></p><p></p><p>Please provide your source for your information so that I might learn something new here. I have never read nor heard of any such standard. If you can't provide a source then I am left to assume your information is speculation and arbitrary.</p><p></p><p>I am not sure "all you need is a DSP that can do about a 2ms FIR". Nor am I sure you can put an absolute value on the required DSP latency to perform an FIR, at least one where the industry as a whole would accept as a standard.</p><p></p><p>But, let's assume you are right and it does require "about 2ms". And because you included the modifier "about", I'll throw in plus and minus 25% error in your favor, so that leaves 2.5ms to 1.5ms.</p><p></p><p>As an example of using your low number (1.5ms), I have been studying commercial DSP engines for some time and came across this one ( <a href="http://www.symetrix.co/wp-content/uploads/2011/01/Jupiter_ds_EN1.pdf" target="_blank">http://www.symetrix.co/wp-content/uploads/2011/01/Jupiter_ds_EN1.pdf </a>) which is capable of FIR calculations and to their credit, they publish their idle time as 1.6ms. Which means, it takes 1.6ms for the audio signal to go from input to output with no DSP processing of any kind. According to your numbers on the low side (1.5ms), this DSP is not sufficient to be used for FIR processing, because 1.5ms (required time for FIR) is less than 1.6ms (idle time). In other words, it takes more time sitting idle than it does to perform the needed FIR processing.</p><p></p><p>This leads me to conclude your low number is off.</p><p></p><p>Now let's consider your high side number of 2.5ms. What would be the penalty of a DSP calculating an FIR which takes 2.5ms or longer? To keep the math simple and ignoring any atmospheric interference, let's assume the speed of sound in air is one foot per millisecond. Then the penalty of a 2.5ms FIR calculation is simple. It would have a similar affect as moving your coax wedge 2.5 feet farther from your ears, and the SPL would not change. That is it. Seems like a benign penalty to me. Even if the calculations took twice or three times longer than 2.5ms, it would still appear to be benign.</p><p></p><p>Pictures are worth a thousand words, so I have attached a simple diagram to show the affect of no latency, 2ms latency, and 4ms latency. </p><p></p><p>To quote David Gunness' paper ( <a href="http://fulcrum-acoustic.com/wordpress/wp-content/uploads/2008/03/improving-loudspeaker-transient-response-with-digital-signal-processing-2005.pdf" target="_blank">http://fulcrum-acoustic.com/wordpress/wp-content/uploads/2008/03/improving-loudspeaker-transient-response-with-digital-signal-processing-2005.pdf</a> ):</p><p></p><p>"All loudspeaker systems are subject to significant latency, because of the relatively slow propagation speed of sound in air. Therefore, a small amount of added latency is usually inconsequential."</p><p></p><p></p><p>[ATTACH]149674[/ATTACH]</p></blockquote><p></p>
[QUOTE="Kimo Lewis, post: 26540, member: 497"] DSP latency NOTE: What I'm about to say does not only apply to FIR calculations in a DSP, but applies to all DSP calculations such as crossovers, parametric EQ's, Graphic EQ's, time delay, limiting, and compression, etc. GIVEN: It is well established and accepted that DSP latency should be minimized as much as possible and any DSP developer is well aware of algorithms (FFT vs DFT for example) which affects the throughput of a DSP engine. Please provide your source for your information so that I might learn something new here. I have never read nor heard of any such standard. If you can't provide a source then I am left to assume your information is speculation and arbitrary. I am not sure "all you need is a DSP that can do about a 2ms FIR". Nor am I sure you can put an absolute value on the required DSP latency to perform an FIR, at least one where the industry as a whole would accept as a standard. But, let's assume you are right and it does require "about 2ms". And because you included the modifier "about", I'll throw in plus and minus 25% error in your favor, so that leaves 2.5ms to 1.5ms. As an example of using your low number (1.5ms), I have been studying commercial DSP engines for some time and came across this one ( [URL="http://www.symetrix.co/wp-content/uploads/2011/01/Jupiter_ds_EN1.pdf"]http://www.symetrix.co/wp-content/uploads/2011/01/Jupiter_ds_EN1.pdf [/URL]) which is capable of FIR calculations and to their credit, they publish their idle time as 1.6ms. Which means, it takes 1.6ms for the audio signal to go from input to output with no DSP processing of any kind. According to your numbers on the low side (1.5ms), this DSP is not sufficient to be used for FIR processing, because 1.5ms (required time for FIR) is less than 1.6ms (idle time). In other words, it takes more time sitting idle than it does to perform the needed FIR processing. This leads me to conclude your low number is off. Now let's consider your high side number of 2.5ms. What would be the penalty of a DSP calculating an FIR which takes 2.5ms or longer? To keep the math simple and ignoring any atmospheric interference, let's assume the speed of sound in air is one foot per millisecond. Then the penalty of a 2.5ms FIR calculation is simple. It would have a similar affect as moving your coax wedge 2.5 feet farther from your ears, and the SPL would not change. That is it. Seems like a benign penalty to me. Even if the calculations took twice or three times longer than 2.5ms, it would still appear to be benign. Pictures are worth a thousand words, so I have attached a simple diagram to show the affect of no latency, 2ms latency, and 4ms latency. To quote David Gunness' paper ( [URL]http://fulcrum-acoustic.com/wordpress/wp-content/uploads/2008/03/improving-loudspeaker-transient-response-with-digital-signal-processing-2005.pdf[/URL] ): "All loudspeaker systems are subject to significant latency, because of the relatively slow propagation speed of sound in air. Therefore, a small amount of added latency is usually inconsequential." [ATTACH=CONFIG]149674.vB5-legacyid=1207[/ATTACH] [/QUOTE]
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