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23rd December 2012, 10:00 AM  #351 
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23rd December 2012, 10:04 AM  #352 
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Location: Hypex's doorstep


23rd December 2012, 10:29 AM  #353 
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As I see it the TAS stores coefficient with only 23 bit resolution whereas a 32 bit fixed point processor has 31 bits of resolution. I'm sure the SHARC is going to perform transparently with 24 bit ADC's and DAC's. Even with 32 bit floating point it has a 23 bit mantissa and 40 bit floating point accumulator with 80 bits of multiply precision.

23rd December 2012, 10:32 AM  #354 
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I shall test the hypex dsp agains my other active filters.
I use them for my special speaker project, a emarald physics CS1 clone. Speakers designed to use with a dsp. i already have,  Behringer DCX2496 with analoge volumecontrol.  Behringer CX3400 analoge active filter( moddified).  Nanodigi with pcm4222 ADC and PCM1798 DAC. So there will be enough to compare. 
23rd December 2012, 02:48 PM  #355  
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Quote:
It would be similar to Phase arbitrator in its implementation I think (inverted overlapped buffers). Concerning rePhase, if you stay at 48khz and use a rectangular window (best window if you only want to do phase correction and moderate amplitude corrections) 1024 taps should be enough to correct even a LR 48khz at 100Hz with only minor phase deviation down low (and of course any other additional filter at higher frequencies). Current openDRC convolution implementation is direct, which limits it to something like 12000 taps at 48khz (depending on the additional power you want to keep for biquads sections), but fft convolution lead to much more taps available, still with less rounding errors than biquads. I hope they will stay with a direct convolution, and let the user distribute taps between channels and downsample the frequency of the low channels to make the best use of the available taps (as per Four Audio's technique). Anyway, this is a bit OT, but I think (direct) convolution is the most elegant (and mathematically optimal) way to do filtering (linear phase or minimum phase, or a mix of the two with a given target delay...) and will probably become more and more affordable in the future. Most of the price of these crossovers is already in the converter and analog path... I think FIR has a bad reputation because it is always associated with semiautomated correction tools (which are sometimes really good, but require far more knowledge than it seams to operate correctly and not introduce more problems than solutions), steep filters (brickwall...), and delay. Fact is anything that can be done in IIR can be done in FIR (with no additional delay), with less potential errors and sound degradation. And of course much more can also be done (phase correction being just one of these), with arbitrary amplitude and phase responses... 

23rd December 2012, 02:53 PM  #356  
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The LS1 uses a derivate of the DLCP in a 4way configuration, and has enough power to operate a short convolution to correct the LR24 @ 1.5khz. That type of filter does not require a lot of taps, but more taps would be required to correct a filter lower in frequency (but with more audible gain probably) Last edited by pos; 23rd December 2012 at 03:19 PM. 

23rd December 2012, 03:08 PM  #357  
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23rd December 2012, 05:38 PM  #358  
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Thanks! As time reversed IIR and FIR are both out of scope to the DLCP I agree we're getting a bit off topic for this thread. I'll reply in the rePhase thread in a bit. (Though the TAS3108's 135MMACs is enough to do about 200 taps per channel on the DLCP; maybe a future update will have support...)
Quote:
The TAS3108 uses 28 bit coefficients, not 23; please refer to TI's documentation for the part. You're correct accuracy relative to a biquad fully implemented with higher precision diminishes as coefficients are truncated but, as a basic approximation, there's no difference between a truncated coefficient and a higher precision coefficient whose least significant bits happen to be zero. Coefficients are about how accurately the requested filter is implemented and, at 28+ bits, the results are generally pretty decent. Numerical limitations in an IIR's feedback path want more caution as they result in limit cycles and other forms of nonperiodic steady state noise. The TAS3108's 48 bit data path provides 48 bit feedback so calling it a 28 (or 23 bit) part neglects that it's numerically better conditioned than the more common implementations with 28 bit coefficients and feedback and 56 bit MAC or 32 bit coefficients and feedback with 64 bit MAC. Also I think maybe you misread the SHARC documentation. The floating point multiplies yield 40 bit results, not 80. The fixed point multiplies are 64 bit and the accumulator's 80 bit. This allows slightly less conservative fixed point scaling than in DSPs without a 64 bit saturating MAC, potentially moving the numerical noise floor down by one or two bits if the implementer decided the extra shift instructions required to realign the data were worth it. Almost nobody does this so it's probably more realistic to view the SHARC's extended accumulator as a feature for avoiding FIR overflow that doesn't really apply to IIR biquads. A biquad chain can be made somewhat FIR like to reduce the scaling overhead and take better advantage of extended accumulatorssearch for merged biquadsbut, again, it's unlikely most implementations will code this. So, in the majority of cases, the extra 16 bits in the SHARC's accumulator probably don't make a difference with respect to IIR accuracy. However, as I mentioned a few posts back, I'm not aware of results as to whether allowing the numerical noise floor to rise above the DAC's noise floor is subjectively audible. Hence my interest in your analysis. Don't get me wrong here; I'm not saying the miniSHARC is going to be bad, just that a look at the DSP suggests the DLCP has a reasonable chance of being subjectively preferable even in cases where the DAC isn't being pushed hard (as an aside, I looked into DIYing my own SHARC based DSP platform a while back but couldn't justify the cost of the compiler). 

23rd December 2012, 10:35 PM  #359  
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Quote:
There is a good article about the sharc here Extended Precision Floating vs Fixed Point  Sharc  Comp.DSP  DSPRelated.com Quote:
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23rd December 2012, 11:08 PM  #360 
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So no particular analysis, got it, thanks.
The product announcement's in the December 2012 newsletter. Nothing not already available in the OpenDRC. really. Last edited by twest820; 23rd December 2012 at 11:11 PM. 
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