FIXED POINT TO FLOATING POINT
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From: mukeshp11 on 17 Apr 2010 09:35 Dear All, I am currently using fixed point DSP F2810. Now what advantage i will get if i get shifted to floating point. I mean i use IQ math library in fixed pt. DSP so virtually its floating point DSP. So is there any diffrence or there will be speed difference as there will be no conversion of IQ math is required in floating point DSP. so more speed. I guess if i use IQ math library in fixed pt. DSP the accuracy will be almost same as floating point. Pls. Comment. Rgds Mukesh
From: Tim Wescott on 17 Apr 2010 15:40 mukeshp11 wrote: > Dear All, > > I am currently using fixed point DSP F2810. > > Now what advantage i will get if i get shifted to floating point. Mostly you'll be able to think less about the range of the individual variables; with floating point you'll retain what precision you have for almost any reasonable value. > I mean i use IQ math library in fixed pt. DSP so virtually its floating > point DSP. On the contrary, it is fixed point. 1e-6 in 1r31 fixed-point only has 12 bits of precision left to distinguish it from its neighbor. 1e-6 in IEEE 32-bit floating point has 25. > So is there any diffrence or there will be speed difference as there will > be no conversion of IQ math is required in floating point DSP. so more > speed. Depending on the problem, you may never need to convert. Speed differences depend on the processor. Two processors, each offering a single-cycle MAC at the same clock rate, are going to be about the same speed. But implementing a single-cycle floating point MAC takes a lot more hardware than doing the same thing with fixed-point. So -- all else being equal -- you can expect the floating point processor to be bigger, more costly, and more power-hungry. > I guess if i use IQ math library in fixed pt. DSP the accuracy will be > almost same as floating point. Floating point representation takes bits away from the mantissa for the exponent, reducing the precision available in the number in return for a gain in range. For IEEE 32-bit floating point you end up with 24 bits of storage for the mantissa, with an implied leading 1, giving you 25 bits of effective precision. If you use 32-bit fixed point vs. 32-bit floating point and design well, for most DSP problems the accuracy will be better with fixed point, because you're not using up 7 bits worth of precision for the exponent. For many of the closed-loop control systems I design, 32 bits of precision is perfectly adequate, yet 25 is not -- so I either have to allow a tremendous speed hit by going to 64-bit floating point or I have to use 32 bit fixed point. Fixed point often wins, unless the control loop is a slow and/or minor part of the system and as such doesn't get a chance to drive cost. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: glen herrmannsfeldt on 17 Apr 2010 20:45 mukeshp11 <mukeshkumar.chaudhary(a)n_o_s_p_a_m.cgl.co.in> wrote: > I am currently using fixed point DSP F2810. > Now what advantage i will get if i get shifted to floating point. > I mean i use IQ math library in fixed pt. DSP so virtually its floating > point DSP. > So is there any diffrence or there will be speed difference as there will > be no conversion of IQ math is required in floating point DSP. so more > speed. I believe that most DSP algorithms are best implemented in fixed point. In some cases, keeping more bits for intermediate values then needed for the result is useful. Much of the time, you will need fewer extra bits than you would for an exponent in a floating point value. > I guess if i use IQ math library in fixed pt. DSP the accuracy will be > almost same as floating point. -- glen
From: steveu on 18 Apr 2010 05:45 >mukeshp11 <mukeshkumar.chaudhary(a)n_o_s_p_a_m.cgl.co.in> wrote: > >> I am currently using fixed point DSP F2810. > >> Now what advantage i will get if i get shifted to floating point. > >> I mean i use IQ math library in fixed pt. DSP so virtually its floating >> point DSP. > >> So is there any diffrence or there will be speed difference as there will >> be no conversion of IQ math is required in floating point DSP. so more >> speed. > >I believe that most DSP algorithms are best implemented in fixed point. >In some cases, keeping more bits for intermediate values then needed >for the result is useful. Much of the time, you will need fewer >extra bits than you would for an exponent in a floating point value. Some things only work in fixed point. Various sliding algorithms only work if you extract from one end of the window exactly what you injected into the other. In general, floating pointing arithmetic causes these algorithms to drift off. >> I guess if i use IQ math library in fixed pt. DSP the accuracy will be >> almost same as floating point. Steve
From: Al Clark on 18 Apr 2010 13:28
glen herrmannsfeldt <gah(a)ugcs.caltech.edu> wrote in news:hqdko4$3dh$4 @naig.caltech.edu: > mukeshp11 <mukeshkumar.chaudhary(a)n_o_s_p_a_m.cgl.co.in> wrote: > >> I am currently using fixed point DSP F2810. > >> Now what advantage i will get if i get shifted to floating point. > >> I mean i use IQ math library in fixed pt. DSP so virtually its floating >> point DSP. > >> So is there any diffrence or there will be speed difference as there will >> be no conversion of IQ math is required in floating point DSP. so more >> speed. > > I believe that most DSP algorithms are best implemented in fixed point. > In some cases, keeping more bits for intermediate values then needed > for the result is useful. Much of the time, you will need fewer > extra bits than you would for an exponent in a floating point value. > >> I guess if i use IQ math library in fixed pt. DSP the accuracy will be >> almost same as floating point. > > -- glen Some algorithms are much easier in floating point. For example FFTs allow you to avoid scaling between butterflys. In general with fixed you gain precision, in float, dynamic range. I probably split 50 - 50. Floating point DSPs usually are also fixed point processors with similar computation speed. For example SHARC is 32 fixed and 32/40 bit float. Both types do a SISD MAC in 1 instruction. You choose whatever makes sense for the situation. Floating emulation wth a fixed pont DSP is usually very computationally expensive. Often fixed point DSPs are limited to 16 bit precision. This means that you are likely to need double precision (from a 16 bit word perspective). This may also be true with the 24 bit 56K stuff. Double precision increases computation requirements by 4x. Floating point DSPs usually consume much more power, however ADI's latest SHARC, the ADSP-21478 or ADSP-21479 are interesting exceptions. Al Clark www.danvillesignal.com |