将32位数转换为16位或更少

Question

On my mbed LPC1768 I have an ADC on a pin which when polled returns a 16-bit short number normalised to a floating point value between 0-1. Document here.

Because it converts it to a floating point number does that mean its 32-bits? Because the number I have is a number to six decimal places. Data Types here

I'm running Autocorrelation and I want to reduce the time it takes to complete the analysis. Is it correct that the floating point numbers are 32-bits long and if so is it correct that multiplying two 32-bit floating point numbers will take a lot longer than multiplying two 16-bit short value (non-demical) numbers together?

I am working with C to program the mbed.

Cheers.

Answer 1

I should be able to comment on this quite accurately. I used to do DSP processing work where we would "integerize" code, which effectively meant we'd take a signal/audio/video algorithm, and replace all the floating point logic with fixed point arithmetic (ie: Q_mn notation, etc ).

On most modern systems, you'll usually get better performance using integer arithmetic, compared to floating point arithmetic, at the expense of more complicated code you have to write.

The Chip you are using (Cortex M3) doesn't have a dedicated hardware-based FPU : it only emulates floating point operations, so floating point operations are going to be expensive (take a lot of time).

In your case, you could just read the 16-bit value via read_u16() , and shift the value right 4 times, and you're done. If you're working with audio data, you might consider looking into companding algorithms (a-law, u-law) , which will give a better subjective performance than simply chopping off the 4 LSBs to get a 12-bit number from a 16-bit number.

Yes, a float on that system is 32bit , and is likely represented in IEEE754 format . Multiplying a pair of 32-bit values versus a pair of 16-bit values may very well take the same amount of time, depending on the chip in use and the presence of an FPU and ALU. On your chip, multiplying two floats will be horrendously expensive in terms of time. Also, if you multiply two 32-bit integers, they could potentially overflow, so there is one potential reason to go with floating point logic if you don't want to implement a fixed-point algorithm.

Answer 2

如果处理器中不存在特殊硬件（浮点单元），则假设将两个32位浮点数乘以比两个16位短值相乘的时间更长是正确的。