efficient bit stream parsing

[Timothy 123]

Hi,

What is the most efficient way of doing bit stream parsing in labview. I am acquiring data from a pxi 6561 and it comes in as unsigned u16 arrays with each array element as a sample from the 6561. The 6561 is a 16 channel device for acquiring digital data. As the data comes in the elements of the u16 array look like as follows

b15, b14, b13, ..., b0 for each sample. I am converting this to strings to parse it. But creating a string multiplies the amount of memory used since each element of the array is becomes 16 * 1 bytes = 16 bytes since each bit in the u16 array element becomes one character of a string which is 8 bits. For an array with many elements such as 10^6 elements, this becomes memory intensive. Is there a more efficient way to do bit stream parsing in labview.

Thank you,

-Tim

[bfarley]

Tim,

Not sure why you are converting to strings - but that is a huge resource hog.  If you need to see certain bits, the most efficient way to di it is using bit masks to 0 the bits you are not interested in and then us shift operations to convert to a number if you need to.

An example would be if bits 3-5 represented a uint between 0-7, you would logical AND the16 bit word with 0000000000111000 binary.  Then use a logical shift to move bits 3-5 to bit positions 0-2.

Bitwise operations like these are very fast because they are done at the register level

Bill F

[TCPlomp]

If you are interested in a 2d array of booleans, you could typecast the U16 into an array of booleans (make you sure you have 4.x conversion) right click on the type cast

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A little background:
In LV 4.x and earlier an array of booleans were bit-wise in memory, each bit was one element.
After that they changed it to bytewise, each byte is one element.

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Depending on you purpose this might be faster.
Another way might be AND-ing like Bill said, then do a conversion into U8. And then typecast it into an array of booleans (NOT 4.x style0) this works on one bit, because false is declared 0, and true non-zero.
Ton


See here

[Ben]

The very first LV Challenge was the "Bit Twidler". See here.

http://zone.ni.com/devzone/cda/tut/p/id/5307 

There was plenty to learn in that challenge!

Ben