Data AcQuisition And Real-Time Analysis
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The following is from the Daqarta Help system:



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DAT Format (Signed, Unsigned)

Controls: DD/Open >> DAT Format >> Format
Macro: DATsgn=Sgn, UnSgn

These buttons select whether the data format is Signed (two's complement) or Unsigned (offset) binary. Signed values use binary zero to represent zero signal, whereas unsigned values use half of full-scale to represent zero signal. Sound cards always use signed values for 16-bit data and unsigned for 8-bit data, but lab-type data acquisition boards may use other schemes; 12-bit boards are often unsigned.

The binary values corresponding to maximum negative, zero, and maximum positive are shown below:

                   Neg     Zero    Pos

8-bit  Signed:     -128      0      127
     Unsigned:       0      128     255

12-bit Signed:    -2048      0      2047
     Unsigned:      0       2048    4095

16-bit Signed:   -32768      0      32767
     Unsigned:      0      32768    65535

If you are trying to open a file with an unknown format, but you have the Bits and Channels set correctly, you may see waveforms that appear to be full-scale with their tops and bottoms flipped over. Try using the other Format button.

The preceding assumes all original signals were bipolar, having both positive and negative portions. That is always true with data from sound cards, since they are AC-coupled so that the signal always has an average value of zero. (But see DC Measurements And Outputs for alternative approaches.)

But some data is naturally unipolar, having only positive values. If your .DAT file is from such a system, it was very likely recorded in an unsigned format to avoid wasting half the ADC range of unused negative data. Daqarta is not really set up to handle this properly, but if you are lucky and none of the signals went above half-scale on the original unsigned range, you can set Signed; Daqarta will show it properly, with zero signal at zero on the waveform Y axis and all values going up from there. However, if there are any peaks above half-scale, they will "wrap" and appear to be negative.

Macro Notes:

DATsgn=Sgn or DATsgn=0 sets the format to Signed, DATsgn=UnSgn or DATsgn=1 sets it to Unsigned. DATsgn=x toggles the current state.

Note that if you test DATsgn with an IF statement, you must use the numeric form: IF.DATsgn=0. (But you can use variables or expressions for the value.)

See also File Menu, Open Existing Data File, .DAT File Format Dialog


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