Signal Continuability

In some cases, Modulation Toolkit palette-level VIs are continuable, meaning that they can operate on continuous signals that have been broken into blocks or frames. This division can separate the large dataset of the digitized signal into smaller blocks. Breaking signals into blocks or frames can help in situations where there are PC memory limitations or a limited amount of hardware acquisition memory.

Because Modulation Toolkit VIs are continuable, you can utilize them in two distinct modes:

• One-shot mode—Processes all data with a single call to the appropriate Modulation Toolkit VIs.
• Continuous mode—Processes data block-by-block as available or appropriate.

When using continuous mode, consider the action of VIs that include a reset? parameter. This parameter is present in VIs that store state information when called in a loop, as when processing a continuous stream of data. In these situations, the VI is typically reset when initially called, and on subsequent calls information is retained about data processed in prior calls to the same instance of the VI. After several iterations, you can reinitialize the VI to return it to a known state. The MT Generate Bits (poly) VI, shown in the following figure, is an example of a VI that utilizes the reset? parameter in this way. The VI can generate a continuous PN sequence, which is a pseudorandom bit stream that is seeded for easy continuous reproduction.

MT Generate Bits VI in One-Shot Mode

MT Generate Bits VI in Continuous Mode

The preceding examples show two ways you can call the MT Generate Bits (poly) VI. The top section shows the VI being called a single time to generate the first 32 bits of a PN sequence. The bottom section shows the VI being called inside a For loop and generating the same 32 bits by concatenating two 16-bit chunks. Because the MT Generate Bits VI is continuable, the same instance can be called multiple times, with the first call being reset and subsequent calls maintaining state information from previous calls so as to continue each new operation where the last call left off.

Note  By default, all continuable Modulation Toolkit VIs reset when an instance of the VI is first called and operate thereafter in a continuable manner. The reset? parameter default values are set accordingly, which means that the VIs in the preceding example would reset automatically even if the reset? parameters were not explicitly wired.

Note  Some Modulation Toolkit VIs require a minimum input block length before they can run. After a reset condition, these VIs do not return data until the minimum input length has been exceeded. For instance, the MT Convolutional Decoder VI requires at least D × n input bits (where D is the traceback depth and n is the code word length) after a reset before it can begin to generate decoded bits. Each call to the VI with less than the appropriate input length results in the input data being buffered and appended to any input data from previous iterations that followed the last reset.