page_title: Stochastic Signal Density Modulation
section_title: Principle and signal creation
hierarchical_section_title: Stochastic Signal Density Modulation / Principle and signal creation
caption_attribution_description: English: Demonstrates SSDM signal generation. 5-bit pseudo random number sequence is shown with a desired threshold, and a SSDM signal generated from the sequence. Also the averaged SSDM signal is shown to demonstrate how the density approaches a desired value.
context_page_description: Stochastic Signal Density Modulation is a novel power modulation technique primarily used for LED power control. The information is encoded - or the power level is set - using pulses that have pseudo random widths. The pulses are produced in such a way that on average the produced signal will have a desired ratio between high and low states. The main benefit of using SSDM over for example Pulse-width modulation, which is usually the preferred method for controlling LED power, is reduced electromagnetic interference. Figure 1 illustrates a SSDM signal and demonstrates how the average signal density approaches desired value. The pseudo random pulses in the signal are clearly visible.
  SSDM can be seen as a special case of Pulse-density modulation or Random Pulse Width Modulation.
context_section_description: Producing SSDM signal requires a Linear-feedback shift register (LFSR) or similar source providing a sequence of pseudo random numbers, a signal density register and a comparator. On each clock cycle, the LFSR provides one pseudo random number. This is compared against the signal density register. Should the signal density register be at or lower than the generated pseudo random number, output is set to high. If the signal density register is higher than the generated pseudo random number, the output is set to low. This is illustrated in figure 2.
  When LFSR is used to provide pseudo random numbers with maximum possible period, the sequence will contain each of the numbers in the range once (except the zero), and eventually repeat. For a 4-bit LFSR the sequence is [1 2 5 3 7 6 4]. If the signal density register would be set to 3, would the corresponding SSDM signal be repeating the [1 1 0 1 0 0 0] pattern.
  Figure 3 demonstrates the signal generation process. Generated sequence of pseudo random numbers is shown in the first sub-plot along with a desired 30% threshold. The lower plot is the produced SSDM signal. It can be seen that the output is high only when the generated pseudo random numbers are at or below the desired threshold. The SSDM signal will average at 30% of the maximum. Also the repetitive nature of LFSR is clearly visible in the pseudo random number sequence and in the resulting SSDM signal.
caption: Figure 3: creation of 5-bit SSDM signal with 30% signal density.
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