Below are 3 plots showing selected demodulator results from KB4YZ's recording of the 1155 UTC 06 April 2001 transmission by VK3LM. Each plot spans 10 symbol periods. Together, the 3 plots span 30 consecutive symbol periods. One symbol period is 96 "sound card clock" periods. For the, nominally, 11,025 Hz sound card clock rate, each symbol period is about 8.707 milliseconds. The horizontal spacing between grid lines on the plots below is one nominal symbol period.
Demodulator output from 2 adjacent subcarriers is graphed on each of the plots below. The nominal subcarrier frequencies for the two, which are graphed, are 1050 Hz and 1280 Hz. For comparison purposes, the "ideal" demodulator output is graphed, along with the the demodulator output from KB4YZ's recording of VK3LM's transmission. The "ideal" demodulator output was generated by using the sound card INPUT file as the input to the demodulator-decoder program. The color code used to identify the various traces on the plots below is listed in the following table
Color Subcarrier Source
(Hz)
----- ---------- -----------------
red 1050 ideal case
blue 1050 VK3LM Tx to KB4YZ
magenta 1280 ideal case
green 1280 VK3LM Tx to KB4YZ
The vertical axis on these plots is labeled in units of "phase change". The information in each particular subcarrier is conveyed by which of the 9 possible "phase changes" that subcarrier made during each symbol period. On these plots, the 9 possibilities are: -4, -3, -2, -1, 0, 1, 2, 3, and 4.
Look at the magenta and green traces on the first plot. At the left edge, they are both closer to "-4" than to any other of the 9 possibilities. Continuing on (horizontally) with the magenta and green traces, at the next 4 grid lines, they both indicate the following sequence of phase change values: 3, 0, 1, -1. Except for the end of the third plot, the green (from VK3LM) and the magenta (ideal) traces are very similar to each other. Since what we care about is which of the 9 possible "phase changes" the demodulated signal is "closest to", at the proper sampling time, the green trace (actual received signal) is conveying the same information as the magenta trace (ideal case).
The relatively close match between the green and magenta traces over this 30 symbol period shows that the path taken by the modulated energy centered on 1280 Hz was stable during this time. This is in stark contrast to the case for the modulated energy centered on 1050 Hz, just 230 Hz lower.
Look at the red (ideal case) and blue (from VK3LM) traces on the first plot. Just to the right of the center of the plot, between the horizontal labels of "66432" and "66528", where the blue trace dips down to "0", then, for the next symbol, rises to "1", the blue trace seems to be LEADING the red trace by about half of a symbol period. Three symbol periods later, between the horizontal labels of "66720", and "66816", the blue trace seems to be LAGGING the red trace by about half a symbol period. Thus, over a 3 symbol period, about 26.12 msec, the energy centered at 1050 Hz was delayed by about an extra 8.7 msec, as if it switched to a path that was 1,620 miles longer that the path it had been taking.
This second plot is included to maintain continuity.
This third plot shows that whatever was causing the "corruption" shown on the blue trace in the previous two plots has "disappeared", so that the blue trace is now following relatively closely the red (ideal) case.
Below is a shrunken version of the spectrogram of a signal received by W9NTP, from a transmission by KB4YZ, on 40 meters. The full spectrogram is 844,880 bytes in length, and has been removed, to sav space.
Note that near the 4th horizontal tic mark, all 8 subcarriers fade to near background noise levels. This transmission was successfully decoded.
Below is a plot of a 10 symbol interval showing the demodulator outputs for two of the subcarriers. The blue trace is the demodulator output for the 1280 Hz subcarrier. The green trace is the demodulator output for the 1740 Hz subcarrier. Also shown are the "ideal" demodulator outputs for the same two cases. The red trace is the "ideal" trace for the blue. The magenta trace is the "ideal" trace for the green. The "ideal" traces are obtained by demodulating the .wav file used as input to the sound card at the transmitter.
The vertical grid lines are spaced one symbol period apart.
Compare the delay of the blue trace with respect to the red trace during the first symbol period shown here (at the left side of the plot below), with the corresponding delay during the last two symbol periods shown here (at the right side of the plot below). At the left side, this delay is about 20% of a symbol period. At the right side, this delay is about 66% of a symbol period. This is a change in delay of about 4 milliseconds. Thus, it seems that the energy near 1280 Hz was taking a path about 740 miles longer at the time show on the right of the plot below, than it was at the time show on the left of the plot below. The right edge of the plot below is 87 milliseconds later in time than is the left edge of the plot below. As the crow flies, W9NTP and KB4YZ are about 68 miles apart.
Below is the image transfered from KB4YZ to W9NTP via the transmission whose spectrogram is shown above.
For those whose browser does not know what to do with a .png image file, the same image in .gif format is provided.
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