Rx Operational Considerations

Before recording, set your sound card to sample at 11,025 Hz and 16 bits per sample.

If you forget to set your sound card to 11,025 Hz and 16 bits, convert your recorded files to these values before trying to decode them with "WAV2BIN". The "WAV2BIN" program depends on the sampled data having these parameters, but DOES NOT CHECK them. If the sampling parameters are not 11,025 Hz and 16 bits per sample, "WAV2BIN" will fail, and not tell you why it failed.

The basic principal to follow is to keep your equipment within its linear range of operation.

If you operate with the RF gain control at maximum and let the AGC limit the signal level, you will get poorer results than if you operate with the RF gain control backed off to the point where you have the best signal to noise ratio. Having a loud signal is counterproductive, when the loudness comes at the expense of non-linear operation.

A two tone signal has been developed to aid in setting the audio level. This two tone signal is also useful in setting the level going into the transmitter. It is the file named "2TONE-12.WAV".

The two tone test signal consists of the sum of an 1180 Hz sinusoid and a 1520 Hz sinusoid. Note that these frequencies straddle the 1200 Hz and 1500 Hz lines on the MMSSTV spectrum display. During the first 2 seconds of the two tone signal a constant amplitude is maintained. At the 2 second point in the signal, the amplitude steps down, such that the power drops by 3 db. This level is maintained until the 4 second point in the signal is reached. At the 4 second point, the amplitude drops again, so that the power drops by another 3 db. This level is maintained until the 6 second point in the signal. At the 6 second point, the amplitude steps back to its original level, resulting in a 6 db increase in power. The last 6 seconds of the two tone signal are a repetition of the first 6 seconds.

If you can receive the transmission of the two tone test signal, from someone you believe is operating within the linear range, then you can adjust the RF gain, AGC, and audio level on the receiver, so that you can hear the -3db, -3db, +6db, -3db, -3db pattern of the two tone signal. Alternatively, if you have a program that displays the spectrum of an audio signal going into your sound card, you can adjust things so that substantially all of the energy in the two tone signal is at only the two frequencies. If you see significant energy at more than two frequencies, when monitoring the two tone signal, then something is operating in the non-linear range.

I found that the linear range of operation of my sound card was essentially the middle half of its dynamic range. When signals with sample values in the upper quarter, or lowest quarter of my sound card's dynamic range were digitized by my sound card, they showed evidence of non-linear operation. Since then, I have tried to keep signals I record in the middle quarter to middle half of my sound card's dynamic range.

When recording, make sure that only the desired input to your sound card is providing the audio signal being recorded.

Spectra of leader, one transmission, three different recordings

The next three plots show the spectrum of the leader section of the same transmission, as received by three different people. All three recordings of this transmission were decoded successfully.

Since the first plot shows significant energy at only 12 frequencies, we know that the transmitter and this receiver were both operating in the linear range.

This second plot shows significant energy at more than 12 frequencies, thus, the receiver and/or sound card were being operated in the non-linear range.

In the plot below, note that the two largest spikes above 2200 Hz are spaced at 230 Hz intervals, and thus are not due to random noise, but to non-linear operation

The spectrum below shows no significant energy at 230 Hz, nor above 2200 Hz. Thus, this receiver was operated within its linear range.

Below is a list of the errors that were corrected in each of the three recordings, whose leader spectra are shown above.

An explanation of the error list format is elsewhere.


For data file: VK3LM13-pm8a.wav
	using Hamming window and /tmp/work/Wyman1x-demod-decode program
Block inner  |      outer code      | outer   |       inner code
  #   code   |       erasures       | code    |         changes
      changes|1st   2nd   3rd   4th | changes |    0   1   2   3   4
----  ------ |---   ---   ---   --- |   ---   |  --- --- --- --- ---
  0     341     0h    0s    0t    6f      1  ||   59 159  82   6   0   +
  1     334     0h    0s    4t    8f     10  ||   61 156  77   8   0   +
  2     282     0h    0s    3t    8f      4  ||   97 138  60   8   0   +
  3     412     5h    0s    3t   20f     13  ||   27 150 101  20   0   +
  4     412     2h    0s   18t   22f     22  ||   24 134 106  22   0   +
  5     434     2h    0s    9t   33f     15  ||   29 131 102  33   0   +
  6     439     2h    0s    6t   34f     18  ||   28 135 101  34   0   +
  7     253     0h    0s    0t    2f      2  ||  110 141  53   2   0   +
Decoded result is file: robot-vk3lm-x2hs.png
  0 bad blocks detected, out of   8 total blocks.


For data file: vk3lm14ap6-pm8a.wav
	using Hamming window and /tmp/work/Wyman1x-demod-decode program
Block inner  |      outer code      | outer   |       inner code
  #   code   |       erasures       | code    |         changes
      changes|1st   2nd   3rd   4th | changes |    0   1   2   3   4
----  ------ |---   ---   ---   --- |   ---   |  --- --- --- --- ---
  0     331     1h    0s    3t    3f      5  ||   65 146  88   3   0   +
  1     413     3h    0s   13t   25f     26  ||   25 142  98  25   0   +
  2     371     3h    0s    6t   13f     14  ||   36 164  84  13   0   +
  3     383     8h    0s   10t   31f     25  ||   48 128  81  31   0   +
  4     380     2h    0s    4t   16f      7  ||   39 158  87  16   0   +
  5     419     4h    1s    7t   22f     15  ||   26 139 107  22   0   +
  6     316     1h    0s    1t    4f      7  ||   74 148  78   4   0   +
  7     331     0h    0s    6t    6f      7  ||   54 167  73   6   0   +
Decoded result is file: robot-vk3lm-x2hs.png
  0 bad blocks detected, out of   8 total blocks.


For data file: 3lm13-pm8a.wav
	using Hamming window and /tmp/work/Wyman1x-demod-decode program
Block inner  |      outer code      | outer   |       inner code
  #   code   |       erasures       | code    |         changes
      changes|1st   2nd   3rd   4th | changes |    0   1   2   3   4
----  ------ |---   ---   ---   --- |   ---   |  --- --- --- --- ---
  0     402     5h    0s    2t   22f     18  ||   37 144  96  22   0   +
  1     369     5h    0s    4t   13f     14  ||   39 160  85  13   0   +
  2     419     4h    0s    5t   17f     19  ||   22 148 110  17   0   +
  3     354     4h    0s    4t   19f     17  ||   44 173  62  19   0   +
  4     366     0h    0s    1t   20f      9  ||   64 136  85  20   0   +
  5     388    11h    0s    5t   34f     14  ||   59 108  89  34   0   +
  6     329     4h    0s    1t   13f      9  ||   70 146  72  13   0   +
  7     294     9h    0s    0t   18f     16  ||   93 132  54  18   0   +
Decoded result is file: robot-vk3lm-x2hs.png
  0 bad blocks detected, out of   8 total blocks.

  

The middle case, in which the receiver was operated in its non-linear range, had the lowest average number of error free symbols, 45.875 compared to 54.375 for the first case and to 53.5 for the third case.

Overall envelop of recorded signals

Following are plots showing the overall envelope of each of the three recordings, whose leader spectra are shown above.

At the beginning of the first envelope plot, there is evidence of amplitude changes, due to a 3 second version of the two tone signal transmitted just before the chirp signal and the phase modulated signal.

The plot below shows that the amplitude variations from a 3 second version of the two tone signal near the beginning of this recording were compressed so that all 3 levels came out the same. Non-linear operation removed the amplitude variation that was transmitted during the first three seconds of the signal.

This is the loudest of the three recordings of the same transmission, and it had the smallest average number of error free symbols. Loudness, at the expense of linearity is counterproductive.

Below is the plot showing the envelope of the third recording of this same transmission. This plot starts after the 3 second version of the two tone signal. It is the least loud, of the three recordings, and had about the same average number of error free symbols as the first recording above.