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.
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.
The next three plots show the spectrum of the (unmodulated) 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 8 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 8 frequencies, thus, the receiver and/or sound card were being operated in the non-linear range.
In the plot below, note the spike at 230 Hz (the spacing between adjacent subcarriers) due to intermodulation distortion. Also, note that the three spikes above 2200 Hz are spaced at 230 Hz intervals, and thus are not due to random noise. The smaller spikes near: 2050 Hz, 1820 Hz, and 1590 Hz continue this 230 Hz spacing.
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.
The middle case, in which the receiver was operated in its non-linear range, had the most errors, and used up the largest fraction of the error correcting capability encoded in the transmission.
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.
This plot 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 largest number of errors to correct. Loudness, at the expense of linearity is counterproductive.
Below is the plot showing the envelope of the third recording of this same transmission. It shows proper amplitude drops due to the 3 second version of the two tone test signal near the beginning. It is the least loud, of the three recordings, and it had the fewest errors of the three recordings.
Keeping everything within its linear range of operation yielded the best results for this transmission.
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