Difference between revisions of "Ka3jjz"
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| − | + | ==How do I decode the signal when the center frequency drops to a very low level (i.e. 65 hz)?== | |
| + | * Most soundcards won't go down below 200 hz or so. To get around this, you must use a sideband capable radio or a SDR. Listen in either USB (preferable) or LSB. On LSB you must tune 1 kHz higher. On USB you must tune 1 khz lower. Because AM signals have two sidebands, the 65 Hz signal then becomes 1065 Hz and 935Hz, and the AM carrier becomes 1000 Hz (which is the amount you offset your tuning), which you will also hear. So you will actually get three signals, two of which are useful for decoding. But the 935 Hz tones will be REVERSED, so use the 1065 Hz tone or click the "Rv" button on Fldigi if using 935 Hz. | ||
| + | ** The problem is mostly due to the IF and demodulator filtering. Many SSB receivers have a low passband edge of only 300Hz, and some AM receivers have a highpass of 150Hz. This applies during demodulation (like when making an audio recording or performing a live decode, or when demodulating an IQ recording to audio for decoding.) | ||
| + | ** Another issue that can impact decoding under these conditions is if the transmitting station's low frequency audio response cuts off above the frequency you are trying to tune. This would effectively prevent any possibility of decoding. | ||
Revision as of 20:37, 30 August 2019
How do I decode the signal when the center frequency drops to a very low level (i.e. 65 hz)?
- Most soundcards won't go down below 200 hz or so. To get around this, you must use a sideband capable radio or a SDR. Listen in either USB (preferable) or LSB. On LSB you must tune 1 kHz higher. On USB you must tune 1 khz lower. Because AM signals have two sidebands, the 65 Hz signal then becomes 1065 Hz and 935Hz, and the AM carrier becomes 1000 Hz (which is the amount you offset your tuning), which you will also hear. So you will actually get three signals, two of which are useful for decoding. But the 935 Hz tones will be REVERSED, so use the 1065 Hz tone or click the "Rv" button on Fldigi if using 935 Hz.
- The problem is mostly due to the IF and demodulator filtering. Many SSB receivers have a low passband edge of only 300Hz, and some AM receivers have a highpass of 150Hz. This applies during demodulation (like when making an audio recording or performing a live decode, or when demodulating an IQ recording to audio for decoding.)
- Another issue that can impact decoding under these conditions is if the transmitting station's low frequency audio response cuts off above the frequency you are trying to tune. This would effectively prevent any possibility of decoding.