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| howto:hambasics:sections:digitalconcepts [2020/10/07 08:01] – ↷ Page moved from howto:hambasics:digitalconcepts to howto:hambasics:sections:digitalconcepts va7fi | howto:hambasics:sections:digitalconcepts [2020/10/07 08:30] (current) – ↷ Links adapted because of a move operation va7fi |
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| For example, here's how "VE7" (•••− • −−•••) could sound like if dots were tones of 600 Hz and dashes tones of 800 Hz (with some dead space in between each note for us to hear the breaks): | For example, here's how "VE7" (•••− • −−•••) could sound like if dots were tones of 600 Hz and dashes tones of 800 Hz (with some dead space in between each note for us to hear the breaks): |
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| {{ howto:hambasics:afsk_ve7.ogg }} | {{ howto:hambasics:sections:afsk_ve7.ogg }} |
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| {{ howto:hambasics:afsk_ve7.png }} | {{ howto:hambasics:sections:afsk_ve7.png }} |
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| The **second** thing we need to fix is that the number of 0s and 1s (number of bits) that a letter needs is different for different letters. For example: "V" has four bits (0001) where as "E" only has one (0) and "7" has five (11000). | The **second** thing we need to fix is that the number of 0s and 1s (number of bits) that a letter needs is different for different letters. For example: "V" has four bits (0001) where as "E" only has one (0) and "7" has five (11000). |
| If we download the audio file and open it in [[https://www.audacityteam.org/ |Audacity]], we can see the wave form visually: | If we download the audio file and open it in [[https://www.audacityteam.org/ |Audacity]], we can see the wave form visually: |
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| {{ howto:hambasics:afsk_sync.png }} | {{ howto:hambasics:sections:afsk_sync.png }} |
| The first half of the transmission contains the same pattern of one long wave (1200 Hz) followed by 12 short waves (2200 Hz) repeated 76 times. I separated these in red. Since each of these sections lasts 1/150th of a second and each note lasts 1/1200th of a second, each section contains 8 notes (separated here in yellow). | The first half of the transmission contains the same pattern of one long wave (1200 Hz) followed by 12 short waves (2200 Hz) repeated 76 times. I separated these in red. Since each of these sections lasts 1/150th of a second and each note lasts 1/1200th of a second, each section contains 8 notes (separated here in yellow). |
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| Let's take a look at the beginning of the message that follows the synchronization period: | Let's take a look at the beginning of the message that follows the synchronization period: |
| {{ howto:hambasics:afsk_data.png }} | {{ howto:hambasics:sections:afsk_data.png }} |
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| From here, there's a few ways to decide how we should encode the information: do we want each note to literally represent a 0 and a 1? or do we want the //change// in note to represent a change in bit? See this short but very instructive [[http://n1vg.net/packet/ |page]] for more details. | From here, there's a few ways to decide how we should encode the information: do we want each note to literally represent a 0 and a 1? or do we want the //change// in note to represent a change in bit? See this short but very instructive [[http://n1vg.net/packet/ |page]] for more details. |
| * All notes are transmitted back to back without spaces in between so a sync method is required. | * All notes are transmitted back to back without spaces in between so a sync method is required. |
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| In a sense, FSK is the digital analog of [[howto/hambasics/wavemodulation#fm |FM]], where as [[wp>Amplitude-shift_keying |ASK]] (Amplitude-Shift Keying), which we're going to skip here, would be the analog of [[howto/hambasics/wavemodulation#am |AM]]. | In a sense, FSK is the digital analog of [[howto:hambasics:sections:wavemodulation#fm|FM]], where as [[wp>Amplitude-shift_keying |ASK]] (Amplitude-Shift Keying), which we're going to skip here, would be the analog of [[howto:hambasics:sections:wavemodulation#am|AM]]. |
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| Now, there's a distinction we need to make that's not clear yet (but will be soon). There are two ways to describe the speed of the transmission: | Now, there's a distinction we need to make that's not clear yet (but will be soon). There are two ways to describe the speed of the transmission: |
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| In a nutshell, the phase of a wave describes where it starts. Let's take a look at an example: | In a nutshell, the phase of a wave describes where it starts. Let's take a look at an example: |
| {{ howto:hambasics:psk_phase.png }} | {{ howto:hambasics:sections:psk_phase.png }} |
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| Supposing we use the blue wave as a reference wave, the green wave is 90° out of phase, while the red wave is 180° out of phase. Notice that all three waves have the same frequency (the number of cycles per second) and the same amplitude (the height of the wave), but they start at different points. To the human ear, all three notes would sound exactly the same, but a computer can be made to detect the difference, which means we can use two of those waves to represent different bits. | Supposing we use the blue wave as a reference wave, the green wave is 90° out of phase, while the red wave is 180° out of phase. Notice that all three waves have the same frequency (the number of cycles per second) and the same amplitude (the height of the wave), but they start at different points. To the human ear, all three notes would sound exactly the same, but a computer can be made to detect the difference, which means we can use two of those waves to represent different bits. |
| ===== Constellation Diagram ===== | ===== Constellation Diagram ===== |
| {{ howto:hambasics:bpsk_gray_coded.png?200|}} | {{ howto:hambasics:sections:bpsk_gray_coded.png?200|}} |
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| A useful way of representing the different "states" that the waves can be in is using a constellation diagram.((BPSK Constellation diagram was modified from [[https://en.wikipedia.org/wiki/File:BPSK_Gray_Coded.svg |Wikipedia]])) For example, using the Blue and Red waves to encode a message, the constellation diagram would have the two dots on the horizontal axis (one on the right at (1,0) and one on the left at (-1,0)). | A useful way of representing the different "states" that the waves can be in is using a constellation diagram.((BPSK Constellation diagram was modified from [[https://en.wikipedia.org/wiki/File:BPSK_Gray_Coded.svg |Wikipedia]])) For example, using the Blue and Red waves to encode a message, the constellation diagram would have the two dots on the horizontal axis (one on the right at (1,0) and one on the left at (-1,0)). |
