3 ms·

It might be useful to study the techniques that modems used to transmit data over phone lines. I seem to recall trellis coded modulation being used: https://e

It might be useful to study the techniques that modems used to transmit data over phone lines. I seem to recall trellis coded modulation being used: https://en.wikipedia.org/wiki/Trellis_coded_modulation https://en.wikipedia.org/wiki/Trellis_coded_modulation The acoustic channel is bound to suffer from multipath too, so some equalization may be needed too. https://en.wikipedia.org/wiki/Equalization_(communications) https://en.wikipedia.org/wiki/Equalization_(communications) https://www.ti.com/lit/an/spra140/spra140.pdf https://www.ti.com/lit/an/spra140/spra140.pdf In order to receive the signal far from the transmitter, some form of spread spectrum encoding could be used, like CDMA. The spreading factor could be negotiated. https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum https://en.wikipedia.org/wiki/Direct-sequence_spread_spectru...

cogman10 1y ago
Another step to look into if you really want to have fun is implementing some sort of QAM.
ghurtado 1y ago
Did somebody say Spectrum? https://softspectrum48.weebly.com/notes/tape-loading-routines https://softspectrum48.weebly.com/notes/tape-loading-routine... I always assumed that PWM was the go-to method for this kind of low bandwidth / high noise medium, I wonder why the author didn't go that route and used FM instead
- nomel 1y ago
  > Tape data is encoded as two 855 T-state pulses for binary zero, and two 1,710 T-state pulses for binary one. Is that not FM, more specifically FSK, just with some extra harmonics?
  - ghurtado 1y ago
    keep in mind I don't know much about waves, so all of this could be wrong, but I think PWM works by modulating the width of the pulse, in other words, the "duration of the note" if you will, so the frequency of the square wave remains constant. You have a high pulse of width t to represent a zero and a high pulse of t*2 to represent 1. The human ear might hear this as a modulating frequency, but that's only because the pulses are changing faster than our brains can recognize pitch, if that makes any sense . I don't know what a t state is, but I suspect the number is the duration of the pulse in microseconds or something of the sort. I believe IR remotes work on a similar principle: a series of blinks of two different durations, which represent 0 and 1
    - MobiusHorizons 1y ago
      What you are describing is basically CW which is how Morse code is transmitted over radio. It works well at the low data rates of Morse code. The higher the symbol rate, however, the higher resolution you need for measuring pulse length, which requires sharper edges from off to on. In practice it is usually easier to use different tones for each symbol (fsk) since the pulse length is less critical in that case. Quadrature modulation like bpsk is more efficient at the cost of additional complexity, and spread spectrum techniques allow for much better performance in noisy environments at the cost of using more bandwidth. So as usual in engineering, it’s a trade off that has to be decided on a case by case basis.
    - nomel 1y ago
      The spectrum of the shorter pulses would have a higher frequency peak show up, which would also a slightly reduced carrier (PWM frequency) peak. This is why I say it's kinda like FSK: the spectral energy somewhat shifts between two frequencies (well, really a bunch of harmonics). To decode it, with a simple setup, you could just look at the amplitude of the higher frequency peak relative to the carrier peak. In fact, you don't even need the carrier! You can just look for changes in the amplitude of the higher frequency peak.

cogman10 1y ago

Another step to look into if you really want to have fun is implementing some sort of QAM.

ghurtado 1y ago

Did somebody say Spectrum? https://softspectrum48.weebly.com/notes/tape-loading-routines https://softspectrum48.weebly.com/notes/tape-loading-routine... I always assumed that PWM was the go-to method for this kind of low bandwidth / high noise medium, I wonder why the author didn't go that route and used FM instead

nomel 1y ago
> Tape data is encoded as two 855 T-state pulses for binary zero, and two 1,710 T-state pulses for binary one. Is that not FM, more specifically FSK, just with some extra harmonics?
- ghurtado 1y ago
  keep in mind I don't know much about waves, so all of this could be wrong, but I think PWM works by modulating the width of the pulse, in other words, the "duration of the note" if you will, so the frequency of the square wave remains constant. You have a high pulse of width t to represent a zero and a high pulse of t*2 to represent 1. The human ear might hear this as a modulating frequency, but that's only because the pulses are changing faster than our brains can recognize pitch, if that makes any sense . I don't know what a t state is, but I suspect the number is the duration of the pulse in microseconds or something of the sort. I believe IR remotes work on a similar principle: a series of blinks of two different durations, which represent 0 and 1
  - MobiusHorizons 1y ago
    What you are describing is basically CW which is how Morse code is transmitted over radio. It works well at the low data rates of Morse code. The higher the symbol rate, however, the higher resolution you need for measuring pulse length, which requires sharper edges from off to on. In practice it is usually easier to use different tones for each symbol (fsk) since the pulse length is less critical in that case. Quadrature modulation like bpsk is more efficient at the cost of additional complexity, and spread spectrum techniques allow for much better performance in noisy environments at the cost of using more bandwidth. So as usual in engineering, it’s a trade off that has to be decided on a case by case basis.
  - nomel 1y ago
    The spectrum of the shorter pulses would have a higher frequency peak show up, which would also a slightly reduced carrier (PWM frequency) peak. This is why I say it's kinda like FSK: the spectral energy somewhat shifts between two frequencies (well, really a bunch of harmonics). To decode it, with a simple setup, you could just look at the amplitude of the higher frequency peak relative to the carrier peak. In fact, you don't even need the carrier! You can just look for changes in the amplitude of the higher frequency peak.