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Rf basics, digital vs analog info tx

  1. Jan 7, 2015 #1
    Sir, i need some basic information about analog vs digital signal/information transmission.
    i have few Questions and and idea to discuss weather its feasible or not.

    [Q1] can digital data requires same bandwidth as analog data contains as if i convert analog information to digital using DAC and transmit?

    [Q2] can i send same information in digital form on 100MHz carrier using FM (or other modulation types) as i send analog information on same carrier using FM.

    my idea is to transmit voice[0- 3.4kHz] signal on FM 100MHz carrier but in encrypting form so no one can hear this, by doing this i need to first convert voice to digital using ADC and then play some of my own cryptographic techniques and then send it over air in digital form,
    make me right if i goes wrong, that if i convert that digital data to analog and send it over air then may be some sort of high frequencies and hormones generated.
  2. jcsd
  3. Jan 7, 2015 #2


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    depends on the transmission baud rate -- the higher the baud rate the wider the bandwidth needed

    hormones are chemical things in the human etc bodies

    I think you mean harmonics ;)
  4. Jan 7, 2015 #3


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    You can certainly do what you are suggesting, and it is done routinely in communications. Digitize audio or video with ADC, (optionally compress), encrypt, TX, RX, decrypt, (optionally decompress), DAC to audio or video.

    There are a couple other issues to keep in mind. First, you cannot just start transmitting your own signal on 100MHz without a license. In the US, the government agency in charge of airwave frequency allocations is the FCC. You can apply for a license to transmit in certain bands, with limitations on TX power and signal bandwidth, harmonics, etc.

    Second, you should check to see if there are any restrictions on using encryption (and how strong the encryption is) for any particular band that you want to use in your country. In the US, I would think that sending encrypted data over the airwaves would attract the attention of the FBI and DHS. There are certainly lots of encrypted transmissions being sent routinely, but they are generally sent between government agency radios.
  5. Jan 7, 2015 #4
    Thanks, yes i suppose the bandwidth required to TX live audio stream might be 12bit date @ 8k, i dont know what bandwidth is utilized, question is is this possible with normal hardware, i mean not complex one.

    yes it was a typo mistake, sorry.

    Thanks, yes you are right the frequency i am suggesting to TX is licensed but what if i make it P2P using highly directional antenna.
    so both of issues may resolved if its possible, the frequency i am going to use is easily achievable so i'm not going for higher one,
  6. Jan 8, 2015 #5


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    highly directional antennas and 100MHz don't go together
    There is no antenna you can make for anything under at least 1GHz ( let alone 100MHz) that will not spread its signal over a really wide area
    and will be easily detected by the authorities and those legally licenced to use that frequency

    highly directional P2P requires microwave links

    speaking from personal experience here :)

    there are a world wide set of frequencies that are set aside for miscellaneous unlicensed, low power transmission, eg 315MHz, 433MHz, 2.4GHz
    These are called the ISM bands .... read up on them, and do things properly :)

  7. Mar 4, 2015 #6
    Q1 If you convert an analogue signal to digital using a DAC it will require more bandwidth. For instance, if you sample 3kHz audio at 6kHz and then use 16 bits per sample, you will require to transmit 96kb/s.
    Q2 If you simply modulate the carrier with both analogue and digital signals, they will interfere with one another.
  8. Mar 4, 2015 #7


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    I corrected a typo in your reply :smile:

    And in terms of bandwidth, it's easier to perform compression on digital data, so digital data may take less bandwidth in some cases...
  9. Mar 4, 2015 #8


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    Digital requiring higher bandwidth than analog is a rather presumptive statement. The beauty of digitizing an analogue signal is that the data does not need to be sent at the same rate the analogue signal would be naturally sent in real time assuming buffer sizes are adequate. This of course means that it can be sent in a very small bandwidth and also means that we will wait longer for the data to be transmitted. Without knowing whether or not this is acceptable in this case not much else can be said.
  10. Mar 5, 2015 #9
    My take on the question of digital bandwidth is that we can reduce the required bandwidth if there is carrier-to-noise margin to spare because we can use several levels for coding, such as 64QAM. Other techniques always lose something. For instance, buffering the transmission will increase latency. Digital compression by removing information we do not think the recipient will want does not, of course, transmit full information. The use of a vocoder, as with mobile phones, is limited to sounds for which TX-RX co-operation has been set up.
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