Advice on: Duplex Walkie Talkie / Interphone

Wireless & IoT
1

Hello,

I’m attempting to start on a project to build a helmet mounted Walkie system that approximates the functionality of these motorcycle rider-to-rider intercom systems. http://www.revzilla.com/interphone.

The intended use is for communicating while rock climbing without having to yell at my partner 100m below/above on a rock face.

The basis requirements of the project:

  • - Outdoor Range of at least 100m. 400-500m ideally.
  • - Continuous voice from all parties, with the ability to hear while transmitting (like a conference call, not like a PTT Radio)
  • - Two people would be great, but the ability to have 3-4 in one "conference room" would be terrific.
  • - A few hours of talk time

    • Based on my limited understanding of the project right now, I see the following fork in the road:

    1. Go analog transmission, essentially attempting to build a VHF Radio. Since this technology can’t duplex, I’d have to use either VOX instead of a PTT button, or by oscilating between transmit and receive fast enough that the user perceives it as duplex (like cellphones used to fake full duplex with Time Division multiplexing). This can be done either with i) a HAMshield, ii) hacking an arduino to a radio module to act as the oscilator, or iii) attempting to build a VHF circuit from scratch.

    2. Digital transmission. This means an analog to digital module on either end to compress the audio, shoot it over the air, and another DAC decompresses. This seems to be much cheaper to implement, with two options that seem attractive to me: i) Xbee+FioV3 because of the great range and excelent documentation, and ii) A Moteino,http://lowpowerlab.com/moteino/ an arduino clone with integrated RF module that seems to achieve excelent range with a much lower cost than the Xbee system.

    My Question:

    What reason would drive me to chose between analog / digital?

    At the moment, both approaches seem to promise similar range, and a similar difficulty of implementation. A hamshield seems to be the simplest solution, but the cost at $100/ piece is unnactractive. Based on my limited understanding or RF, I’m inclined to chose the Moteino board and go from there. It seems robust enough, has decent range and reasonable cost.

    Will any digital transmission system have enough bandwidth for voice?

    My research found that for voice with compression, you can get by with 1kbps. Distortion isn’t a huge problem as long as you can make out the words.

    What else do I need to consider?

    Thanks for your time! I’m looking forward to your replies. :slight_smile:

    2

    Analog - vastly simpler.

    Voice Operated Transmission (VOX) allows transmitter to be keyed only when there is speech.

    A good VOX is the hardest part and likely justifies buy don’t make.

    I’m assuming this is wireless for inter-motorcycle rather than intra-motorocycle in which case VOX would just mute the mic.

    Road / wind noise is the devil in this.

    Don’t do digital. And you’d need about 8KHz of bandwidth for 4KHz voice.