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Topic: "Digitizer" for two-way handheld radio (Read 2265 times) previous topic - next topic

EYYE

Oct 14, 2015, 11:44 am Last Edit: Oct 14, 2015, 11:45 am by EYYE
Hi,
I'm not sure where to ask. Maybe I should put this question into "Networking, Protocols, and Devices" category, in that case, please move it there.
Let's start with my problem. What I have are some two-way handheld radio walkie-talkies (such as the Baofeng UV5R). Those are using analog communication. Taking advantages of digital communication into account made me came up with the idea of "digitizing" the signal. I want simply make a device which will be connected between headset and radio. This device will convert my voice via ADC to radio. That radio will transmit digital information. On other end, second radio will receive this signal, convert it via DAC to analog signal into speaker. And I want to controll it with arduino (for posibillity of sending gps coordinates, or other no-voice data, and to recognize different types of data received). Can someone please help me to find the easiest way to archieve it? What problems may occur (without law problems)?

DVDdoug

#1
Oct 14, 2015, 07:19 pm Last Edit: Oct 14, 2015, 07:21 pm by DVDdoug
That's theoretically possible but probably impractical...

What you need in addition to ADCs & DACs on each end are modems to convert the digital data into a sound-signal that the analog radio can handle.   

If you look at the picture in that Wikipedia link you'll see an original acoustic modem that not only converted to/from a sound signal  but actual sound.*

The transmitted sound is digital data (beeps representing ones & zeros) so at the receiving-end it has to go through another modem and then through a DAC to get the original sound back.

And although there's no more information  in the digital data than there was in the original analog sound, you'll probably need some data compression to get the same end-to-end bandwidth.





* There was a time when it was illegal to plug anything to the phone line that wasn't supplied or approved by the phone company, so it was done without any electrical connection.    There were answering machines that mechanically lifted the receiver off the hook to answer the phone!!!! 

EYYE

Yes, it's impractical. But I have a group of people with analog radios like Baofeng UV5R, who need to communicate in very bad terrain. Unfourtunatelly, we are all on low budget, so I'm trying to find alternative path then simply buying new radio stations :) Thank you for your reply.

jremington

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But I have a group of people with analog radios like Baofeng UV5R, who need to communicate in very bad terrain.
How do you think "digitizing" voice would help in bad terrain?

It is possible to send text messages via voice radios using two tones for the 1s and 0s, for example using 300 baud or 1200 baud modems. Look up APRS for examples from amateur radio, and see this device, which sends audio encoded GPS coordinates.

EYYE

I think it will help improve the quality of communication. In that location, full of hills, digital signal have much lower possibilitty of losing its information than analog device, right?

Grumpy_Mike

I think it will help improve the quality of communication. In that location, full of hills, digital signal have much lower possibilitty of losing its information than analog device, right?
No wrong.

EYYE

Well, why? When I'm sending two tones with 0 or 1 meaning, it's just a digital information. Frequency modulated signal transporting analog information must be more sensitive to deformation of signal. Why do you think it will not help?

Grumpy_Mike

I know it will not help or in fact even work. If you are modulating tones then the digital data rate is limited by the frequency of those tones. Using an analogue demodulator you will have to have at least one cycle of that tone to represent each binary state. This limits the data rate you can send. At a guess I would be surprised if you could get a data rate of 300 baud.

But even worse while a corrupted FM signal can be partly demodulates by the signal processing power between your ears with a digital signal you have no chance of recovery once an error has occurred.

I used to design digital TV set top boxes and know about the modulation techniques used. There is a lot of complex operations to ensure packet integrity, even so digital TV signals will suffer from reflection problems that stop it working when over the same path an analogue TV is visually perfect.

I am also a radio ham and know about propergation.

EYYE

Oh well, you mean Nyquist-Shannon sampling theorem. OK, that's right. Thanks for bringing some light in the problem :)

Grumpy_Mike

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Oh well, you mean Nyquist-Shannon sampling theorem.
Well that is why it will not work.

It does not explain your incorrect impression of digital modulation being better than FM.

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When I'm sending two tones with 0 or 1 meaning, it's just a digital information.
If you apply tones to the input of a voice FM transmitter you will produce AFSK - Audio Frequency Shift Keying, not FSK - Frequency Shift Keying.
FM is the best at coping with multi path reception. With AM - Amplitude Modulation multi path can produce fading by alternating constructive and destructive reinforcement. Where as FM will lock on to the strongest signal and ignore the others. If you do get problems with FM it will because of low signal levels which is why you are having problems. Also with AM you are only transmitting the maxim power on the peaks of the speech where as FM transmits maximum power all the time.

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