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Transmitting sound through radio digitally

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Integrate

New Member
Hello. I'm new here, and I want to do a little project for fun but I'm not too experienced with some of the components of the project. I was hoping that I could get some help/suggestions on what to do:

I want to make a walkie-talkie like device. I want a transmitter that will take AtoD signals from a microphone + amplifier into a pic (18f1320), which I plan to sample at 8000 times a second. Then, the pic will send the AtoD data serially through a RF transmitter.

The reciever will have a RF reciever, which will be connected to a pic. It will then use a R-2R resistor ladder to create an analogue signal, which will be sent to an amplifier and speaker.

Now I have a few problems that I need to work out. The first of which is speed. I've browsed around on some past posts and other places online and I found that a AtoD sample rate of 8K is phone quality sound. I do not know if this is true or not, so I need some confirmation on that. Also, what would it take to do mp3 quality sound?

I also need some sort of RF transmitter and reciever. I have no knowledge about how to make these, I tried this one once for another project and it didn't work at all for me:
RF Remote Controller
I also have some RF transmitters and receivers from sparkfun that can do 4800bps, but that is not fast enough to transmit 8K*10bits per second. So I need a module that can do 80K bps. I don't know of these things exist or not, but I'm hoping that they do.

The amplifying parts I think I would be able to do. I have a little experience with opamps, and I can just use a capacitor as my low pass filter.

I was hoping to keep the digital part of the project since I can apply it to something else I'm working on.

Thanks for your time!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
A number of points here:

1) You need to sample at least 2.5 times as fast as the highest frequency.

2) You must filter the input to remove frequencies below that.

3) Use 8 bit mode instead of 10 bit, it's fine for the audio quality you're using.

But regardless, you're talking HUGE quantities of data, for only low quality.
 

MikeMl

Well-Known Member
Most Helpful Member
Ham radio enthusiasts do this all the time; its called Packet Radio or APRS (Google it).
A modem-like device is connected between a digital source and a transmitter, and then to the receiver to recover the data stream. However, the baud rate achievable with reasonable signal (and IF) bandwidths and without using special broad-band radios is only 1200 baud. Building a high-speed modem into a cheap radio is a major undertaking...
 
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Integrate

New Member
Thanks for the help. I guess I'm going to have to change my plans. I was hoping that high baud radios would be within my reach, bah.
 

unclejed613

Well-Known Member
Most Helpful Member
there's also a ham mode called DRM (not to be confused with Digital Rights Management) that uses OFDM modulation to transmit sound files and picture files. it's probably along the lines of what you were thinking. the only problem with sending digital data over the air is that at some point, that data exists as one or more analog signals, and it's the modulation scheme that is the most important part of the puzzle. the data stream requires quite a bit of bandwidth, since it's aomething like 64 carrier waves spaced about 30hz apart, for close to a 2khz bandwidth, and that's SSB, not AM or DSB (which are simpler to implement).
 
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