sending digital signals over radio

He might want to add error-correction/detection - there are chips around that will add a five bit checksum to the eight-bit data, which will allow you to correct all single-bit errors, and detect many multi-bit errors.

There is also the approach used in the Taxi chips, where eight-bt data is recoded into the 256 ten-bit words that have lowrst DC content .....

------------- Bill Sloman, Nijmegen

Hmm..8 bits..ASCII..Baudot..RS232 ports...sounds simple to me.

What range / frequency / data rate do you need? You can also get off-the-shelf RF modules for short range serial data links.

You could use a UART (Universal Asynchronous Receiver/Transmitter). This chip does it for you. They're used in RS232 serial ports. They work by adding a "start bit" to the front of your data. From the middle of the start bit, the receiver counts 8 bit periods to find the centre of each data bit. Micro-controllers (e.g. PIC) often have a UART built-in.

I'd use a UART at the transmitting end (FIFO if you have bursts of data) - parallel in, formats the data into a serial stream, output serial stream (+5/0V) can modulate a carrier and be transmitted, demod at other end to recover the formatted serial stream from the received carrier, and straight into another UART to recover as 8 parallel bits.

Most UARTS should handle the upper limit of digital speed that a modulated signal could cope with.

Chris.

Yo,

Check out the units by Radiotronix. Just key the transmitter with serial output and connect the reciever to a serial input. I have used these with PIC's all the time.

Or you could just send it all twice (normal then inverted) and detect virtually all errors.

......

The method I mentioned above also helps to remove the DC especially if you interlace the normal and inverted data bit by bit.

If you plan to use "dumb" RF modules like FSK transmitters/receivers then an UART will not be enough. You need :

- Some kind of preable sequence to let the receiver set its AGC

- Some kind of "start of frame" header (these two point may not be necessary if you have a continuous transmission, but this is often not autorized by regulatory agencies... at least in Europe)

- Some way to balance the signal (as many "0" as "1"), and some way to minimise the number of successive 0 or 1, if not the receiver will fail to decode the signal. The "inverted/interleaved bit" mechanism proposed by Anthony is a simple solution, but divide the throughput by two and may make the identification of start bits more difficult. More robust solutions exists (NRZ, etc).

- And very probably some error correction firmware.

As a starting point you may have a look at application notes published by Chipcon

Of course everything is dependant on your requirements : how many errors can you tolerate, what is the distance and power budget, what is the throughput, etc.

Friendly yours,

LOL

Check out a copy of the ARRL Radio Amateurs Handbook from your library, and look for Digital Modes. It's all been done. Basically, you transmit tones in the voice bands. Simplest hardware consists of a PC sound card; lots of software available for that.

Tam

No you don't need a shift register or IMO any flashy digital stuff to send it by radio. I don't think you have in mind digital radio because thats way too complicated. For normal analog radio here goes: A trick you can use on the transmission side to reduce the chances of detection errors on the receiver side is passing the binary bits through a high pass filter. If you observe the waveforms on a scope you'll see why ... The peaks on the outputs of your binary signals will correspond to the 1's and the dips to the 0's but the peak to peak voltage will be about double.

You don't seem to have any problems on the transmission side but if you do we can go into that. So ..... Use a normal AM/FM transmitter to get your high pass filtered digital signals transmitted

On the receiver get the demodulated amplified signal connect it to a low pass filter with gain connected to 2 opamp buffers in parallel then get the output connected to a flip flop like the 4093 and voilà your digital signal.

The circuit is a bit more complicated then that with some math to do but If you need more details let me know.