RF project, designing 433MHz digital low power walkie-talkie

Basically my degree is telecomms and computing. i have covered most topics but of late the degree has been on computing. i find that i forget what i have learnt very easily. i am reading my notes on modulation but u see the degree is alot of theory with little practical work. so allthough i know about dsp and modulation i don`t know how to put it in practice.

been doing alot of searching on the web and come up trumps with u guys. now back to business

CVSD, these i feel are similar to vocoders in the way they are ment purely for speach/music and so are more efficent. but looking on the CVSD tutorial there is a blosck diagram that makes this a great device but for my application(i will get marked on this)is that it simplifies everything too much. i think i will go for the option of programming a pic with a standard code(manchester code) this way ill get a better understanding of the project

DSP probs
been reading up in my books but can`t find anything about the application of code. know all about what the different codes do but i need to be able to program this onto a pic?? looked for hours. please help me.

any advice on the programming of code would be greatful

If you take another look at the link I posted about the RS232 link you will see code for a PIC doing manchester encoding/decoding.

Or you could get rid of the PIC and do it with logic. Or you may be able to get a manchester encoder/decoder IC but this would make it too easy :smile:

Hi,steviemidnight
There is already mono-chip that can realise all the function of CVSD ov PCM and FSK modulation at 2.4 GHz,
so U donot need to know how to realise it .
what U need is just to use it.

ok i saw the programming but did not understand it? know of any tutorials that could give me an understanding of what to do.

his program runs off the PIC 12C509 which i have used before(chipping my c12 phone and PS2) but as i don`t understand his code and the fact that his transmitted data comes from the serial port i beleive that his code will not work for me. i understand that i will need to amend this but i don`t know how to do this. the PIC 12C509 i used have all been programmed in hex(which i did not program) this program the guy has written is not in hex so as i understand it it cannot be programmed onto a pic in its current form.

my walkie talkies will be mobile devices. the serial port in gives the infomation like a ADC would in 1s and 0s then his pic encodes it with manchester code.

i have emailed the guy so ill see what he says

On 2002-02-18 01:35, yellowtooth wrote:
Hi,steviemidnight
There is already mono-chip that can realise all the function of CVSD ov PCM and FSK modulation at 2.4 GHz,
so U donot need to know how to realise it .
what U need is just to use it.

Click to expand...

Which 2.4GHz chip did you use ??
Thanks

I don't know if this can be helpful but in the past Motorola have done two ic for CVSD.
They were both codec for 3 and 4 bits but I didn't remember p/n, they was MC-something...
If you need it I'll try found p/n.

Let me know...

Syzzler

Lanwve compay.
Microlinear company

can`t find the Lanwve company??

Also stilll none the wiser about programming dsp pics. im off to ask a DSP lecturer in an hour so ill tell you all what he says.

looked on the motorola site and found: which has some good info and as third party code. i just don`t understand the code.

http://dspvillage.ti.com/docs/dspvillagehome.jhtml

this is what my DSP lecturer had to say:

Steve, Hi.
Anti-alias is a good low-pass filter at 4kHz. Chebyshev has quite a steep
roll-off; look in "The Art of Electronics" for designs.
The PIC should have an ADC on board. All you have to do then is to take the
8 data bits and send them out serially using say the RS232 convention. The
PIC will probably do this for you. At the receiver, put the data stream into
a UART and you should get your 8-bit data back out in parallel. DAC it and
put it into a loudspeaker. You shouldn't need a specialist DSP chip.

Apparently there's plenty of PIC support free on the web. I don't know the
sites but use Google and some PIC keywords and you should find it.
Sorry I was late back
Chris

So im off now to find some code. hopefully something will come up. seems like everyones bored of my problems but thanks for your help any way

You've almost gone back to square one and it won't work. RS232 was designed to be sent down wires and not through the air; you can't just transmit it as it is!

Take a look at these pages:
**broken link removed**
http://www.piclist.com/techref/io/manchester.htm

But if you add start and stop bits and then manchester encode your data you will end up with data too fast for the tx/rx.

So, use the CVSD chips!

hum... this may explain the problems i am having wwith wireless rs232

i know im going round in circles, but if been doing that for 3 months. the more i go round the more i learn.

i have printed out the articles and read them all, i have increased my knowledge alot but im still having problems as you can tell by the number of replys by me. all the code i have managed to find is for interfaces with a pc not for an embedded pic on a standalone circuit. i know i need to send a serial line of bits through the Tx.

im really getting frustrated( likewise i can imaging)with my lack of understanding. im trying to get a grips with the theory which i belive i have now thanks to you guys. but i also am trying to understand code written in hex/c and asembly code. when programming is deffinitly not my strong point. i know i need to packetize my data and add a stop byte, but how? if u think that u have given me all that you can, then please tell me so. i know that i must be boring everyone so feel free to pass me on to other electronics forums( as i keep getting told its just simply connect this do that then ??? etc - and i don`t understand, am i stupid for not finding this easy??)

has any one got the full MPlab software? i just don`t want to download all 8 1.4Mb files!!

the anti-alising filter will be a 5th order Chebyshev low pass filter.

sorry if this gets taken the wrong way, but im just tired and got too many things to do and my brain hurts :smile:

ok im going back to basics.
design plan.
i will first get speech sent via a RS232 wire link to a speaker. using a ADC pic and a DAC pic.

then i will encode some kind of error checking and parity stop bits. before finally adding the FM modules.

i will be researching the pic family for pic with a onboard ADC and one with a DAC. the ADC needs to have serial output which i can then simply encode with a small program. the DAC needs to be serial in with a anologue out.

once i find the chips i post them up/ in the mean time please post up your suggestions