Analog sound over FM frequency?

Wireless & IoT
1

Hi everybody.

I am at the moment working on a little project, where I sent sensor data wireless to my notebook with the help of an Xbee. However, I also want to sent audio back to the location where the sensors are measuring, which is about 10 to 20 meters from my notebook. The audio is generated by my notebook with the help of Max/MSP and right now it is just played over the notebook speakers. I already know that the XBee will not be suited for sending the audio back.

So I figured the easiest way would be to connect a transmitter to my 3.5 jack on my notebook. The question is how and what? FM frequency sounds as the most logical option. I could just use a FM transmitter board, or even an iPod FM transmitter. Does this make any sense?

On the receiving side however, things are a bit more complicated. The model that houses the sensors and Xbee is fixed in size, and not too big. The question I thus have is: what would suit my needs best? Could I receive the FM frequency with the (http://www.sparkfun.com/commerce/produc … ts_id=8770) and just connect a little amplified speaker? How do I set the right frequency then?

And what is the difference with http://www.sparkfun.com/commerce/produc … ts_id=8972?

I just need analog audio, no controls or other data, just audio. Small samples of 2 seconds at the time.

Please, if somebody could help me out I will be forever grateful! :smiley:

2

cmegens:
I already know that the XBee will not be suited for sending the audio back.

Xbee could send a digital data stream, that is

decoded to audio (analog) by a CODEC. Possibly

utilizing an MP3 decoder chip. The data would also

be required at the sending end (your notebook?).

cmegens:
So I figured the easiest way would be to connect a transmitter to my 3.5 jack on my notebook. The question is how and what? FM frequency sounds as the most logical option. I could just use a FM transmitter board, or even an iPod FM transmitter. Does this make any sense?

Agreed. The iPod FM transmitters are very

similar to the IC used in the Sparkfun radio modules,

but have the burden (in your case) of iPod stuff

you don’t need.

cmegens:
The question I thus have is: what would suit my needs best? Could I receive the FM frequency with the … and just connect a little amplified speaker? How do I set the right frequency then?

And what is the difference with …

Either module would work. WRL-08972 is a breakout board

(designed by SFE) to help evaluate the IC, but could be

made part of your final design. WRL-08770 is the same IC

in a more project friendly format. Through this bus the

radio is tuned to a particular channel in the broadcast FM

band. Any FM broadcast radio would also pick up your

signal, so you could look for a novelty FM radio as an

alternative to developing one from these modules.

Either module will require some set up over the serial

bus, and therefore you need a uC or add a function to

your existing uC (assuming one is present at the remote

end). It might make sense to add a self-contained

transmitter at the sending end of your system, to avoid

development of a new application to run on your NB.

3

Xbee could send a digital data stream, that is

decoded to audio (analog) by a CODEC. Possibly

utilizing an MP3 decoder chip. The data would also

be required at the sending end (your notebook?).

Ehm yes, that sounds possible. To me however this sounds a bit more complex than just sending an analog signal, which is why I didn't look into this. Besides, the mp3 decoders are bigger than the WRL-08770, which is why it caught my interest. And then there is the case of a constant stream of data already taking up bandwith on the Xbee..

Agreed. The iPod FM transmitters are very

similar to the IC used in the Sparkfun radio modules,

but have the burden (in your case) of iPod stuff

you don’t need.

Not really a problem. I have a FM transmitter with a 3.5 jack, on which I can select the transmitting frequency.

Either module would work. WRL-08972 is a breakout board

(designed by SFE) to help evaluate the IC, but could be

made part of your final design. WRL-08770 is the same IC

in a more project friendly format. Through this bus the

radio is tuned to a particular channel in the broadcast FM

band. Any FM broadcast radio would also pick up your

signal, so you could look for a novelty FM radio as an

alternative to developing one from these modules.

That’s clear. I know i will be transmitting a radio signal that can be picked up, but for this purpose that is ok. What do you mean by novelty FM radio? Just an existing and working FM radio? Yeah figured that too, but it doesn’t suit my size criteria, since I need to add a amplifier and speaker. Besides, the WRL-08770 also runs on 3.3v, which is exactly the same as the Xbee and my sensors. This might not be the best reason, but it works for me :D.

Either module will require some set up over the serial

bus, and therefore you need a uC or add a function to

your existing uC (assuming one is present at the remote

end). It might make sense to add a self-contained

transmitter at the sending end of your system, to avoid

development of a new application to run on your NB.

Now you lost me. With uC you mean microcontroller? isn’t there any way to lock the receiver into a certain band, like 88.3 MHz? If not, is it possible to do this with the Xbee on initial setup, or does it need a constant information flow? There is no microcontroller present at the sensoring part, just the Xbee that is sending raw sensor data to an Xbee connected to my notebook, which interfaces with Max/MSP.

Sorry I’m asking all these newbie like questions!

[/quote]

4

cmegens:
To me however this sounds a bit more complex than just sending an analog signal, which is why I didn’t look into this. Besides, the mp3 decoders are bigger than the WRL-08770, which is why it caught my interest. And then there is the case of a constant stream of data already taking up bandwith on the Xbee.

An all analog receiver and amplifier is your easiest

solution. Canabalize an old FM radio. If you send data

over the Xbee it would not be all the time, as you

indicated you only have brief audio messages. Or,

you could save the messages in EEPROM or an SD

card and just send a trigger (with message number).

cmegens:
I know i will be transmitting a radio signal that can be picked up, but for this purpose that is ok.

I wasn't indicating security, only that you can

use any ‘found’ radio to complete your project.

Try looking in a thift store.

cmegens:
What do you mean by novelty FM radio? Just an existing and working FM radio?

Yes.

cmegens:
Yeah figured that too, but it doesn’t suit my size criteria, since I need to add a amplifier and speaker. Besides, the WRL-08770 also runs on 3.3v, which is exactly the same as the Xbee and my sensors. This might not be the best reason, but it works for me.

Okay, but 3.3V isn't really enough to drive a speaker.

You can salvage a small size FM radio from many things.

I had one in a pedometer. That’s a novelty radio.

cmegens:
Now you lost me. With uC you mean microcontroller?

Yes.

cmegens:
isn’t there any way to lock the receiver into a certain band, like 88.3 MHz?

Doubtful. You need to get the Ar1010 datasheet. The radio

module has a control pin for I2C, and three pins for some

form of serial bus (not just I2C format).

cmegens:
If not, is it possible to do this with the Xbee on initial setup, or does it need a constant information flow? There is no microcontroller present at the sensoring part

I don't know much about Xbee modules. To config the

FM radio transmitter you might only need a POR (Power

On Reset) and sent a configure message of a few bytes.

The easy way is to pair your FM Tx with an 8 pin uC.