Cheap and Simple Audio Transmission

[wildgoose]

Greetings all,

I'm trying to put together a system where I can talk to my house. I'm aiming for a microphone in each room so I can give voice commands to the house such as "dim the lights" or "turn the TV on". I'm fine with the speech recognition side of things and I've cobbled together a simple test system using a small FM transmitter and an FM radio connected to the mic in on the soundcard of my PC.

What I'd like to do now is build some transmitter modules for each room. The FM I'm using is clearly illegal and rather too close on the dial to Radio 2! So what I'm after is a cheap, simple and legal transmitter and receiver. So far my googling has come up with modules like this **broken link removed**
which looks great except it seems to be designed to transmit data rather than audio. Does anyone know of something similar for audio/analogue? The simpler it is the better. Ideally connections for mic, aerial and power and that's it.

WG.

 

[Nigel Goodwin]

So far my googling has come up with modules like this **broken link removed**
which looks great except it seems to be designed to transmit data rather than audio. Does anyone know of something similar for audio/analogue? The simpler it is the better. Ideally connections for mic, aerial and power and that's it.

It's licence free data only, audio is specifically banned.

 

[n.house]

Talking to your house!!!!!!!!!!!!!!

**broken link removed**

This may be helpful I can tell you about tuning, but I think this is legal. But its on eBay @ 99p and only 3hours left

 

[audioguru]

I have heard about crazy people who talk to themselves, and now here is somebody that wants to talk to his house!

Does the stick-on label from E-Bay "filter noise" or "boost" the transmission of a cell phone? I doubt it.

 

[JimB]

**broken link removed**

This may be helpful I can tell you about tuning, but I think this is legal. But its on eBay @ 99p and only 3hours left

I am not sure whether to shake my head in amazement, or just laugh until my bits drop off!

JimB

 

[philba]

In the US, there is a carve-out for <10 mW radiated power. Don't know about the UK, though.

Is FRS available in the UK? it runs in my mind it is. I bet thouse could be adapted. In the US, they are pretty cheap.

 

[Nigel Goodwin]

In the US, there is a carve-out for <10 mW radiated power. Don't know about the UK, though.

No, any unlicenced transmitter is illegal, except for the specific unlicenced radio modules of course, which have to be approved for use. There have been rumours recently that it may change though, in order to make the little IPod transmitters legal - currently people illegally import them from outside the UK.

Is FRS available in the UK? it runs in my mind it is. I bet thouse could be adapted. In the US, they are pretty cheap.

There are some cheap low power walkie-talkie units available (not called FRS here though), which again are licence free - and available pretty cheaply!

 

[philba]

yeah, I think they use the family radio service band (frs).

 

[wildgoose]

So, not so easy then.

Well thanks for the (rather disappointing) info about the legality of this and hence availability of easy to use modules.

Anyone got any ideas how to get fairly low quality mono audio from multiple rooms to one central point? Running new wiring is not really an option.

I hear that transmission over existing mains cabling is dreadful, plus I'd probably kill myself.

How about sticking a cheap ADC on the front of a radio module designed to send data and then a DAC at the receiving end?

Oh yeah, to some above, I feel I should point out I'm not planning on having long conversations with my house. Just issuing a few simple commands like:

"Lounge lights On"
"Coffee On"

or

"Open the bay doors Hal"

 

[JimB]

The cheap walkie talkies which you see for sale everywhere may be a solution for you. They are legal, licence free, but be aware that some local comedian may record your voice and replay it at odd times, "bedroom lights ON" at 03:15hrs may not seem like a joke at the time!

The radios are generically referred to as PMR446.
PMR = Private Mobile Radio (If I remember correctly)
446 = The frequency, 446Mhz

There are eight channels and 38 CTCSS codes.

Google PMR446 to get a load of information.

JimB

 

[wildgoose]

Hi Jim,

Walkie talkies are a good idea, thanks. I found a pair for £15 from Maplin which I may have a play with.

I take your point about other people being be able to tune in and while I have to admit that I lead the kind of life where anyone eavesdropping would probably die of boredom, I'd not considered they might be so sneaky as to record the house commands and re-transmit them. I may have to omit the "Unlock Front Door" and "Open the Safe" commands.

I'm still thinking that a DAC coupled with a data radio module might work and would also get round the eavesdropping problem. I think I’ll have a play with that first but if it doesn’t work I’ve got the walkie talkies to fall back on. “House, turn the lights on, over”

 

[JimB]

It occurs to me that the voice recognition system seems a bit of overkill when you have to pick up the radio and talk to it. You could use VOX, (where the radio switches to transmit when it hears sound), but then it would transmit every sound in the house, not a good idea!

Why not create a keypad which sends tones via the radio to do the switching? Again, someone could record and playback the tones, but at least they would have no idea what it was for, and so would not bother.

JimB

 

[fingaz]

Just a thought. . .

If you use your pc to 'act upon' the voice commands, maybe you could use bluetooth? It's secure because you have to pair devices and it's reasonably cheap. A bluetooth dongle for the pc is only about £10 and bluetooth headsets are not much more.

Just a random thought. . . I seem to have a lot of those recently, don't we?

 

[Sceadwian]

I'm not sure using walkie talkies for this application is legal, as the FCC limits the types of communication that can occur on the band not just the devices. So human to human usage is okay, but human to PC is probably technically illegal. It's considered a remote control application and covered under a different ruleset. The unlicensed bands (Wifi and Bluetooth) would likley have to be used to make it fully legal. A bluetooth setup would probably work best, but would be frightfully expensive and difficult to set up as you'd basically need a repeater for ever room of your house. Then again I don't go priceing this stuff on a regular basis so I'm not sure how technically feasable it is using bluetooth to set up a pico net of audio devices. bluetooth audio device specs are standard though so you should be able to get away with some pretty basic hardware. Try googling for basic bluetooth microphones.

 

[JimB]

I'm not sure using walkie talkies for this application is legal, as the FCC limits the types of communication ...

The FCC have absolutely no authority over this whatsoever.

The OP is in the UK, and while Teflon Tony is forever sucking up to GWB, I dont think that he has handed control of telecommunications to the FCC just yet.

Whilst I agree that the application is a bit on the edge from a legal point of view, it is a lot better than ising a dirty little "part 15" type device which is splattering all over the VHF FM broadcast band. That is certainly illegal, and rightly so.

JimB

 

[wildgoose]

Hi Jim,

The keypad idea would simplify things but I've got my heart set on the voice thing. Besides its getting far too close to just walking over and pressing the light switch to turn on the light or walking into the kitchen and switching on the coffee machine. "Don't worry Hal I'll open the bay doors myself", it just doesn't sound right!

 

[wildgoose]

Not sure about Bluetooth, I always thought that was very short range, like 10 meters max and not very good at going through walls.

The more I think about it the more concerned I find myself becoming about broadcasting my life to the whole street. I think its going to have to be:

Audio In ---> ADC ---> Transmitter --------- Receiver ---> DAC ---> PC

at least that way the radio part will be digital and although not secure it should prevent anyone tuning in accidentally.

I think I'm going to have a play with a pic chip (got a few kicking around) for the ADC part and something like **broken link removed** for the radio bit. No idea whether the data rate will be enough or what kind of sound quality to expect but it sounds like fun and I'm sure I'll learn something, most likely along the lines of "well that was a dumb idea".

 

[fingaz]

The more recent bluetooth stuff has a range of up to 100 meters! or so some manufacturers claim. Though I have the older spec, with 10 meter range, and can connect my headset to my pc and walk anywhere in the house and still stay connected.

The other benefit of bluetooth, is it would be difficult for someone to 'hi-jack' the system by recording and playing back commands since all devices have to be 'paired'

 

[Sceadwian]

Bluetooth has various 'classes' of devices which determine the power available for broadcasting, and as fingaz said it's difficult to hi-jack because the device to device communication requires a basic authentication procedure that is difficult to reproduce and impossible to 'casually' overhear. If you want to eliminated legality and reduce complexity to the lowest possible level, a bluetooth system would probably be your best bet. But finding the right products to do what you want might take a bit of research, if not simply time to see if the products are on the market right now.