Digital data and voice on FM or AM signal?

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Ironic

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Hi, this is my first post, and i hope it´s in the right place. Tried search but din´t find anything similar to what i need.

My objective is to transmit a voice signal at least a few hundred feet. It can be mono, and of reasonable quality. I also want to transmit digital data, in this case, to control a few servos, without the digital data being heard on the voice signal.

I has hoping to transmit at least 16bytes per second. (the higher the better, but reliability is more important). I can include check sum´s in the sent data to avoid errors.

One idea i have to do this is to use a standard FM or AM transmitter (like the ones found in ebay), and use a micro-controller to add a high frequency component to the transmitted signal (Frequency-shift keying). The receiver would then have a filter to separate the high frequency from the voice signal, and feed it to another micro-controler, that would decode the signal.

Also i would like the signal to be able to penetrate walls and buildings (the lower the frequency the better right? AM?).

Don´t want to burden anyone, but would appreciate suggestions/critics, and to be pointed in the right direction. The details i think i can workout on my own.

Thank You
 

ilgrant

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You may want to look at how sub-carrier radio and RDS work with respect to the FM band. (I'm assuming you're in the U.S.) This may give you a better understanding of how to implement your idea.
 

Ironic

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in europe, but rds is used here also

but by my understanding, rds works on a harmonic of higher frequency that the fm signal.
since it´s a higher frequency,because of planned indoor use, i assume i will loose the signal even if still receiving the voice channel. In my experience in car radios, in areas of low reception, we can still hear the fm station while the rds is gone...

i am hoping that a am or fm transmitter, since are designed for music, should be able to emit close to 20khz(human hearing range), but since voice only goes up to 4khz(used on telephone lines) i should be left with at least 10kHz off bandwidth to work with...
 

Squad10

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I've never done it, but I'm thinking it may be possible to send 16bytes per second (128baud) FSK and voice at the same time using a common NB(direct)FM transceiver. The FSK modulation will occupy bandwith below 300 Hz, the voice above 300 Hz.

To encode, the microprocessor 16 bytes per second FSK output to a 300 Hz lowpass filter. Inject the low pass filter output into the direct FM transmitter's modulator having a low frequency response, less than 1 Hz.

To decode data, receiver discriminator audio to a 300 Hz low pass filter then data detector and finally to the microprocessor.

A 300 Hz high pass filter can be used to remove the FSK modulation from being heard if a speaker is used.
 

Ironic

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is there an advantage to use the lower frequencies?

my idea was to use the higher frequencies. I will have more bandwidth meaning i could send a lot more data then i need, so i could use redundancy and/or error corrections codes to improve reliability.

or can the higher frequencies of the modulated signal be degraded faster then the lower ones?
i´m not criticizing your suggestion, but want to understand the reasons behind it.

for my application the data signal is more important then the voice, when signal strength is low.

Thank You
 

Squad10

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is there an advantage to use the lower frequencies?

my idea was to use the higher frequencies. I will have more bandwidth meaning i could send a lot more data then i need, so i could use redundancy and/or error corrections codes to improve reliability.

or can the higher frequencies of the modulated signal be degraded faster then the lower ones?
i´m not criticizing your suggestion, but want to understand the reasons behind it.

for my application the data signal is more important then the voice, when signal strength is low.

Thank You
If you want to employ CRC or FEC, overhead will require more bandwidth than 300 Hz. I suggest you examine (for your specific application) the Signal to Noise Ratio that will determine the Bit Error Rate of the type of modulation needed for the Data Rate you require for your application. These three factors will hold pretty much true for the RF band that best suits your application and that are legally available for your use. I would also research different forums. RR is not the place for this discussion.
 

Squad10

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Forgot to mention, look at cmlmicro.com for 1.2k to 19.2k NBFM modem devices and white sheets.
 

dnoyeb

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Aren't the DPL codes digital? Im not sure how many BPS they are sent, but I do believe they are digital and sent in the same frequency as the voice but sub-audibly somehow.
 
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