Audio over RF???

darb1972

darb1972

Senior Member
Hi All

Does anyone know of (or have experience with) Audio over RF? I'm referring to analogue left and right Audio between 20Hz to 20kHz. I need to cover about a 20m distance so a Dorji transceiver might be suitable(?). In my part of the world the unlicensed bands are 433MHz, 2.4GHz and 5.8GHz. The FM band is out of the question due to communication restrictions.

I'm not sure if a PICAXE can do this or if there's too much information converting from analogue to digital, transmitting, receiving, error checking(?) and convert back from digital to analogue for use in and Audio system.

Can anyone give some guidance on this? There are plenty of "AV Senders" on the market, but, speaking very frankly from first hand experience on many other jobs and using a number of different brands, they are generally crap. I've only found one ready made unit that is apparently "digital" and apparently provides "crystal clear audio" but I do remain slightly skeptical. The unit retails for $109-00 so it's not outrageously priced if it actually works, but, I am still interested in seeing if a PICAXE could do it or if others have further suggestions.

I look forward to hearing from everyone.

Regards,
Brad
 
hippy

hippy

Technical Support
Staff member
I would not have thought it would be easy to create a transmission system for carrying audio which would be better than a commercial system.

To carry a 5kHz audio signal, that requires sampling at 10kHz, so one has to sample every 100us, and half that for stereo; 50us. For 10-bit audio sent digitally that would be 4us per bit, 250K baud.

As well as AV senders there are Bluetooth senders, and some wireless headphones use IR.
 
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MikeM100

Member
Use of the the Licence Exempt Bands for the transmission of audio is discouraged as such signals occupy the frequency continually and thus block other users.

Some time ago commercial manufacturers (Phillips) used to market 433 MHz 'audio senders' which lead to formal complaints by other users. When the European 868 MHz band plan was being devised there was made a specific allocation (300 kHz) for such devices operating at 5mW and up to 100% duty cycle. (Other sub bands only allow between < 10% to < 0.1% duty cycle)

For some reason few commercial manufacturers manufacture audio RF modules but an example here:

http://www.cdt21.com/products/audio/

An interesting article on the technical considerations here:

http://www.cdt21.com/resources/pdf/TI_003.pdf

As you will see it is not 'easy' which may explain why there are few offerings.

In practice most audio 'senders' use 2.4GHz but performance is far from perfect with poor range being often being reported.
 
darb1972

darb1972

Senior Member
@ Hippy. Thanks for the detailed reply. I gather that 250k baud is out of the question for PICAXE and the general solution is to use a commercial product.

@ Tex. Nope. Not all of Australia. As mentioned in the first post, I need about 20m of range, barely a fly poo when compared to the vast range that is Australia!!!
 
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hippy

hippy

Technical Support
Staff member
darb1972 said:
@ Hippy. Thanks for the detailed reply. I gather that 250k baud is out of the question for PICAXE and the general solution is to use a commercial product.
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The PICAXE can do 250k baud, but it is the conversion and sending bytes quickly enough which it would struggle to do.

Perhaps explaining the application - why you need 20Hz to 20kHz and crystal clear clarity - will allow the most appropriate solution to be suggested.
 
techElder

techElder

Well-known member
darb1972 said:
@ Tex. Nope. No all of Australia. As mentioned in the first post, I need about 20m of range, barely a fly poo when compared to the vast range that is Australia!!!
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Then, how about modulated light for a transmission medium?

You didn't describe your application.
 
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inglewoodpete

Senior Member
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Goeytex

Senior Member
Hi Darb,

In Australia there are 2 newer bands that are allowed for wireless audio devices such as wireless microphones. The law changed last year according to the article below .These are 520-694 MHz and 1790-1800 MHz.

http://acma.gov.au/Citizen/Spectrum/About-spectrum/Wireless-microphones/new-arrangements-for-wireless-microphones

Here is something to look at from Silicon labs. So yes it is possible but will require some engineering. Not something you can slap together in a few hours. Not sure how a Picaxe would fit in as it will likely not be able to process data fast enough, but could be used for glue logic.

https://www.silabs.com/Marcom Documents/Resources/audio-applications-guide.pdf

Other manufacturers may have similar offerings.
 
darb1972

darb1972

Senior Member
MikeM100 said:
Interesting article here:

http://www.shuredistribution.co.uk/dms/shure/support_downloads/frequencies/pdf_guides/frequency-guide_sml/Shure Frequency Guide - February 2013.pdf
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Hello Mike

Thanks for the links and information. You are dead right, it's definitely not an easy process for one to undertake. Although your articles mainly relate to the European based spectrum, we have a similar structure with a reshuffle (restack) of DTV channels and a sell-off of the UHF Spectrum above our RF Ch51 for use with 4G (and probably eventually 5G) mobile phones and associated communications. I guess I was just curious to see if it could be done with the amazing little PICAXE controllers that despite having some limitations, seems capable of almost anything we can dream up.

I think I'll have to stick with a commercially available solution working at 2.4GHz and hope it works.
 
darb1972

darb1972

Senior Member
hippy said:
The PICAXE can do 250k baud, but it is the conversion and sending bytes quickly enough which it would struggle to do.

Perhaps explaining the application - why you need 20Hz to 20kHz and crystal clear clarity - will allow the most appropriate solution to be suggested.
Click to expand...
Hello Hippy

Thanks for your reply. The general concept is to take the audio band (it could be narrower than 20Hz to 20kHz if one was to sacrifice some fidelity), in stereo (left and right channels) at line level (up to 1Vpp) from either a CD player or DVD player, convert this audio to an RF signal (preferably digital, not remaining as analogue for transmission), transmit it over a distance (20m to 30m would be great), receive the digital signal, error check it if possible, and convert it from digital back to an analogue line level stereo signal to feed into a remotely located amplifier.

This challenge arose due to a building which is basically impossible to cable (no roof or wall cavities) and the "client" (this is basically classed as a "love job") wants their source unit located at one end of a small building while the amplifier is located at the other end. Before anyone issues a WTF, this has come about because an electrician cabled it this way (during a minor extension/renovation about 12 months ago) and I'm working through a few issues in an attempt to make it operate how the client thought it was going to work as opposed to how it is (incorrectly) cabled.

Does that make sense?
 
darb1972

darb1972

Senior Member
Texasclodhopper said:
Then, how about modulated light for a transmission medium?

You didn't describe your application.
Click to expand...


Hi Tex

For the application description, please refer to my last reply to Hippy.

Thanks for the suggestion of modulating light. I gather you might be referring to IR? I don't have total direct line of sight. This signal would have to penetrate one gyprock wall and one window/glass door (not tinted). I've not used the likes of IR for such an application, but I believe it has been done before by a number of commercial companies. Other than LOS and directional issues, IR can be effected by ambient light and one of the rooms (where the signal would originate from) is rather well light from sky lights.

Maybe I'll have to stick with RF and might be forced to use a commercially available unit due to complexity.

It's not necessarily a case of not wanting to use a commercial unit, it's more a case of getting something reliable, but, maybe more importantly, being able to build it myself if possible and reasonably feasible!!! Call me odd, but I enjoy designing stuff and bringing it to life!!!
 
darb1972

darb1972

Senior Member
inglewoodpete said:
433MHz is out of the question. You are only allowed to transmit for a few seconds every hour. Otherwise, you could block everyone in the street from unlocking their cars or opening their garage doors:).

Edit: A good starting point (plain language) is https://www.communications.gov.au/what-we-do/spectrum/wireless-audio-devices
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Hi Pete

Thanks for the link. I'm an Electronics Tech so I've been involved with the restack for 4G for a few years now (I install DTV antenna systems, Satellite and other forms of comms gear). I know a reasonable amount about most of the spectrum. I didn't however realise that only 433MHz comms was for intermittent transmissions only, but, now you mention it that makes complete sense. Doh. I should have realised that.

A commercial unit I have been investigating works on 2.4GHz at a transmission of 17dbM. That should be o.k. provided the client doesn't have 2.4GHz WiFi or a cordless phone on the same channel/s. I have a Fluke WiFi meter that will detect WiFi (2.4GHz and 5.8GHz) and non WiFi devices on all of the given channels so I'll have to investigate further to avoid a clash with local and neighbouring devices. RF can be a bit of a nightmare at times!!!
 
darb1972

darb1972

Senior Member
Goeytex said:
Hi Darb,

In Australia there are 2 newer bands that are allowed for wireless audio devices such as wireless microphones. The law changed last year according to the article below .These are 520-694 MHz and 1790-1800 MHz.

http://acma.gov.au/Citizen/Spectrum/About-spectrum/Wireless-microphones/new-arrangements-for-wireless-microphones

Here is something to look at from Silicon labs. So yes it is possible but will require some engineering. Not something you can slap together in a few hours. Not sure how a Picaxe would fit in as it will likely not be able to process data fast enough, but could be used for glue logic.

https://www.silabs.com/Marcom Documents/Resources/audio-applications-guide.pdf

Other manufacturers may have similar offerings.
Click to expand...
Hello Goey

Good to hear from you. Thanks for the links. As per my other responses, I have been involved in a fair bit in the frequency reallocation which has included replacing a fair number of "illegal" wireless microphones in PA systems. I'm not sure if the 520MHz to 694MHz band requires a license for use in my Country but I would be inclined to think that is the case. I didn't realise that the 1790-1800MHz band was up for grabs, but again, I'm guessing that one might require a license to use it.

I will have to do some more research on these available bands and note the restrictions, power levels and licenses (if needed).

I'll take a look at the link for Silicon Labs and return for further comment if needed. It's a shame that PICAXE might not totally meet the needs of the system as it's my preferred platform for a PIC (I'm not overly experienced with PICs despite there being a large number of platforms to chose from these days). It's basically a case of too many toys, too little time!!! Anyway, I plan to win Lotto this week, retire, and become a full-time tinkerer! ;)

Even if I select a commercially available solution for this client (and it actually proves to be reliable), from an academic stand point I think the whole Audio over RF link is still worth some further investigation. I've had a number of cases in the past where it could have been extremely helpful. I also checked with one of my PA suppliers today and discovered that Denon do one. It retails for about $500-00 to $600-00 AUD so it depends on the needs of a client and what they are prepared to pay!
 
hippy

hippy

Technical Support
Staff member
It is entirely possible to stream audio with no loss of quality over Wi-Fi so that is the approach I would likely take. There are ready made receivers but it's easy enough to build one's own with a small SBC such as the Pi and it is likely easy enough to build a transmitter as well. It should be possible to do exactly the same with Bluetooth.
 
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eggdweather

Senior Member
To transmit 20Hz-20KHz you would need a lot of bandwidth at 433Mhz and I have not seen any devices on the market that would achieve what you need, bandwidths are typically 12.5KHz or less, so about half of your requirement. Modulation would need to be optimised to make use of the bandwidth available, so usually 20-20KHz bandwidth is transmitted as L+R channels (mixed) and a sub-carrier just above the bandwidth maximum, lets say 22KHz is modulated with L-R channels so that the receiver has L+R and L-R then the resultant signals are either added or subtracted. Right = (L+R)-(L-R)=2R and Left is (L+R)+(L-R)=2L the twice amplitude is of no consequence.

Creating that modulating signal to reduce bandwidth (20-20KHz plus 5Khz) is not easy to do. A monaural system is so much easier, almost trivial to achieve. You can buy broadcast FM transmitters for a few $, often referred to as FM bugs, , the audio stages of which provide the transmitter drive, which could then be applied to a 433MHz module, but here's the rub, nearly every module on the market is OOK (On-Off Keying) or Amplitude Shift Keying (ASK) and are totally unsuitable for audio signals. FM types could achieve the modulation, but their deviation (that needs to be 2xmax audio F=40KHz) are limited to 5KHz at best, so you hit a brick wall. Broadcast that level of audio fidelity requires typically 200KHz of bandwidth as the side-bands extend out a long way either side of the carrier to convey the upper frequencies towards 20KHz.

Doing this legally is not possible IMO, well not without spending comparably large amounts of time and design effort and cash, but doing it illegally is perversely quite easy if you look around for suitable transmitters. I have seen FM Stereo broadcast units on eBay for less than $30 from China... and you could mix their output (88-108MHz) with a 335MHz RF source = 433Mhz, but that takes a lot of skill.

You can buy audio video senders for the 2.4GHz band, I would use some of those, but not all are broadcast they tend to be point-to-point closed links.
 
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inglewoodpete

Senior Member
eggdweather said:
To transmit 20Hz-20KHz you would need a lot of bandwidth at 433Mhz....
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As I mentioned previously, transmission of more than a few seconds per hour is illegal on 433MHz in Australia and many other countries. The band is primarily for use with remote controls Eg. cars, garages, gates, door bells etc. Officially, it is called "ISM use" - Industrial, Science and Medical.
 
darb1972

darb1972

Senior Member
hippy said:
It is entirely possible to stream audio with no loss of quality over Wi-Fi so that is the approach I would likely take. There are ready made receivers but it's easy enough to build one's own with a small SBC such as the Pi and it is likely easy enough to build a transmitter as well. It should be possible to do exactly the same with Bluetooth.
Click to expand...
Thanks Hippy. I'll look into this further.
 
darb1972

darb1972

Senior Member
eggdweather said:
You can buy audio video senders for the 2.4GHz band, I would use some of those, but not all are broadcast they tend to be point-to-point closed links.
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Hello Eggweather

Thanks for the comprehensive and informative reply. I appreciate the information. I'm looking into a 2.4GHz model (off the shelf solution due to what looks like a complex issue to address). This is plan A. I'll let the forum know how this option performs before considering other alternatives.
 
T

tmfkam

Senior Member
darb1972 said:
Hello Hippy

Thanks for your reply. The general concept is to take the audio band (it could be narrower than 20Hz to 20kHz if one was to sacrifice some fidelity), in stereo (left and right channels) at line level (up to 1Vpp) from either a CD player or DVD player, convert this audio to an RF signal (preferably digital, not remaining as analogue for transmission), transmit it over a distance (20m to 30m would be great), receive the digital signal, error check it if possible, and convert it from digital back to an analogue line level stereo signal to feed into a remotely located amplifier.

This challenge arose due to a building which is basically impossible to cable (no roof or wall cavities) and the "client" (this is basically classed as a "love job") wants their source unit located at one end of a small building while the amplifier is located at the other end. Before anyone issues a WTF, this has come about because an electrician cabled it this way (during a minor extension/renovation about 12 months ago) and I'm working through a few issues in an attempt to make it operate how the client thought it was going to work as opposed to how it is (incorrectly) cabled.

Does that make sense?
Click to expand...
In the past I've done the following:
Take the digital PCM audio feed from a CD or DVD, feed this signal into the Video input of an RF 'Video Sender', connect a D-A converter to the Video output of a tuner unit (scart socket of old video recorder) and then feed the audio from the D-A into the line inputs of an amplifier.

I ran this type of setup for some time. My intention was to send audio to my workshop 2-3 miles away, sadly by the time I'd got hold of a high (enough) powered modulator, I worked further away! It worked extremely well around the house though, if used a lot of old video recorders. As my job was repairing those, I could get as many of those as I needed. One reason it interested me was that the RF carrier, modulated by PCM, looks like random noise, much like an unmodulated carrier and I thought for high power transmission it would be difficult to spot, reducing the likelyhood of being caught running an illegal transmitter.

I have seen software for converting local audio signals to a 'stream' which can be broadcast across the internet, or in your case local net. Using WiFi and two computers might work for you?
 
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inglewoodpete

Senior Member
erco said:
Any way to get the Picaxe's clock speed output to a pin to an antenna? M1=1 MHz, right center of my AM dial.
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Only way I know is to use a 1MHz resonator/crystal on an X2 PICAXE and tap off the resonator connection. The chip will run a bit slow @ 4MHz but it's better than nothing.

You could also use a PICAXE M2 series with resonator and reprogram it in Microchip C but that's a bit radical!

Later: another thought - can you poke some SFRs in an M2 and get the PWM leg to output at 1MHz?
 
darb1972

darb1972

Senior Member
tmfkam said:
In the past I've done the following:
Take the digital PCM audio feed from a CD or DVD, feed this signal into the Video input of an RF 'Video Sender', connect a D-A converter to the Video output of a tuner unit (scart socket of old video recorder) and then feed the audio from the D-A into the line inputs of an amplifier.

I ran this type of setup for some time. My intention was to send audio to my workshop 2-3 miles away, sadly by the time I'd got hold of a high (enough) powered modulator, I worked further away! It worked extremely well around the house though, if used a lot of old video recorders. As my job was repairing those, I could get as many of those as I needed. One reason it interested me was that the RF carrier, modulated by PCM, looks like random noise, much like an unmodulated carrier and I thought for high power transmission it would be difficult to spot, reducing the likelyhood of being caught running an illegal transmitter.

I have seen software for converting local audio signals to a 'stream' which can be broadcast across the internet, or in your case local net. Using WiFi and two computers might work for you?
Click to expand...
Hello tmfkam

That's an interesting and somewhat curious way to do it! I never really thought of using the Video input of a sender for audio. I guess if the video amp/output section has got the correct BW (presumably 0MHz (DC) to 7MHz) then I guess it could work as the audio is only a fraction of the BW.

I also didn't really consider transmitting the Digital Audio and then converting back with a DAC for analogue use. Presumably you were using/transmitting 2.0Ch of Digital Audio? Otherwise decoding 5.1Ch might have been a bit complex. Did you have to separate the L and R channels??? I'd like to know more about how you pulled this off!!!

Regards
Brad
 
darb1972

darb1972

Senior Member
@erco and Phil

Thanks for the information. I'm a big fan of Paeansonic. Great stuff. :rolleyes:

I don't know what the restrictions are for using the FM band in Australia but you can bet I'm (sadly) not allowed to use 7W of power. That would cover a big area!!! I'm not even sure if I can transmit anything for that matter! I believe that small FM modulators (like used in a car) are only going to be in the milli-watts range. I would have to investigate. My guess is the same would go for the AM band down at 1MHz. Yes, beleive it or not, in Australia some radio broadcasters still transmit on the AM band. It has crappy fidelity but I believe it can cover big distances, particularly out West in very flat areas (plains).

Although it's tempting, I might have to avoid the AM and FM bands. Until we go entirely DAB in Australia (which might take several decades to happen in regional areas), dabbling in AM and FM bands might not be wise in my part of the world. Having said that, it will be interesting to see what (if anything) the ACMA (our Governing body) does with these bands. Maybe they hand them over for public use? That would be awesome!
 
erco

erco

Senior Member
darb1972 said:
I don't know what the restrictions are for using the FM band in Australia but you can bet I'm (sadly) not allowed to use 7W of power. That would cover a big area!!! I'm not even sure if I can transmit anything for that matter! I believe that small FM modulators (like used in a car) are only going to be in the milli-watts range. I would have to investigate.
!
Click to expand...
Per the listing, power can be between 0.1 W and 7 W maximum power continuous adjustment

See also http://www.ebay.com/itm/New-FM-Radio-Transmitter-Module-PLL-Wireless-Digital-Microphone-Stereo-87-108MHZ/272315850379
 
hippy

hippy

Technical Support
Staff member
erco said:
inglewoodpete said:
Later: another thought - can you poke some SFRs in an M2 and get the PWM leg to output at 1MHz?
Click to expand...
Exactly the kind of magic we need. Technical? Hippy?
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You can use the PWMOUT Wizard to determine settings for PWM up to 8MHz at 64MHz operating speed.

However we would not recommend its use for wireless transmission without considering the legal and regulatory aspects of doing that.
 
T

tmfkam

Senior Member
The PCM stream contains both left and right channels as one. My 'transmitter' really was as simple as feeding the PCM out (or S/PDIF) signal from the CD player into the video input of a video/UHF modulator, I already had a D-A converter which was used at the remote end for turning the digitally modulated video signal back into the audio left and right. I see no reason why this shouldn't work for 5.1, so long as you have the appropriate encoder (or existing stream from a CD or DVD) and a matching decoder. The bandwidth required is probably 1-2MHz which is why I used the video modulator, on most AM or FM bands you wouldn't be able to use any existing transmitter design as I doubt they would cope with the required bandwidth needed to keep the digital signal from rounding off, causing the decoder to misread the stream. You might possibly be able to transmit the digital signal as a series of differing tones then use those tones to regenerate a copy of the original stream, but it could be hard work. Many recent amps have built in 5.1 decoders, if so, these will often be capable of resolving 2.0 as well. A-D encoders are less common, but easier and cheaper to obtain than they were ten years or more when I experimented with mine.
 
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