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Rarkov 9th September 2002 07:29 AM

Radio Remote HiFi Speakers - University Project
I'm embarking on my final year university project soon and am doing a bit of research on the subject.

After recently moving house I found that I couldn't place my speakers where I wanted them, due to cable constraints amongst others. I decided to design a remote HiFi radio system. I would like to make a 5.1 system buut also to be easily expandable. To limit the interference, I was thinking about transmitting digital data, although I know that would being to get difficult.

Any ideas/comments would be greatly appreciated!
Gareth Bradley

skaara 9th September 2002 08:32 AM

Its doable, but I dont know what will happen to sound quality.

Rarkov 9th September 2002 09:01 AM

This is what I was concerned about.
The more speakers connected to the system, the longer the period of time until it receives its next bit of data.

The only way to sort this is to have 5 transmitters running in parallel, each with different encoded signals. Unfortunatly this would require digital transmission, that I think would get expensive.

I could send a PWM signal, but how could I let the speaker know I was talking to that one, if I were to use only one transmitter.


skaara 9th September 2002 09:40 AM

For the quality i think its better to have 5 transimitters, each on its own frequency.

mrfeedback 9th September 2002 03:50 PM

Economical Method
Gareth, I have repaired cheap Asian surround sytems in the past that ran FM radio coupled rear speakers.
Simple FM low power transmitters with audio compression (NE 570/572) and recievers with audio expansion did the job ok but not hifi, but then again 100 Hz - 5 kHz is all that is required for Dolby surround.


Rarkov 9th September 2002 03:57 PM

I had thought about that, but like you pointed out, 100Hz-5kHz isn't really hifi. That was one of the advantages of the digital transmission.

Last year, at uni, I designed a class D amp from first principles (Sawtooth and Comparator = PWM, amplify PWM, LPF, Speaker). I have really grown to like the PWM code as it is almost as easy to 'see' the music as when it is a sine wave (if that makes any sense!). Could I send a PWM code over an FM radio?!

Would I be able to then pass it through a shmitt trigger to get my original code back out?


mrfeedback 9th September 2002 04:27 PM

The deviation of an FM transmitter determines the bandwidth that is transmissable (broadcast FM is 15 kHz).
High deviation is required to transmit a digital data stream.


hifiZen 10th September 2002 09:55 AM

Yeah, I wouldn't bother trying to modulate FM with a PWM signal... the PWM waveform will contain high harmonics that will push your bandwidth limits too far. It would be far easier to do a direct FM modulation straight from the analogue waveform. 5 channels, one for each speaker.

If you really want to go digital, you'll have to think upscale, in terms of more innovative concepts... perhaps 802.11 transmission of PCM samples taken directly from the CD source? Maybe bluetooth can handle it (I once did a rough figuring in my head that bluetooth could handle about 2 channels of 16 bit CD audio).

Let's see.. you could also try some hair-brained scheme involving wireless video transmitter/receivers... that would give you plenty of bandwidth to play with, and you could try different techniques for encoding the audio, either in analogue directly, or digitally. Maybe you can borrow ideas from the wireless technology used by telcos? Telco technology, wether old or new will tell you all about multiplexing many many channels together. How about infrared? I believe there's a high bitrate ethernet standard which uses infrared, can't remember if it's part of 802.11 or another section of 802...

Narcisse91 10th September 2002 10:51 AM

I had looked into something similar, although I was going from a PC to my receiver, instead of receiver to speaker. I just dug through my stacks and stacks of datasheets and schematics and found some application notes from my research. Take a look at this:

I looked into this a while ago, and to be honest, don't remember too many specifics, but that modulation scheme had looked like it would be pretty effective for a project like this. If you look into it, and find any serious downfalls of it, post them here (because I'd like to know about them, in case I revive my project).

Rarkov 10th September 2002 11:28 AM

Thanks for all those suggestions! HifiZen, u hit upon something there! Bluetooth really caught my eye. The 10m range is plenty - fast and can be implemented in /very/ small packages (My mobile has it).

Bluetooth will be a main point of research I think...I'll let you know how I get on!


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