Radio Remote HiFi Speakers - University Project

Hi,
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!
Thanks
Gareth Bradley
 
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.

Thanks,
Gareth
 
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?

Thanks
Gaz
 
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...
 
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:

http://www.national.com/an/AN/AN-899.pdf

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).
 
What are your power requirements?

Thus far, transmission of the audio has been discussed. What are you going to power your receiver/demodulator units and speakers with? You mention that the cabling is an issue. Are power cables also an issue?

If so, better factor in a power source. :D

Years ago, had a student ask me about an interesting idea, a wireless communications link for motorcycle helmet headsets. Many larger touring motorcycles use stereo headsets, plus a microphone for communication between the rider and passenger, as well as CB radio between bikes.

There were several problems:

The idea was to employ infrared transmission. In daylight hours, the sun provides enough IR noise to blank out most any signal. The signals required were L + R audio, and a Mike signal.

I normally mounted the helmet speakers away from the ear, like a Stax headset. The speakers were cut into the foam shell of the helmet, at least 50mm off-axis (above) the ear, and 30mm away, allowing the rider to hear as much ambient sound as possible. Many riders wanted closer installation, but I never agreed to mount the drivers any closer! In this configuration, "cranking up" the system would require higher power, and the perceived sound was like speakers "floating" a foot or so away from the ear.

With this in mind, the problem was providing adequate power for the system. Granted, one could have cheated by using efficient "ear bud" Walkman type speakers, but that would be both illegal and unsafe.
 

Yoda

Member
2001-07-30 1:35 am
Bluetooth wireless

I too have been investigating the possibilities of using bluetooth to transport digital audio. How is this coming along for you? are you sending SP/DIF signals or something else? is latency an issue? My idea is to send digital audio signal to the speaker that has an internal class D amp fo reffiecency to run on battery power, what are your ideas?
 
Hi,
My problem with cabling is the long speaker cable stretches. Latency is an issue but I would think it'd be negligable for bluetooth. As long as my speakers are all relativly in phase I'm OK with that.

I suspect that not sitting dead central or leaning over to pick something up off the floor, or a speaker slightly out of line would cause more problems than latency...

I have found Bluetooth to be very cheap. I found 5 chips (BGA package :( ) for £45 or $75US...It is a cheap method!

Gaz
 
a point not yet brought up...

volume control It always seemed to me to that the best way to transmit the signal is in spdif or something akin to it. That part while technically challenging seems fairly straight forward. The problem I ran into while thinking about this was the volume control. If you use spdif or its equivelents you would get HiFi for the price of a few a/d, d/a, and a transmission system. But if you do this how to you get the volume signal out? Perhaps there is a really simple digital attenuator, but it seemed to me that this was throwing away a fair amount of data. I came up with two or three systems, the perhaps easiest was to get a few frequency hopping portable phones and rework them a bit. The second would be to transmit spdif digitally and then use an FM signal to transmit the gain(volume). The third and best solution would be to re-encode the spdif to include volume information. But this requires programming a computer for each transmitter and reciever.

Hopefully this is all moot because of a good way to do digital attenuation, but I haven't seen one (because I haven't yet looked for one seriously)
 

Yoda

Member
2001-07-30 1:35 am
rarkov, upon reading the bluetooth standard manual, it appears to be a packetized standard. How could you send a stream of data along in packets without a buffer or computer at the other end. I assume that there will be no bufffer in the speaker itself because it would cause time delay issues if a packet didnt get through. we need a wireless interface that does not use packets. can you "hack" up one of those bluetooth modules to send raw data and recieve raw data? any more details about your project are welcome!
 
Yoda said:
rarkov, upon reading the bluetooth standard manual, it appears to be a packetized standard. How could you send a stream of data along in packets without a buffer or computer at the other end. I assume that there will be no bufffer in the speaker itself because it would cause time delay issues if a packet didnt get through. we need a wireless interface that does not use packets. can you "hack" up one of those bluetooth modules to send raw data and recieve raw data? any more details about your project are welcome!

The stream of data may use packets, but from what I remember, a proprietary Bluetooth receiver will, with the help of some external logic, depending on the Bluetooth component used, turn this back to data usable by a DAC.
 
That's interesting...
Bluetooth chips aren't all that complicated...It was meant and designed to be a one-chip-does-it-all solution to wireless networking peripherals...As a result of it being developed by many companies, info is easy enough to come by...

The problem is with the miniturisation of bluetooth components. It is mainly for mobile phones and other handheld devices...We are not interested in that and it is far more important to be solderable...Well, for me at least...

Any one seen BT chips in DIP packages?!

Gaz