PROPOSAL: wireless digital loudspeaker with remote volume and on/off control - diyAudio
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Old 22nd May 2013, 12:15 AM   #1
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Default PROPOSAL: wireless digital loudspeaker with remote volume and on/off control

I've been playing around with both some real components and some circuit sims and I think I have a working concept for a wireless active "digital loudspeaker" based on the MiniDSP 2x4-crossover and MiniDIGI boards (one of each per speaker). This includes:
  • S/PDIF digital audio sent wirelessly to the loudspeakers
  • Remote digital volume control
  • remote shutdown and power-up of most of the electronics in the loudspeaker (the control circuitry in the loudspeaker must remain powered up for this to work)
The advantage of this approach is that the signal remains in the digital domain through the crossover before becoming an analog signal and then sent to the onboard amplifiers. The only cabling that must go to the loudspeakers is the power cable.

So, how to do this as a DIYer? Here's what I have come up with:
  • Use an inexpensive "AV Sender" set consisting of one transmitter and two receivers. There is one receiver in each loudspeaker.
  • The digital audio signal is sent over the "video" channel of the AV Sender in S/PDIF format.
  • Analog circuitry is used to send other control signals via the L and R audio channels of the AV sender. These allow wireless control of the volume, and other functions such as the remote on/off. Several channels like this could be accommodated, but I have only seen need for two.
I've designed several circuits that generate the control signals that are connected to the transmitter, or convert them into useful signals that can interface with other equipment in the loudspeaker.

I have tested the concept using an inexpensive AV sender set to send the digital signal and this works well so I'd like to move on with the build.

Are there other here that might be interested in this kind of project? If so I can eventually design some PCBs for the control circuitry.

-Charlie
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Old 22nd May 2013, 04:42 PM   #2
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Default control circuit concepts

Here are some additional details about the control circuits:

Additional circuits control volume of each speaker, and remotely turn the system on and off. The signals for the circuits are sent as AC waveforms over the analog channels of the AV sender, the "left" used for the left speaker and the "right" channel used for the right speaker. I chose to use an AC signal over the wireless connection because a typical AV sender will have a limited bandwidth (e.g. 70Hz - 15kHz, @-3dB) and will most likely not pass DC signals. To keep it simple, I use a 556 timer configured in astable mode as a square-wave oscillator. This is sent to the transmitter. After reception, the signal is sent to a bandpass filter having the same frequency as the square wave. This results in something like a sine wave output with reduced amplitude. A precision rectifier circuit and RC filtering then provides a DC output. Since the amplitude of the AC signal sent using the AV sender has a dynamic range of 40dB or more, the signal amplitude can be used to trigger relays to energize the power amps, or as an input to the crossover board digital volume control interface. Additional control channels could be implemented as long as the bandpass filters are designed to provide the necessary attenuation so that adjacent bands do not interfere.

This seems to be a pretty general method for wireless SPDIF transmission and reception. Without the remote volume control the system is not all that practical for loudspeakers, since the signal would emerge at full volume. At this time, digital volume controls is not widely available, so I developed the solution described above that works with the onboard digital volume control of the MiniDSP 2x4 platform.

The control section (volume control, on/off switch) could be located with the main audio system components, just like with a preamplifier. Source selection could also be implemented using an SPDIF (coax and toslink) switcher, and an A-to-D converter for analog sources (tuner, turntable, tape, etc.).

-Charlie
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Old 24th May 2013, 01:06 PM   #3
magdrop is offline magdrop  United States
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I am definitely interested in this project. I don't have hands-on experience with either SPDIF or the wireless or the MiniDSP 2X4, but I think I do understand most of the requirements.

What are you looking at for the AV sender?
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Old 30th May 2013, 08:32 AM   #4
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Hi,
This is a very interesting idea having looked into a set-up just to be able to control the volume remotely with miniDSP.
My concern regards the stability of wireless for HR files.
What is your opinion on that (interference with other signals at home, can it handle the bits etc.)
My set-up is Genelecs 8260 with internal DSP and digital signal in over AES.
Configuration would be:
- two receivers powered by the G's?!
- could the receiver be somehow built into it?
The AV senders could be attached to say Squeezebox/Mac/Sonos/Unitiserve/Streamer with fixed SPDIF/Toslink/AES out (with varying degrees of quality I believe). Or to miniDSP digital preamp with multiple gear?
If going through the Mac probably the volume control and file playing could be integrated in one "App"? What about other gear?
Lots of questions but I find your approach very interesting.
Good idea!
What would be the cost?
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Old 30th May 2013, 05:47 PM   #5
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Quote:
Originally Posted by aboekhoudt View Post
Hi,
This is a very interesting idea having looked into a set-up just to be able to control the volume remotely with miniDSP.
My concern regards the stability of wireless for HR files.
What is your opinion on that (interference with other signals at home, can it handle the bits etc.)
My set-up is Genelecs 8260 with internal DSP and digital signal in over AES.
Configuration would be:
- two receivers powered by the G's?!
- could the receiver be somehow built into it?
The AV senders could be attached to say Squeezebox/Mac/Sonos/Unitiserve/Streamer with fixed SPDIF/Toslink/AES out (with varying degrees of quality I believe). Or to miniDSP digital preamp with multiple gear?
If going through the Mac probably the volume control and file playing could be integrated in one "App"? What about other gear?
Lots of questions but I find your approach very interesting.
Good idea!
What would be the cost?
Although the specification includes single ended, the only AES/EBU I have seen is a balanced interface. The AV sender has only a single "digital" channel, so unless you can use a SPDIF signal on both ends (or somehow use unbalanced AES/EBU) this may not work for your particular monitors if you need the balanced version. You could I suppose use two parallel systems operating on different channels and implement a balanced digital line that way... but it's twice the cost, for no real added benefit.

Last edited by CharlieLaub; 30th May 2013 at 05:49 PM.
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Old 3rd June 2013, 09:15 AM   #6
minidsp is offline minidsp  Hong Kong
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That's a neat idea Charlie. Keep us updated on your progress...
We don't have any wireless audio solution so providing your system works, we can certainly build a section of our website to advertise your final tweak.

Keep us updated.
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Old 3rd June 2013, 10:22 AM   #7
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A great idea, but how about the time/phase synchronisation of the two channels? As far as I can see the two separate MiniDSP units operate independently, without their clocks being synced. Or did I miss something?

One solution could, of course, be a master-slave configuration with DSP and amps in one speaker cab, and a speaker cable to the other. A delay configured to match the cable length difference.

A second issue I wondered about is how to get signal from analog sources to the active speaker(s).
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Old 3rd June 2013, 03:03 PM   #8
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Quote:
Originally Posted by selainjuh View Post
A great idea, but how about the time/phase synchronisation of the two channels? As far as I can see the two separate MiniDSP units operate independently, without their clocks being synced. Or did I miss something?

One solution could, of course, be a master-slave configuration with DSP and amps in one speaker cab, and a speaker cable to the other. A delay configured to match the cable length difference.

A second issue I wondered about is how to get signal from analog sources to the active speaker(s).
I'm not following you here - how is there a time/phase problem related to the clocks of each MiniDSP??? If there is, what is the magnitude of the "error"? Your assertion would be much more convincing if you could provide some numbers or calculations.

Also, your "solution" seems to forget that an AC EM field travels quite rapidly in a wire, so that the delay the signal experiences when traveling from one speaker to the other through the wire is so short as to be insignificant. (See: http://en.wikipedia.org/wiki/Speed_o...magnetic_waves) It's also defeating the whole purpose of my wireless system: no additional wires.

-Charlie

Last edited by CharlieLaub; 3rd June 2013 at 03:12 PM.
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Old 3rd June 2013, 03:06 PM   #9
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Quote:
Originally Posted by selainjuh View Post
A second issue I wondered about is how to get signal from analog sources to the active speaker(s).
This done by using an analog to digital converter, then sending the digital signal to the speakers as usual. This would need to happen even if the crossovers were not located in the loudspeakers, because the crossovers are DSP types operating in the digital domain.

-Charlie
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Old 6th June 2013, 12:45 AM   #10
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Default update...

UPDATE:

I built up a version of part of the control circuitry. One part of the circuit didn't perform as expected, so I am designing another version and hope to build it in the next couple of days. In general, the concept is looking pretty good so far.

I will post more info on the next build iteration when I have it.

-Charlie
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