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Old 7th November 2006, 01:48 PM   #1
_atari_ is offline _atari_  Germany
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Default radical idea for an active digital speaker

Hi,

in discussion with a colleague I came up with an interesting idea, since I want to know if it s feasible.

At home I mainly listen to digitized music - stored on my hard drive.
I came up with the idea that the best playback can be achieved if it is transfered as long as possilbe as an jitter free, error corrected digital audio stream and converted to analogue audio as late as possbile in the process.

So my basic idea is to build a speaker with an built in dac (2 or four for each chassis), some built in mono amplifiers to make a bi- (or quad-)amping active speaker.

The music should be transfered over tcpip (wireless) to get an error corrected stream directly to the speaker.

In the speaker a small linux bo picks up a channel, acts as an all digital frequency-separating filter and delivers the signal to the DAC(s).

In Theory this should deliver the best possbile reproduction of an digital signal.

My question is:
What do you think?
Has somebod done this before?
And if not - why?

Best Regards
_Atari_
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Old 7th November 2006, 01:55 PM   #2
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TDA2020 does something similar. It includes one digital amp.
Generally you would want a digital amp so you go digital all the way to the speaker - in effect the speaker acts as the DAC.

http://www.tripath.com/

Jan Didden
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Old 7th November 2006, 05:24 PM   #3
bwaslo is offline bwaslo  United States
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Volume control is another factor to consider. If you are sending the signal to the speaker as a digital stream, you DON'T want to turn it down by reducing the digital values in the stream at the sending end. That way loses DAC resolution fast. If you are say, playing 30dB from the highest value the digital path can represent (and the highest level the speaker can presumable play), then you've lost 30dB of dynamic range that the DAC is capable of, and that's not good.

You'd best send the stream over scaled so that the peaks are near full-scale, and then also send some digital instruction to the speaker to specify how loud it should be playing. Control the volume AFTER the DAC.
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Old 7th November 2006, 05:57 PM   #4
heater is offline heater  Finland
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There have always been jokes about linux running on this and that, but now "My speakers runs linux"

Given the small size of systems I am working on that run linux, why not ?

So basically what we have is sound being broadcast over an IP network (wireless or otherwize) to the speakers which then do crossover and whatever other processing. How about Ambisonics, for example, after all our network can have many more than two nodes.

I might suggest using FPGA's. Put a CPU running Linux into part of the FPGA to handle the networking. Put the crossover filters and whatever else into the rest of the FPGA as VHDL or Verilog to get good speed.

Now the big problem here is time synchronization. Each linux OS is sucking data down from a central source and they all need to output nice jitter free samples samples in sync. No matter if there is some difference between one and another, after all you get that from normal distance variations betwen speaker and listener. But that difference should be too big or change much. some how a common clock has to be distributed to all those DACs.


Interesting.
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Old 7th November 2006, 05:59 PM   #5
el`Ol is offline el`Ol  Germany
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I believe digital amplifiers start to become interesting when x-over is also digital, because with passive x-overs frequency response becomes load-dependent. And since nobody does, digital amps don`t have the reputation they should have, except among some fullrange lovers like me.
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Old 8th November 2006, 11:52 AM   #6
_atari_ is offline _atari_  Germany
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Yes, synchrnoization is a problem!

because you need a confident world clock between all speakers to prevent any kind of phase shift.
One idea I had was to use ping to measure timeshift to the source and use some kind of timing information in the audio signal - so that the speakers could use buffers to deal with network lag and network distortion.

Is there any prebuilt circuit that could help me?

To answer the questions:

Putting Linux in FPGA? Way above my head - so I am very interested in it!

Yes x-over should be done digital -since then you can do any funky equalizer x-over or even room adjustment in the digital domain to not disturb the signal.

Is anybody interested in helping me to come up with a real-world-speaker running linux

Thanks for the encouraging response!

Marcus
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Old 8th November 2006, 01:55 PM   #7
_atari_ is offline _atari_  Germany
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Just as an addition:

This type of Xover should be done in software: active crossover
And this software could be possibly used for network transport: netjack

(You must know: I am working in sofware development - so my approach is to get a cpu and add software )
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Old 8th November 2006, 05:45 PM   #8
el`Ol is offline el`Ol  Germany
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"ralf" has software x-over, software EQ and Panasonic digital receiver. You could ask him about his experiences.
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Old 8th November 2006, 06:03 PM   #9
soongsc is offline soongsc  Taiwan
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There are a few chips out there that take standard serial input and drives the speakers in class D. Some people have used those to develope USB based amps.
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Old 8th November 2006, 07:26 PM   #10
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Check out Meridians DSP8000 speakers.

http://www.meridian-audio.com/p_d8k.htm

Rob.
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