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antedeluvian
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Switched Capacitor filters
antedeluvian   4/17/2014 6:39:25 PM
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Max

Have you thought about switched capacitor filters instead of your linear ones. Now I have little experience in this and I fully expect to be shot down for something or other I do not know. The advantage of a switched capacitor filter is that the pass frequency/cutoff frequency is set by a clock frequency so it can be changed at a moment's notice. It may be possible (and you will need to investigate) to have one filter and clock it according to your "bins" looking at each one in turn. Processing speed would seem to play into this approach, but still..

The LMF100 from National/TI is a long established part with 2 channels, but Linear Tech and no doubt Analog Devices and others have parts and app notes.

Of course if you used a PSoC1 you could have it built into the chip.

B.V.Rao
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You could be better off driving the display with FPGA
B.V.Rao   4/18/2014 3:14:28 AM
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I suggest using your controller to run DSP and use FPGA to drive the LEDs.  You may not need so many outputs.  all the LED strips may be daisy chained.  Few years back I made a large (16x256) mono-color matrix with a very small FPGA.  it used to display graphic data placed in block RAM by a controller.

If you could lay your hands on a old DIY audio graphic equaliser,  you may probably be able to use it for analog spectrum separation.

Max The Magnificent
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Re: Switched Capacitor filters
Max The Magnificent   4/18/2014 9:39:13 AM
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@Antedelivian: Have you thought about switched capacitor filters instead of your linear ones?

I have to say that I haven't -- but that's because (like so many things) I know more about the theory than the practice.

I'm really starting to think that digital processing is the way to go, because you can do so much more manipulation of the data.

 

Max The Magnificent
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Re: You could be better off driving the display with FPGA
Max The Magnificent   4/18/2014 9:41:15 AM
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@B.V.Rao: I suggest using your controller to run DSP and use FPGA to drive the LEDs.

That's actually a very interesting idea -- the reason I opted for the Arduino MCU driving the NeoPixel LEDs is that the timing on the LEDs is somewhat sensitive and the folks at Adafruit have already done all the work for me (bless their little cotton socks :-)

Max The Magnificent
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Re: You could be better off driving the display with FPGA
Max The Magnificent   4/18/2014 9:44:03 AM
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@B.V.Rao: If you could lay your hands on a old DIY audio graphic equaliser,  you may probably be able to use it for analog spectrum separation.

This is another brilliant suggestion. It struck me while I was writing this column that the control of the display and the extraction of the audio data are two completely different things -- I can happily construct the display now while pondering how to extract the audio dat ain the background. Using an old graphic equalizer to extract the data woudl certainly make my life a lot easier...

Do you happen to have an old 16-channel graphic equalizer lying around? LOL

B.V.Rao
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Re: You could be better off driving the display with FPGA
B.V.Rao   4/18/2014 10:56:34 AM
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I had one 12ch. Never could make out whether it is working. Bad ears! It made to trash can long ago.

Max The Magnificent
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Re: You could be better off driving the display with FPGA
Max The Magnificent   4/18/2014 10:59:19 AM
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@B.V.Rao: Never could make out whether it is working.

LOL That sounds like a lot of my hobby projects :-)

GSKrasle
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Freelancer
UI/Information Display
GSKrasle   4/18/2014 3:52:25 PM
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Max,

I have put a lot of thought into many projects involving the display of information. In particular, I have thought about this precise one. (You're not the only one who has built such a thing!)

But first, on Getting the Data: I recommend a proper analog anti-alias filter and D-A, providing for both channels of stereo (and maybe for the other 3.1); you don't have to use more than one initially, but it's easier to have and not need than the other way 'round. The AA filter is really not a super big deal if you can sample really really fast and then apply a multi-rate ("decimation") filter (from a library: the computation has been amazingly optimized). This allows you to reduce the data-rate to something manageable on-the-fly. Then you will probably want to use a FFT (function from the same library) to get the frequency-(phase)-amplitude-time data. Anyway, with proper AA and fs, you have all the available data and what to do with it becomes a software problem only.

But what to do with the data, how to present it? First-off consider some of the diagrams you have, in particular a, c, d: Like Space Invader, the colours are fixed: you might as well use single-colour LEDs or white ones behind a striped filter: the channel, the dimension, is wasted. It is degenerate. (What a cool term!)

Roughly, you have these dimensions available to manipulate in displaying data: X, Y, R, G, B. But your data, the output of the FFT, only has these instantaneous dimensions: f, A. How to map two variables onto five? A simple bar-graph or cartesian graph gets 2 => 2, {f,A} =>{X,Y} leaving the three colours to be usefully employed other than on/off.

How about adding a dimension? Do the FFT on both stereo channels, and now you can have f, AR, AL. Assign them to X, Y as above, but now R, G can be used to distinguish AR, AL.

Add TIME: have the display fade after a pixel has been "hit": that would allow the R, G dimensions to have more than single-bit resolution!

What about the B dimension, though? Maybe it could represent the instantaneous sum of R, G, so that signals in common would be white, with the tip of the bar's R or G hue indicating the differences.... But R+G already looks yellow.... And it would be NICE if there were some conceivable way to display pure B.... Maybe a "peak" indicator of longer persistence? A "halo" over the bars? A horizontal "peak of all" indicator (but that wastes X)?

Oh, well, I have never been able to think-up a way to use a full-colour display for information derived from stereo audio without some redundancy, some degeneracy. I guess all that is left is to add some "art" just to make it look good.

My two cents!

 

GSKrasle
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Re: UI/Information Display
GSKrasle   4/18/2014 3:55:59 PM
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And the typos are free!

Max The Magnificent
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Re: UI/Information Display
Max The Magnificent   4/18/2014 3:58:08 PM
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@GSKrasle: I recommend a proper analog anti-alias filter and D-A, providing for both channels of stereo (and maybe for the other 3.1); you don't have to use more than one initially, but it's easier to have and not need than the other way 'round.

After writing my blog I started to think this way -- as you say, it's better to have both channels available if I want them later -- if I decide to then start with mono, I guess the simplest way is to just add the outputs from the D/A converters and divide the result by 2.

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