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Old 25th March 2004, 06:56 AM   #1
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Default Digital Volume control for in Car use

Ok, so my in car system is a little more than most and of course has more than it's fair share of problems because of it.

I would have posted this in the car forum but it seems to apply more to digital audio that mobile audio.

I'm in the process of running a DEX-P9 headunit with spdif out to a crystal DAC evaluation board in the trunk of my car. My problem is that the headunit does not have a volume control for the digital output, only the analog.

Since the DAC is located in the trunk and the head unit at the front of the car, my thought is that a digital volume control that could attenuate the spdif signal mounted at the front of the car would be easier than attenuating the signal post DAC and then running some sort of control to the front of the car from the trunk. Is there a device out there that will do this for me, I've done a bunch of searching but can't seem to find a conclusive answer. I realize that doing volume control with digital reduces resolution, but I have to think that the loss of resolution would be far overpowered by road noise and other factors. Help would be greatly appreciated.

For reference, and for fun I suppose my signal chain consists of Pioneer DEX-P9 feeding a Crystal DAC eval board (until I can design my own of course) then fed to a Butler Audio LDX23 tube 3 way crossover which sends signal to 6 PPI tube buffered 250W monoblocks powering a pair of Illusion Audio CH-1 horns and a pair of Illusion Audio ND-10's in the front stage. All balanced too. Thanks again.

Cheers
-Steve
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Old 25th March 2004, 07:16 AM   #2
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Forgot to mention that I have minimal experience with microcontrollers, mostly the ATmega128, and assembly language, so if that's the route I need to go then so be it.
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Old 25th March 2004, 08:11 AM   #3
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Maxim make a range of non volitile digital controlled pots that can be used as a volume if you want to do it after the DAC. They have one that can use push button up/down instead of a uC.
If you want to set the volume whilst the audio is still digital, you'll have to get maybe a texas preprogrammed DSP for audio. They'll do volume but probably only controlled from a serial bus(uC controlled). If you go that way you'll have to fit it between the SPDIF receiver and the dac chip via I2S.
I'd reccommend the maxim digital pots tho, 3 wires run to the front of the car shouldn't be too much problem.
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Old 25th March 2004, 08:17 AM   #4
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I figured it would be a DSP situation if I wanted to do it that way.

Can I get a link to the Maxim stuff?
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Old 25th March 2004, 08:37 AM   #5
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volume control in the digital domain will cost you resolution.

in the analog, i would not use the maxim digital pots, as for audio there are special high quality ICs from Cirrus and Burr Brown/TI , CS3310 and PGA2310 / PGA2311. I have a simple low parts count version http://home.tu-clausthal.de/~tpa/PGA/index.html

an Stereo version with LCD and input selector etc. is on the way. A multichannel version is in diskussion at the moment.
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Old 26th March 2004, 01:59 AM   #6
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Quote:
Originally posted by till
http://home.tu-clausthal.de/~tpa/PGA/index.html

Very helpful info.

Thank you!

speaker
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Old 27th March 2004, 09:50 PM   #7
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yes, as you know, a digital signal is quantized. so you have something like 2^16 levels for a 16bit system that uses all 16 bits. if you turn the volume down, you will only use maybe 2^10 levels, and thus you lose resolution becasue the same wave now must fit in a smaller number of levels.

so digital volume isn't ideal. they do make stepped attenuators that are digiatally controlled, but work on analog signals.
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Old 28th March 2004, 12:08 AM   #8
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Quote:
Originally posted by till
in the analog, i would not use the maxim digital pots, as for audio there are special high quality ICs from Cirrus and Burr Brown/TI , CS3310 and PGA2310 / PGA2311.
What is so special and high quality for audio in the volume control which comprises opamp (what quality of it is actually?) at the signal path against the divider network only as in the case of the Maxim/Dallas (and some others as well)?
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