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RCA to SPDIF/Optical to RCA for subwoofer runs (attenuation of digital signals) - Technical Theory Question (1 Viewer)

andlex12

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Alex Malic
Long time reader, first time poster. Was unable to find the exact answer via search but have gathered lots of little bits of information on the web regarding the situation. Asking for opinions on the theory.

Needing to run some long distance cables for a Elemental Designs A7-450 subwoofer to roll around to find ideal placement to integrate into home theater, and possibly for some other future uses.

I have multiple RCA to SPDIF Optical/Coax ADC's and SPDIF Optical/Coax DAC's in my pile of fun stuff. Was considering taking the RCA sub output from my Denon AVR-3808ci and converting to SPDIF optical for the long run (noise free and complete electrical isolation) and then convert back to analog RCA at the subwoofer (to avoid RF noise/humming, etc.)

The question about theory that I have seen debated, but never found a clear and concise answer on is when you attenuate a digital signal, are you actually are "loosing" audio information, or "bits" from the stream. I believe this would fall into the argument on "volume control of digital signals", or how is volume controlled digitally when the audio information is encoded in all 1's and 0's. Debate being to attenuate a digital signal you have to chop away "bits" to control volume. Rather than attempting to pass a full bandwidth 5.1 signal via the optical cable, I would just be passing the volume controlled LFE output from the AVR.

I know the correct volume level is digitized by the ADC and it's level is reflected at the output of the DAC at the the end of the run, it does work. However what I don't know is exactly how compromised is the signal, how much fidelity is lost? Is it negligible when dealing with a low bandwidth LFE signal? Signal delay is not an issue as room correction will compensate for any delay issues automatically or I can program in delays to keep everything in sync via a DCX2496 processor.

I don't currently have access to my PICO scope or precision audio measurement equipment at the moment so I cannot take any real-world measurements. Obviously I can attempt to judge by ear, but I would love if anyone more technical than myself has any input on the topic.

Any thoughts?
Thanks!
 
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Wayne A. Pflughaupt

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First, welcome to the Forum!

The short answer to your concerns, as long as the A/D’s and D/A’s are 24 bit, input levels don’t matter. If you want more detail, check Part 5 of my article on gain structure, which you can find in my signature.

Regards,
Wayne A. Pflughaupt
 

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