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  • jimhead
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SPDIF to analog conversion and vice versa
Trying to solve a long analog cable length problem.  My subwoofer is in the front of my family room and the home theatre receiver in the back.  Connecting those two are a 20-30 foot cable terminated by RCA jacks, all in the wall.  Yes, there are lots of electrical noise problems to the point that it's unusable.  It's two-conductor single-ended, looks like coax.

I'm thinking about using analog->spdif and spdif->analog converters in order to use that long cable as a spdif transport. That seems to be the most robust solution given my predicament.

Other options and their cons:
- Run line-level --> Lots of electrical noise
- Run speaker level --> Sound quality is terrible
- Wireless transmitter kits --> Don't like latency problems, powering on/off problems, general flakiness, sound quality
- Cannot run new cables through room

Questions:
- What good quality spdif/analog converters are out there?  Throwing a/d and d/a conversion into the mix can't be good for sound quality, but seems less bad than the other options.  Thinking about getting monoprice ones.
- Are there better options for me?

  • DVDdoug
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Re: SPDIF to analog conversion and vice versa
Reply #1
20-30 feet of shielded coax shouldn't normally be a problem with line-level signals.    I believe you, I'm just saying there might be something wrong with the wires...  Maybe they are not shielded.   You say there are two...  You only need one for the sub, so have you tried them separately?  

If you've got AC hum it could be a ground loop, and (electrical) S/PDIF may or may-not help with a ground loop.  (And, obviously you can't go optical.)

Quote
My subwoofer is in the front of my family room and the home theatre receiver in the back.
Maybe try putting the sub in the back?   ;) 


Quote
  Throwing a/d and d/a conversion into the mix can't be good for sound quality,
I'd expect that to be completely transparent.  I wouldn't do "extra" A/D & D/A if there wasn't a reason but since you've got problems now the sound should actually be better.   (I don't know anything about the Monoprice converters...   Monoprice sells good cables at great prices.)



  • Last Edit: 20 October, 2017, 06:24:48 PM by DVDdoug

Re: SPDIF to analog conversion and vice versa
Reply #2

Does the subwoofer use a 3 wire power cable? If yes, try an experiment and temporarily use a ground lifter to disable the protective ground. If the noise goes away, you definitely have a ground loop issue. There are ways to solve your problem AND have the required ground of the sub properly connected. Analog > SPDIF > analog is a possibility. Another is a galvanically isolated feed using  a transformer in the signal feed.  Another possibility is to use a balanced line. This does NOT necessarily require a balanced line transmitter at the source, just a balanced line receiver at the sub. The center signal wire would go to the '+' input and the out ground /shield goes to the '-' input. Any noise on the ground is also on the signal and is considered common mode signal and is rejected. Properly designed it can reach 100dB of isolation.  Analog Devices, Burr Brown (now T.I.) and THAT corp all make  ICs specifically for balanced line receivers with excellent common mode rejection though with modest care, a plain old opamp can do the job.

Another source of your noise could be RF pickup from a nearby radio station. 50,000 Watt AM stations can cause LOTS of problems under the right conditions. A common and effective fix for RF noise is a ferrite bead over the signal lead to prevent the RF from getting into the unit in the first place. You've probably seen HDMI or DVI cables with a cylindrical lump on the cable. That's a ferrite  to prevent radiation of noise.


  • schmidj
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Re: SPDIF to analog conversion and vice versa
Reply #3
Try disconnecting your cable TV connection from the street.  This often creates circulating currents that show up as 6o Hz (in the US) hum, particularly in subwoofers.  There are transformers you can insert in your coax cable feed to the cable box to break the noisy ground path.  I forget who I bought the one I use from, possibly MCM?  It says Isolator,5-2400MHz, Premier Farnell Ltd., which is a British company.  F connectors in and out.  I remember finding a tremendous range of prices for similar devices.

While a balanced audio device at the input to the subwoofer would seem to be a solution, I've found that many of them have lousy "common mode rejection".  The balanced input needs to have pretty good common mode rejection at 60 Hz and harmonics to get rid of the hum and buzz.  I found the isolator transformer a much better solution.

Don't leave a ground lifter installed on any device with a three pronged plug.  That is a serious safety hazard, you risk electrocution, and if connected to a cable service from the street, it becomes a fire hazard in the event of a nearby lightning strike.

You might want to verify that your cable feed has a ground block bonded to the ground of your electric system where it enters your house also, as required by the electric code.  But I found even a properly bonded cable was able to induce significant hum into subwoofer inputs.

  • jimhead
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Re: SPDIF to analog conversion and vice versa
Reply #4
Thanks for the suggestions everyone.

20-30 feet of shielded coax shouldn't normally be a problem with line-level signals.    I believe you, I'm just saying there might be something wrong with the wires...  Maybe they are not shielded.   You say there are two...  You only need one for the sub, so have you tried them separately?  

If you've got AC hum it could be a ground loop, and (electrical) S/PDIF may or may-not help with a ground loop.  (And, obviously you can't go optical.)
To be clear, it’s one conductor with a ground shield around it. Spdif is digital, and with the kind of voltages and frequencies it operates at I really doubt household ground noise would have any effect at all on its signal integrity.

The electrician probably routed the cable right next ac lines. I really should have a lot more attention when they were doing work.

  • krabapple
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Re: SPDIF to analog conversion and vice versa
Reply #5
ran a new cable outside the wall, with a bit of dressing/channel mount to make it look nice?

  • krabapple
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Re: SPDIF to analog conversion and vice versa
Reply #6
ran a new cable outside the wall, with a bit of dressing/channel mount to make it look nice?

*Run*, not ran