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Raspberry Pi clipping on digital outputs?


tboooe
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Hi guys. There is a very interesting thread on another forum about potential clipping with the Pi3.

 

Here is a portion of his latest findings (direct link to the thread here). I am not sure what to make of his findings. The idea of lowering the volume and losing resolution does not sound like a good solution.

When I connect the Pi to the DAC (with software volume control disabled) over either USB, or Coaxial from my Digi+, an issue occurs whereby the digital signal appears to be peak limited. This results in the THD and IMD test signal being truncated, thus causing the 15% THD results. If the Pi were rendering the digital information correctly, the DAC would give the same output as with any other digital renderer. The only reason why the DAC's output would change is if there were a change in the digital data it receives. I have shown that this issue occurs independently of the HiFi Berry product. I have also replicated the exact same results with: my USB drive unplugged from the Pi, using ethernet, wifi with ethernet unplugged, with wifi and bluetooth disabled, different power supply, various bit depths and sample rates, and with Volumio as well as RuneAudio. The issue must surely reside at the Pi's i2s stage or earlier.

 

I have found that, by enabling software volume control and setting the Pi's volume to 89%, the issue with the altered test signal goes away and the results in RMAA look normal for my DAC. However, by doing this you are essentially removing 2 bits of resolution from the signal. Using the RMAA 16/44 test with the Pi at 87% (the sweet-spot, see results up-thread) gives a dynamic range output of 85 dB which is the equivalent of 14 bits. Disable the volume mixer (so output is 100%) and you get ~16 bits of resolution, but 15% THD.

 

So, my question is: Is the Pi doing something at the i2s (or earlier) rendering stage that results in a clipped signal? I would be inclined at this point to say 'yes' as, to repeat what I wrote earlier, the analogue output of a single file from a single DAC should be exactly the same - regardless of the device used to render the digital information to it - so long as that rendering device is operating in a bit-perfect fashion.

12TB NAS >> i7-6700 Server/Control PC >> i3-5015u NAA >> Singxer SU-1 DDC (modded) >> Holo Spring L3 DAC >> Accustic Arts Power 1 int amp >> Sonus Faber Guaneri Evolution speakers + REL T/5i sub (x2)

 

Other components:

UpTone Audio LPS1.2/IsoRegen, Fiber Switch and FMC, Windows Server 2016 OS, Audiophile Optimizer 3.0, Fidelizer Pro 6, HQ Player, Roonserver, PS Audio P3 AC regenerator, HDPlex 400W ATX & 200W Linear PSU, Light Harmonic Lightspeed Split USB cable, Synergistic Research Tungsten AC power cords, Tara Labs The One speaker cables, Tara Labs The Two Extended with HFX Station IC, Oyaide R1 outlets, Stillpoints Ultra Mini footers, Hi-Fi Tuning fuses, Vicoustic/RealTraps/GIK room treatments

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Hi guys. There is a very interesting thread on another forum about potential clipping with the Pi3.

 

Here is a portion of his latest findings (direct link to the thread here). I am not sure what to make of his findings. The idea of lowering the volume and losing resolution does not sound like a good solution.

When I connect the Pi to the DAC (with software volume control disabled) over either USB, or Coaxial from my Digi+, an issue occurs whereby the digital signal appears to be peak limited. This results in the THD and IMD test signal being truncated, thus causing the 15% THD results. If the Pi were rendering the digital information correctly, the DAC would give the same output as with any other digital renderer. The only reason why the DAC's output would change is if there were a change in the digital data it receives. I have shown that this issue occurs independently of the HiFi Berry product. I have also replicated the exact same results with: my USB drive unplugged from the Pi, using ethernet, wifi with ethernet unplugged, with wifi and bluetooth disabled, different power supply, various bit depths and sample rates, and with Volumio as well as RuneAudio. The issue must surely reside at the Pi's i2s stage or earlier.

 

I have found that, by enabling software volume control and setting the Pi's volume to 89%, the issue with the altered test signal goes away and the results in RMAA look normal for my DAC. However, by doing this you are essentially removing 2 bits of resolution from the signal. Using the RMAA 16/44 test with the Pi at 87% (the sweet-spot, see results up-thread) gives a dynamic range output of 85 dB which is the equivalent of 14 bits. Disable the volume mixer (so output is 100%) and you get ~16 bits of resolution, but 15% THD.

 

So, my question is: Is the Pi doing something at the i2s (or earlier) rendering stage that results in a clipped signal? I would be inclined at this point to say 'yes' as, to repeat what I wrote earlier, the analogue output of a single file from a single DAC should be exactly the same - regardless of the device used to render the digital information to it - so long as that rendering device is operating in a bit-perfect fashion.

 

 

 

Sounds like this person uncovered a potential hardware failure. Recall perhaps?

Synology DS214+ with MinimServer --> Ethernet --> Sonore mRendu / SOtM SMS-200 --> Chord Hugo --> Chord interconnects --> Naim NAP 200--> Chord speaker cable --> Focal Aria 948

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I am actually surprised this issue has not be reported on sooner. Makes me wonder if the tester either has a bad unit or his testing methodology is flawed in some way?

12TB NAS >> i7-6700 Server/Control PC >> i3-5015u NAA >> Singxer SU-1 DDC (modded) >> Holo Spring L3 DAC >> Accustic Arts Power 1 int amp >> Sonus Faber Guaneri Evolution speakers + REL T/5i sub (x2)

 

Other components:

UpTone Audio LPS1.2/IsoRegen, Fiber Switch and FMC, Windows Server 2016 OS, Audiophile Optimizer 3.0, Fidelizer Pro 6, HQ Player, Roonserver, PS Audio P3 AC regenerator, HDPlex 400W ATX & 200W Linear PSU, Light Harmonic Lightspeed Split USB cable, Synergistic Research Tungsten AC power cords, Tara Labs The One speaker cables, Tara Labs The Two Extended with HFX Station IC, Oyaide R1 outlets, Stillpoints Ultra Mini footers, Hi-Fi Tuning fuses, Vicoustic/RealTraps/GIK room treatments

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Sounds exactly like what happened with my Sonos earlier this year after one of the updates. (Subsequently fixed) The voltage input to the DAC is too high.

Auralic Aries G2, Ayre QX-5 Twenty, Ayre KX-5 Twenty, Ayre VX-5 Twenty, Revel Ultima Studio2, Iconoclast speaker cables & interconnects, RealTraps acoustic treatments, Sonore opticalModule (X2)

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It sounds strange to me ... something doesn't add up if output via USB and output via i2s are both overdriven it can't be down to the i2s implementation as the USB output (surely) doesn't use any of the "audio" hardware?

 

It sounds more like a software bug rather than hardware issue. But then the analogue output doesn't appear to be affected, and that DOES (afaik) use the same software and i2s implementation!

 

PS. I'm not saying there isn't an issue ... just not sure the issue is identified correctly!

Eloise

---

...in my opinion / experience...

While I agree "Everything may matter" working out what actually affects the sound is a trickier thing.

And I agree "Trust your ears" but equally don't allow them to fool you - trust them with a bit of skepticism.

keep your mind open... But mind your brain doesn't fall out.

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It sounds strange to me ... something doesn't add up if output via USB and output via i2s are both overdriven it can't be down to the i2s implementation as the USB output (surely) doesn't use any of the "audio" hardware?

 

It sounds more like a software bug rather than hardware issue. But then the analogue output doesn't appear to be affected, and that DOES (afaik) use the same software and i2s implementation!

 

PS. I'm not saying there isn't an issue ... just not sure the issue is identified correctly!

 

I agree with you completely. It doenst make sense but I dont know enough about how the Pi is designed. I am definitely going to follow that thread. The interesting thing is I posted my thoughts about using my Pi as a renderer compared to my tweaked pc. In the end I preferred my tweaked pc because I thought the sound was more open and extended. Could this be a result of the clipping in the Pi?

12TB NAS >> i7-6700 Server/Control PC >> i3-5015u NAA >> Singxer SU-1 DDC (modded) >> Holo Spring L3 DAC >> Accustic Arts Power 1 int amp >> Sonus Faber Guaneri Evolution speakers + REL T/5i sub (x2)

 

Other components:

UpTone Audio LPS1.2/IsoRegen, Fiber Switch and FMC, Windows Server 2016 OS, Audiophile Optimizer 3.0, Fidelizer Pro 6, HQ Player, Roonserver, PS Audio P3 AC regenerator, HDPlex 400W ATX & 200W Linear PSU, Light Harmonic Lightspeed Split USB cable, Synergistic Research Tungsten AC power cords, Tara Labs The One speaker cables, Tara Labs The Two Extended with HFX Station IC, Oyaide R1 outlets, Stillpoints Ultra Mini footers, Hi-Fi Tuning fuses, Vicoustic/RealTraps/GIK room treatments

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