High amounts of THD over USB and Coax...

[rastus]

[PeteB]

@rastus: "get" == borrow. As I mentioned before, I've been away from home base for a few months, and really did not anticipate doing much more than plug-and-play until I get back. As you know I also have some other stuff to work on and can't measure noise w/o one... I'm sure there will be a beer tax on the loan...

What I really want for tinkering with the Pi2 and the DIGI+, DAC+ etc. is one of these:



Sadly, I just don't have enough spare time right now. The problem described by the OP is exactly why you need some kind of external measurement instrument. Same goes for previous discussions about power supplies, battery banks, DAC+ v. DAC+ Pro, etc.

[PeteB]

RMAA tests as suggested by robertzombie, above:



RightMark Audio Analyser, Version 6.4.1
USB sound card with line in: Creative Sound Blaster X-Fi HD
External USB DAC: NuForce Icon HDP
Analog cable: standard 1m RCA cable
Digital Coax Cable
Digital Optical Cable

Tests:

1) Loopback test with USB soundcard
2) Laptop > DAC > Soundcard > Laptop
3) Pi 2 w. HiFiBerry Digi+ Coax > DAC > Soundcard > Laptop
4) Pi 2 w. HiFiBerry Digi+ Optical > DAC > Soundcard > Laptop

RuneAudio 0.4, Volume Control disabled
Test file generated in RMAA, and run from a USB flash drive: "Test signal (96 kHz 24-bit).wav"
Sound card incoming signal level set to default

Notes: The Icon HDP is not quite at the level of a full-size Rega or Cambridge Audio dac. It uses a brick-type power supply, so it does not depend on USB for power, but does not have the latest USB async circuitry, and does not fully support hi-def above 96KHz 24-bit. Other relevans specs are 2.6Vrms output, 0.001% THD+N, SNR > 100db. The SoundBlaster X-Fi line input is rated at 2V rms, close but not as high as the output of the dac.

Results:



The highest THD was with the Icon HDP connected to a Laptop through a USB port, but still less than 0.01%. The rest is about what I would have expected. These are such low levels that even a cheap USB cable or a poor USB connection might be to blame, and the laptop used for the test is several years old.

The distortion and noise with the optical cable is only a little worse than with the coax (both cables are ordinary budget 1-meter cables from amazon.com)

[rastus]

@PeteB

"...really did not anticipate doing much more than plug-and-play until I get back."

Stop logging into here, and you might... relax. You know your addicted brother.

"I'm sure there will be a beer tax on the loan..."

I'll take 12% of that spank you berry much. I just got home from a local meet, Sydney Audio Club, they did a "DIY showcase" of members DIY audio stuff. Three Pi's running into various speakers, amps, and God only knows what else, and a MASSIVE PC. Can supply pics. I am, right now, spellbound, spun-out, and gobsmacked.

[robertzombie]

[tboooe]

Thank you so much for taking the time to test. The results look definitely more encouraging. I am very interested to see if it can be found out whats causing the massive difference in test results.

[PeteB]

[PeteB]

@Robert: can you please try a test with HDMI connected and then disconnected, for comparison?

I think I get better results with the HDMI cable unplugged, even if the TV I use as a monitor is still turned on, and on the same ground circuit.

[robertzombie]

[robertzombie]