Cable Distortion and Dielectric Biasing Debunked - page 2
These THD measurements actually have a story behind them. After making my results known, I was informed that some people claimed to have measured plain harmonic distortions on audio cables, more specifically at lowish signal levels. THD levels well over -120dB were reported, with great differences between different makes of cable. These distortions were subsequently attributed to "micro diode effects" in the cable.
I was invited to make a set of measurements to duplicate the test. In first instance, I just grabbed four different cables from my bench and submitted them to distortion measurements.
One was a cheap A/V cable: a triple coax with serve shield, all bare copper.
One was a Japanese manufactured cable with audiophile pretenses by a brand named Hisago. It was the only cable in the test field I've actually listened to, and I remember feeling the sound was exceptionally appalling (without "e" and two "l"). Conductors are bare OFC, insulator is foamed PE.
The third cable was a 50ohm RF coax with solid PE insulator and bare copper conductor. Shield is tinned copper.
The fourth is a coax with Teflon jacket (sheath), teflon insulator and silvered conductors/shield.
Test Set-Up
Source setting was 1kHz, 30mV, 20 Ohms impedance. The plots show 256 times power averaged FFTs of the residual. This means the 30mV fundamental was notched out such that the distortion/noise performance of the ADC does not affect the result. All dBs are relative to the fundamental. Power averaging smoothes the noise floor to be better able to pick harmonic components from the noise.
The graphs are squeaky clean. The generator third harmonic at around -130dB just peeks out. No other distortion products are visible.
These plots are clearly not very helpful - once distortion products are 10dB or so below the noise floor, there's no way of finding them back. Although it is already clear that no distortion products of -120dB levels are present, the inquisitive eye wants to see more.
The Work Around
Luckily, there's a trick called synchronous averaging. It adds up signal records grabbed synchronously with the stimulus, and performs the FFT on the averaged measurement afterwards. All "correlated" components (harmonics etc) will be increased by a factor of n, while noise increases only with the square root of n. The net effect is that the relative contribution of the noise is decreased by the square root of n. Averaging 256 records will thus improve SNR of the measurement by 24dB. Of course, the noise floor is no longer smooth as with power averaging. This makes reading harmonic levels near the (reduced) noise floor somewhat difficult. Otherwise, generator and analyser settings are the same.
Second and third harmonic of the generator are now clearly visible (had they come from the cables, they would be different on each plot). A faint hint of a 5th is visible, but its reading is unreliable due to its proximity to the noise floor.
No Distortion Products Even Remotely Attributable to the Cable
OK, let's now suppose that the low generator output impedance is just reducing dielectric distortion in the same way that it does microphonics, and that the high input impedance of the analyser is preventing the claimed "micro diodes" from becoming sufficiently biased to actually produce distortion. Generator output and analyser input are now set to 600 ohms impedance. This causes the signal level to drop by half, to 15mV. The tests are re-run on all cables.
No News. No Distortion.
A month later, two envelopes leave Steve Eddy's office (Owner of Q-Audio.com ). One is addressed to me, the other to the person who had originally claimed to be measuring distortion.
The envelopes each contain an identical set of four different types of cable.
- An unmarked cable like the ones packaged along with cd players etc.
- An "old style" Radio Shack Gold cable. This cable was laid out as a zip cord.
- A spanking new (totally unused - no "burnin" or what is it) Radio Shack Gold cable. The new cable is sold separate
- An RG174 cable. This cable is interesting in that the conductor is made of copper plated steel ("copperweld"). At least steel is known to have nonlinear magnetic properties so who knows, at sufficiently large signal levels.
This time, no time is lost on making power-averaged plots. All plots are synchronously averaged. Four settings are tried on each cable:
- Amplitude=30mV, Zout=20 Ohms, Zin=high (100k)
- Amplitude=30mV, Zout=600 Ohms, Zin=600 Ohms
- Amplitude=13V, Zout=20 Ohms, Zin=600 Ohms
- Amplitude=13V, Zout=600 Ohms, Zin=600 Ohms
All cables yield frighteningly identical results, including the RG174 cable.
I emailed Steve Eddy to complain that the tests were boring. He replied that this is a sacrifice to make for science. (I felt much better after that).
Cable Distortion Plots and Commentary
Special thanks to Bruno Putzeys, Philips DSL
Chief Engineer Class D Audio at Philips Digital Systems Labs
