Okay, so the other day I was installing my new
BenQ BL2710PT monitor (reasonable monitor for a decent price) and as I was rummaging through my old cables, came across a very old
USB 6' cable that I probably got free when I purchased an old Samsung laser printer back in 2001 during the transition from USB 1.1 to early USB 2.0.
This cable is the thinnest, most flexible, likely most poorly shielded USB cable I have; in other words, about as "bad" as it gets when connected to a quality USB DAC which expects to operate in high speed USB 2.0 mode without completely failing... Behold the "Bad Cable":
Plugging this cable into my desktop
ASUS Essence One provided the opportunity to demonstrate just what a poor USB cable does to the sound... I'm sure this is "old hat" to those who have experience with digital audio, but for those who haven't, have a listen...
I recorded 1 minute of a freely available track from Jason Shaw called "Pioneers"
from here off the Essence One fed into my EMU 0404USB to the usual Win8 laptop using a good quality cable versus the flimsy one above.
Good USB 2.0 cable - well shielded 12', ferrite core on both ends of this specific cable:
"Bad" USB cable as pictured above - poorly shielded against interference and incapable of transmitting at bit-perfect high-speed data rate to the ASUS Essence One:
Even though SoundCloud recompresses the uploaded FLAC audio, I'm sure you can appreciate the
obvious errors in the "Bad Cable" sample. (You can press play back & forth between the two samples to A-B them if you want.)
What you're hearing is what happens with
digital error (ie. not bit-perfect), similar to watching digital TV with the occasional data error leading to macroblocking and bad pixellation as in this sample found off Google (notice the blue stripe due to digital error):
It's worth noting a few characteristics of this poor cable as it pertains to sound:
1.
Poor digital cables leading to digital errors sound like brief pops or occasional static (assuming they do not completely malfunction). They're similar to the errors you get when ripping a CD without something like
EAC or equivalent. Sometimes, you'll hear very brief dropouts. Depending on the data packet disrupted, occasionally they will occur in only one channel but not possible for this to happen consistently in a single channel. Remember that although asynchronous DACs have the capability to buffer, hence improve timing and lower jitter, they do not (at least not in the case of the Essence One with the
CM6631 USB interface as far as I can tell) necessarily error correct or ask for a packet resend. The more data error, the less the amount of "normal sounding" music will be heard. Obviously if the data error occurs every few minutes, it might be difficult to detect, but if it happens frequently, it's
not subtle.
2.
A poor digital cable does not result in overall level changes in the song... This is not like analogue distortion that can consistently alter the volume level or change the dynamic range uniformly or periodically.
3. Similar to the above point,
poor digital cables are not capable of changing the overall tone of the sound. There is no such thing as a digital cable capable of acting as a "tone control", making certain sounds "brighter" or "warmer". A passive digital cable is not capable of acting with some kind of frequency filtering mechanism.
4.
Poor digital cables do not consistently do anything to the soundstage. A poor digital cable cannot make a voice or instrument sound "distant" or move it "forward", or pan the soundstage to the left or right as a whole or in relation to other components of the music.
5. Bad cables cannot cause speeding up or slowing down of the data transfer.
Poor digital cables therefore cannot cause sporadic or consistent timing issues like warble (speed up/slow down pitch changes), "pacing", or rhythm problems.
6.
The concept of cable "break in" makes no sense with digital audio cables. If it carries data accurately when plugged in then the only problem that can happen in time is corrosion at contact points or reactions such as oxidation of the metal over time. This can only lead to transmission errors as demonstrated above, not some magical
improvement due to "break-in".
7. I was
reminded here the other day about the measurements with
a poor RCA cable I used as coaxial SPDIF last year. Indeed, if you use a
very poor, unshielded RCA cable paying no attention to the expected 75-ohm impedance specification with an SPDIF digital interface that's not galvanically isolated (eg. coaxial SPDIF of the ASUS Essence One in that case),
noise can be introduced into the system. However it does not take extravagantly priced cables to make things right (an inexpensive 6' <$20 decent shielded cable from a reputable company will do). As always, noise can be introduced into the analogue domain with any electrical connection (or just being careless like putting your DAC right on top of a noisy desktop computer), so it's not really an issue with the digital system itself.
You might be curious how the 2 USB cables measure in terms of
jitter...
Surprised? As you can see -
not much difference at all! If you monitor the realtime FFT for the J-Test, you will see errors "popping up" with the bad cable due to bad data transfer, but in between, the jitter plots are essentially indistinguishable! This is expected... For an asynchronous DAC like the ASUS Essence One, jitter rejection is handled very well
by design and there's nothing the passive cable can do about that.
Now I'm sure there will be a number of folks who disagree and hear various effects in the list above (see
here,
here and
here for some interesting perceptual accounts and/or creative writing).
The thing is, where is there decent evidence to show that passive digital cables (and I'm talking here not just of USB but also the SPDIF variants like coaxial or TosLink) sound different if they're error free (a.k.a. bit-perfect) and built to specifications (assuming no issue with analogue noise as in item 7 above)? I've never seen manufacturers come up with anything of substance... Or hobbyists/DIY guys show/demonstrate verifiable claims... As usual, I'm happy to change my mind if some kind of
objective evidence exists since I personally have not
subjectively heard a problem except as demonstrated above with digital errors.
(
Digital cable summary from a number of months back for those who might have missed it.
Recent post on EETimes blog on this.)
Recommended album:
- Have a listen to Babatunde Olatunji's posthumous 2005 album
Circle Of Drums.
This is one of the best Chesky albums I have heard. The drums sound fantastic with wonderful tonality and sense of 'space' around the instruments; a lovely exploration of African drums and rhythm. Unless you believe you can hear the difference between 16-bit noise floor and that provided by SACD, IMO there's no need to buy the SACD because it appears to be a 44kHz PCM upsample (
here's the Master List). There is a multichannel mix on the SACD which sounds OK but derived from post-processing. An impressive sounding and quite enjoyable record for those interested in world music nonetheless!
Relax and enjoy the music!
----
Addendum (January 20, 2014):
For the sake of completeness in answering Frans' comment below, here is the J-Test result with the TEAC UD-501 using the poor USB cable vs. good one:
The TEAC uses quite a different asynchronous USB implementation than the Essence One.
It appears to utilize a TMS320 DSP processor rather than "off-the-shelf" USB interface like the CM6631(A). I'm wondering if this interface is actually more robust since the digital errors were even less obviously audible.
In any case, using a different DAC, the jitter test remains unchanged;
two examples now of how an obviously poor USB cable does not appear to affect the jitter from asynchronous DACs in terms of the analogue output (which IMO is the only important measure since that's what we hear!).
