Does USB Cable Affect Sound Quality

Does a USB cable really affect sound quality? I think this question had already been asked many times at many forums and always ended up with long debates. And with the concept of spend only 10% of the budget into accessories, I never really got the chance to verify this issue myself as I do not own such an expensive DAC. However, I finally manage to get myself a RM2.4k Ciunas DAC last month. Therefore, I am eager to try out whether a USB cable really do affect the sound quality or not. The DAC used a A to B USB cable which is commonly found on printer. Hence, I gather around all the cables I am able to find in my house and get ready for testing. Just a side note, this DAC is power up by another USB charger cable which is included. Therefore, the A to B USB cable should only be affecting the sound part and not the electrical part.

USB Cable Do Make A Difference
After testing around with those free A to B USB cables that I found, I am able to hear significant differences when switching the cables. After being convinced on this, I started to look for better audio USB cable. But, it's quite disappointing and frustrated as the audio USB cable market is not very popular and flooded as analogue interconnect. After looking for a while, I manage to purchase a 2m DH Labs Silversonic USB cable and a 1.2m Furutech GT2 USB cable.

Listening Test
I run in these 2 cables each for 100 hours before beginning the test. The DH Labs and Furutech cables are totally opposite characteristic of each other. The DH Labs cable have very good control over the bass but the vocal seem to be a little dark or cold. Meanwhile, the Furutech cable has very good dynamic expression in the vocal but the bass seem to getting bloomy times to times. Then, I switch back to the free cable that I think was the best before. I straight away notice that the bass, treble and vocal totally lack of focus and power when comparing to those 2 cables. The characteristic of this free cable seem to be more of the same as the Furutech cable. Below are the pics of these 3 cables.

Free cable:

DH Labs Silversonic:

Furutech GT2:

Recordings
As usual, I recorded the music using my handphone. But, this time I recorded 2 tracks for each cable; 1 is male vocal while another is female vocal to show the characteristic of the cables. The setup is Ciunas DAC, Marantz pm ki pearl and Kef ls50.

Free cable:


DH Labs Silversonic:


Furutech GT2:

These recordings just serve as references and may not present the live listening condition.

Verdict
- USB cable DO affect the sound quality
- following the concept that only 10% of the budget should allocate for accessories, the USB cable price should be 10% of the DAC price or at most 20%
- the sound of those free cables aren't that bad
- each cable has their own characteristic. There is no best cable and expensive cable may not always be better. It is very important to match the whole setup and your personal listening taste.
- we don't have the return policy that available at some countries and this made us hard to find cables that match our setup and listening taste. Not only USB cable, I am including speaker cable, analogue interconnect, power cable and etc.
- I am thinking maybe the rapid growth and improvement on the DAC market bring forth to this long debate. If you think that USB cable do not make any differences with your old and maybe expensive DAC, just try to borrow a latest model DAC that cost at least RM3k to test it out. Maybe the result will be different.

Below is some theories that I found on a local forum that describes why USB cable affect sound quality even it is passing "0" and "1". It is not proven and lengthy. Read it if you are interested or just skip it.

Basically the USB connection is just an electrical connection. It transmits an electric signal, measured in terms of voltage and current, though voltage is the parameter we are mainly interested in. So the USB cable is susceptible to all those things which affects the standard electrical signal transfer. The main advantage of a digital signal is again that we limit ourselves to two voltage levels, e.g. 0V and 1V. But in real life, the signal can vary anywhere between 0V and 1V. I already explained the issue of setting the threshold for defining digital-0 and digital-1.

Then there is the issue of synchronization. This is the issue of EXACTLY WHEN the signal changes. E.g. if I want to send a digital stream - 001100. Then I modulate the signal in the USB cable to 0V, 0V, 1V, 1V, 0V, 0V. OK then, EXACTLY WHEN do I do that in terms of TIME. And for the purpose of transmission, EXACTLY WHEN do the receiver (e.g. the DAC) 'sample' or measure the voltage in the USB cable to determine what I am sending ? This is the issue of synchronisation.

Now the question of EXACTLY WHEN is the fundamental and most problematic one. TIME is a CONTINOUS entity in so far as our science is concerned, right down to quantum physics. We have a limited ability to deal with it in terms of our maths etc but in quantum physics, TIME and ENERGY levels are continous entities. OK, I will not dwell into this anymore, you can google the net if you want to read more. 

A USB connection works with a certain 'clock rate'. What this means is the 'sampling rate' by which the sender sends and the receiver must measure the signal. For a USB connection this can range from 40 to 60MHz for e.g., i.e. the sender -and receiver - works with a sample, or signal change, of between 40 to 60 MILLION times a second. Put another way, the sender sends a signal - changes the voltage in the connection - once every 0.000000025 to 0.0000000167.... seconds. 

NOTE one anomaly here - the 2nd value above is an approximate. I.e. a 60MHz works out to an exact value of ?.... WE CANNOT express it accurately, it is 0.000000016666666........ INFINITELY. So if we want to work with it, we HAVE to make a decision to adopt a fixed DEGREE of accuray, say 10 decimal places. Therefore 60MHz is 0.0000000167 seconds APPROXIMATELY. This is another issue which forces it way in all scientific and technology discussion. The LIMITATION of our scientific foundation, in this instance, the limitation of the most basic of our science - our NUMBERING system. Again, I won't talk about it anymore and you can google if you want to know more.

So basically the important issue about synchronisation is simply that the sender and receiver MUST work at EXACTLY the same time, or for practical purpose, given the limitations of our technology, at a level of accuracy high enough to ensure there is no error. Meaning that if the receiver tries to sample the signal at the wrong time, then it will be reading a wrong value. And as you can see, it needs to be very accurate. 

So, imagine an electrical signal changing once every 0.0000000167 seconds. Now imagine how sensitive it can be to interference, to cable material quality, to sender/receiver circuitry, and all the other things we know can affect an electrical signal. 

This is why it is NEVER just a series of 1's and 0's. It NEVER EVER was. 

The USB connection has NOT and WILL NOT be adapted by the commercial or military/space environment for their critical communications. I.e. equipments in a massive mainframe computer or a massive server farm are NOT connected using USB. These environments are those which demands the max in accuracy and reliability. So think about what is says about this interface standard. Now translate it to what you need to do to ensure good accuracy and reliability.