Understanding USB

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The issue with USB boils down to noise contamination of the DAC. Noise is generated via the following avenues:

 

1. Noise transmitted from the sending device. The power network inside a computer is a hellstorm of noise, and that gets transmitted along with the USB signal and 5V power wire.

2. Self-noise generated by the USB receiver's controller. USB audio data comes in as lumps (packets) at regular intervals, causing the controller to perform bursts of processing. Also, the crappier the USB signal, the harder the controller has to work to fix it.

 

There is no such thing as "digital audio". What we have are square waves being transformed into sine waves. The human brain can perceive moments of sound far beyond its capacity to perceive frequencies of sound. Even a slight ripple in the square-to-sine wave transformation process can be perceived by humans.

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13 minutes ago, beerandmusic said:

When you say there is no such thing as digital audio, the data is stored on the media as digital (1s and 0s).  The sine wave is a result of the DAC.  It is digital data until it is converted.  It may be digital with noise, but it is not analog.

 

There is no such thing as digital audio. 1s and 0s do not appear anywhere. What we see are square waves. Square waves are 100% analog in nature. There is no fundamental difference between a square wave and a sine wave. What you're referring to as "data" is, in fact, counting a period of time between rises and falls in a square wave. What we see in computer chips are nothing more than very densely packed arrays of various circuits designed to perform advanced counting procedures on square waves. Square waves are useful because they provide a large range of power fluctuation within the moment of time between changes while allowing a circuit to reliably "guess" the value. The square wave will go through the laborious sine wave transformation process every step of the way passing along any noise that gets into power network and that which eventually gets into the output circuit. 

 

Here is another figment of our imagination:

There are no such things as "circles". This shape exists solely in our minds as an imaginary concept -- a way for us to generalize information we perceive.

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2 minutes ago, beerandmusic said:

 

The source file is stored in 1s and 0s.

 

Agree to disagree...they are worlds a part.  A square wave is on or off, a one or zero, where a sine wave (or analog signal) has unlimited variations in value....Tell me you are not an engineer?!

 

A square wave is not "on" or "off". There is a high point and a low point. Although we like to think of it as a binary condition, that is just a figment of our imagination. Between two voltage states there can be 3, 10, a 100, a billion different voltages. Two works well for us because it's reliable.

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1 minute ago, beerandmusic said:

incorrect.

 

In digital it is either on or off.  There is no value 3, 10, or 100...

It is just 0 or 1 (5v or 0v)

 

A signal may have a value of 5 (101), but that would take 3 samples.

With a digital signal there is only ONE value per sample (1 or 0, on or off)

 

To understand audio quality you have to rid yourself of the imaginary concept of "digital audio".  To begin with, square waves and sine waves are representations of electrical currents -- which have absolutely nothing to do with sound. Sound is actually three-dimensional fields of air pressure, and our perception of sound is purely a neruo-chemical phenomena.

 

When you think about digital audio, you think of bits -- pristine 1s and 0s. You believe analog concerns don't matter until the 1s and 0s turn into sound on the other side of a DAC chip. That is intuitive and makes sense -- but that's wrong. There are, in fact, no 1s and 0s, that is just your imagination. The "digital audio" in fact never free of contamination from the moment it leaves its storage location right up until it expends itself as energy pushing a transducer.

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People have to break the illusion of 1s and 0s, or they'll never get it. If you accept digital purity dogma, it's simply too counter-intuitive to understand the effects of noise in a playback system.

 

Back during the TOSlink era, no one knew why quartz glass cables sounded better than cheap plastic ones. SPDIF is digital, the bit either arrived or it didn't, what difference could the conducting material make? Well, we now know the issue with plastic cables is that they didn't have enough bandwidth (plus shortcomings in the SPDIF standard, internal reflections, etc) which lead to jitter issues. 

 

USB doesn't lack for bandwidth, and modern DACs have basically standardized a system for high-quality clock regeneration with the proliferation of very high quality VCXOs. Higher end DACs even use TCXOs. Jitter in the modern era is more-or-less a solved problem. The issue with USB is a problem that TOSlink was largely immune to -- electrical noise. SPDIF controllers still produce self-noise, but it's less of a concern due to SPDIF controllers working at a much lower speed compared to USB controllers. There is a growing awareness in the industry that DACs MUST have galvanically-filtered inputs, and you see a growing number of new products coming out with this feature, but it's still not the norm. Also, galvanic isolation transformers still leak some noise; they attenuate the noise rather than eliminate it.

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USB is an awful audio transport medium -- it's just the only option for very high-rez audio. USB was not designed for audio in mind. It's 5v line is very noisy. There is no power quality requirement, no shielding requirement and no manufacturer standard for cable impedance. The el-cheapo USB cables are designed to work reliably up to 5-6 feet, and anything more is out the window. Infested with dirty power, conducting EMI, impedance mis-matching, etc -- who cares. High quality USB cables are at least built to ensure a reliable 90 Ohms, isolation of the 5v line from the data lines, and good shielding against EMI.

 

A robust USB chain however must include some means to filter out noise from getting into the cable to begin with. The use of linear PSUs and various connector filters on the motherboard is one way (I use a PP OCXO USB controller externally powered off a 5v battery). USB conditioners and isolators are another way.

 

1. A linear PSU will generate much lower ripple than even the best low-ripple ATX power supplies. Also, linear PSUs are less likely to cause ground loops.

2. Elimination of electrical noise from the motherboard's power rails through SATA filters, elimination of fans and the like. I just power my SSD externally into a separate power circuit to completely eliminate SSD power noise from ever getting into the audio circuit. Also there is not a single fan anywhere in my audio PC.

3. Tuning of the OS for dedicated audio use -- reduces self-noise generated by the computer running uneeded tasks.

4. Use of an audiophile-grade USB controller either externally powered like mine, or internally powered from a clean source. Audiophile-grade USB controllers have advanced clocks that will ensure a very high quality transmission. A pristine clock edge means less work for the DAC controller.

5. Isolation of USB from the DAC altogether through various methods -- optical bridging, LAN bridging, DDCs, etc.

 

These tactics should go together to form a strategy. It takes time, learning, and some cost, but the gains are large and should not be overlooked. To see immediate gains right away, start with a USB filter / isolator and once you experience the SQ boost and then realize what you thought you knew about digital audio is false you will be emotionally prepared to take the plunge.

 

 

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52 minutes ago, Fitzcaraldo215 said:

Or, if you do the comparison properly - blind or double blind - you may be surprised that there is no worthwhile difference. 

 

 

 

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39 minutes ago, mansr said:

Sure, if by better you mean more expensive.

 

By better we typically mean sounds better.

 

Even if you don’t want to go through the minor trouble of buying and returning to test your belief system, could you at least make it out an audio show and listen for yourself? Audio shows aren’t the best venues for real listening but some DACs and some rooms are very impressive. Go listen to an Optologic, a Wavedream, Yggdrasil, etc, and come back with your findings. 

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48 minutes ago, unbalanced output said:

How many DACs with built-in optoisolation are out there? You're probably aware that it's limited to 192/24.

 

I can't think of a single one.

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14 hours ago, Fitzcaraldo215 said:

Agreed.  

 

You have done your tests leading you to your conclusions.  I have done my own comparative testing leading me to conclude the Regen, which you value, does not provide preferable sound into my galvanically isolated DAC.  A friend independently concluded the same thing, as I later found out.  He sold his.  Mine gathers dust, unused.  So, what else is new other than the old YMMV?

 

Can you describe your audio chain? I’d like to understand why/how you benefit from noise filtration but not from other conditioning of the signal.