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USB audio cracked... finally!


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9 minutes ago, manisandher said:

Peter now seems to know how to configure a USB cable to voice the sound from it. We are no longer in the realm of trial and error.

That's great but I expect he will retain this information & we are none the wiser as to what changes in the USB signal translate into SQ changes?

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26 minutes ago, One and a half said:

I doubt a cable can suppress noise from the PC, since the prime purpose of the cable is provide little to no loss in the transmission. The OP would prefer the mix of noise from the PC and how it interacts with the DAC, to that end, I have no problem with. 

 

For USB, I'll stick with isolation/PC Card treatment methods to block leakage currents from the PC in combo with a cable that can provide reliability.

 

7 minutes ago, Speed Racer said:

Agreed. USB cracked is to not use it at all......

Really? no USB cable can suppress noise? Hmmmm

http://kgs-ind.com/wp-content/uploads/products/pdf/LNC-Noise-Suppression-USB20-Cable.pdf

 

 

USB cable noise.jpg

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46 minutes ago, Speed Racer said:

I still don't see where it says that cable suppresses noise put on the USB lines by the computer BEFORE they exit the computer. I know why...they don't! That cable is only effective IF there is EFI affecting the sound quality. Other cable can certainly block EFI as well.

 

33 minutes ago, elcorso said:

 

Of course they are... The problem they (sometimes) block musicality too.

 

Roch

 

PS/ I don't own (but like to try)  the "Lush"...life.

There you go!

So who's to say that cable configuration & dielectric material won't effect noise on a USB cable?

 

I can  tell you that the LNC cable above also sounds lush because it cuts off some HF (as well as noise hash)

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37 minutes ago, pkane2001 said:

 

Does that mean that a HF cutoff filter in the digital USB transmission results in HF rolloff in the analog domain? How does that work?

As I said above

 

1 minute ago, mmerrill99 said:

Noise which gets through & how it interacts with the digital to analogue conversion is the final question that needs to be investigated & its action mechanisms determined

 

It may be that the USB signals withing the USB microframes (talking high speed USB 2 here) are essentially square wave electrical waveforms being communicated via the USB signal wires. These squarewaves have a risetime & falltime - anything which changes the shape of these signals from their optimal shape may well have an effect within a USB DAC - the exact mechanism of operation is still unknown 

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16 minutes ago, jabbr said:

 

Hi Mani, really interesting. What is the "USB audio spec?" Is 90 Ohm impedance not optimal? Or is it something else?

 

90 ohm is the USB spec specified between USB D+ & D- signal lines but in reality it is more important to ensure 45ohm impedance from each USB signal line to ground & this is not specified, AFAIK?

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19 minutes ago, pkane2001 said:

So the problem is noise. Be careful filtering noise out using a HF filter on USB transmission -- that will also destroy the shape of the square wave by smoothing out the corners, resulting in exactly the effect you are trying to eliminate.

Yes, bandwidth limiting that affects the signal shape seems to have an audible effect - this includes USB cables which are not of sufficient bandwidth.

 

If we consider USB high speed is 480Mbps or 480MHz fundamental frequency & to represent a half decent square wave requires at least 3 harmonics (some say 5 harmonics) we have a requirement for passing 480MHz X 3 = 1.44GHz bandwidth requirement for a cable or if you want to consider 5 harmonics necessary for a good squarewave then 480X5 or 2.4GHz bandwidth cable is necessary!

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23 minutes ago, scan80269 said:

I suspect there is a fair amount of unavoidable periodicity in this isochronous transfer mode.  For example, packet rate is 8KHz.  In contrast, the bulk transfer mode, by not guaranteeing transfer bandwidth or latency, may actually be less periodic in the way the data fly over the cable from sender to receiver.  Is it possibly what Peter has figured out, by making his DAC support only USB bulk transfer mode, along with a deep FIFO data buffer to smooth out the extreme irregularities of USB data arriving at the DAC?

This would require a new software driver being written so I doubt that is the approach.

The original Hiface used bulk mode USB transfer, btw

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2 hours ago, pkane2001 said:

Scan, can you please elaborate on the 'embedded clock technology'? I know how a clock is derived from an S/PDIF data stream, but how is WCLK embedded in (and derived from) the isochronous USB data packets?

If I can answer for Scan - I believe what he means is not that the audio clock is embedded in the USB data packets but rather that the USB specification states USB frames are sent every 1mS & high speed microframes are meant to be sent every 125uS i.e 8 microframes per 1mS frame or the 8KHz periodic current burst sometimes reported. There is a certain +/- tolerance on these timings in the USB spec. The PC's USB clock usually times this but in asynchronous USB the USB receiver's local USB keeps it's local buffers half full by sending back signals to the PC about how much data should be in the next microframe.   

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1 hour ago, jabbr said:

 

It is perplexing that despite purported improvements in USB isolation there are still ?dramatic? differences in the "sound" of USB cables? I'd think that the goal of USB isolation/reclocking/regeneration is to eliminate the vagarities of cable impedance etc. @PeterSt : I would really like a technical explanation if feasible.

Yea, it's perplexing that there can be audible differences - I wouldn't call them dramatic, though.

 

One of the problems is that with a USB cable change there are a number of variables being changed at once

- impedance & it's variability along the length of the cable

- cable bandwidth

- ground noise

- shield strength

 

If only one of these variables was changing between cables, we might be able to get a handle on what's going on?

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19 minutes ago, jabbr said:

My understanding of USPCB is that great care is taken to conform to the USB specification.

Yes but is it due to other factors that cables suffer from - I don't know, I'm just asking ?

- variable impedance along the cable length

- shielding (how is the USPCB shielded)

- grounding - what is the grounding config?

- 5V supply - how is this configured on the pcb?

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7 minutes ago, scan80269 said:

Good (or bad) implementation can easily make a larger difference than the type of transport interface.

Agreed & it appears that once certain issues are adequately addressed in USB audio, it can be one of the best sounding audio interfaces.

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28 minutes ago, pkane2001 said:

That's right. And that's why I asked the question. If the timing of the actual audio samples is driven by an on-board clock, how does the timing error in the USB packets affect this clock? How does the error in timing due to a poor 'eye pattern' or HF rolloff of the square wave edges result in anything audible after the samples are collected in a FIFO buffer and then doled out using the onboard clock? This question is important in explaining how a USB cable might or might not affect the analog domain.

Your question suggests that a FIFO buffer & onboard clock are ideal, isolated functions not influenced by any noise which may be happening within the USB audio device as a result of signal integrity issues at the USB receiver stage. This is unlikely to be the case & could be a possible way that SI issues become internal noise issues within the system

28 minutes ago, pkane2001 said:

Of course, if timing errors and square wave distortions are not the issue, and instead leaked noise from the PC side is the problem, then perhaps a better isolation between the PC and the USB receiver would eliminate the need for fancy cables. 

It may also be a noise issue & cables can also be responsible for different noise profiles

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18 minutes ago, pkane2001 said:

 

So, are you saying that USB cables improve SQ by reducing noise in the digital transmission?

 

Seems to me that a USB cable is not the best way deal with noise, and that there are much better ways to stop it from entering DAC. Galvanic isolation being one of the obvious ones. 

No that's not what I said - you need to read my post again.

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3 minutes ago, pkane2001 said:

Thank you, Scan! I think you and I have very similar understanding of how these things might work. With proper isolation and circuit design to separate USB receiver from I2S output, all USB cables/regenerators/isolators should begin to sound the same. 

That's not what he said, either.

Unfortunately, what you just posted is not the reality.

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2 minutes ago, pkane2001 said:

 

Sorry if I misunderstood. Please explain how "cables can also be responsible for different profiles" is different than cables improving SQ through noise reduction. Or are you saying that USB cables can only destroy SQ?

 

That was just one bit of what I said & not the totality - it cannot be summarised into what you said in your post - that's a misleading summary.

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1 hour ago, pkane2001 said:

 

You appear to have a reading comprehension issue. This is what Scan said:

 

This is what I said:

 

Where do you perceive this large difference?

The difference is in where the isolation is implemented - now go back & read what you quoted of his post & spot this difference with what you posted.

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5 minutes ago, Speed Racer said:

I can't take any of this very seriously as USB cables don't subtract noise. All they can do is alter it.

If altering this noise tenders it inaudible, is it not equivalent to removing it from audibility?

 

Anyway, cables can reduce noise as I showed with the noise plots of the Lnc cable

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14 minutes ago, Speed Racer said:

 

The LNC plots showed that the reduce noise caused by EMI.  What if there is not noise caused by EMI?? What if all you have is noise put on the cable by the computer itself? 

Why would emi noise not arise from the computer itself? But you said USB cables don't reduce noise, you didn't say don't reduce noise which isn't emi - what noise are you talking about then?

 

More correctly those lnc plots show noise at high frequencies are reduced - emi, RFI - it isn't specified

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13 minutes ago, Speed Racer said:

 

The shielding reducing the EMI noise. So it would be external sources of EMI that would otherwise pollute the signal on the cable.

It's the ferrous impregnated dielectric that affords the reduction in high frequency noise, not what is typically called the shield in a USB cable.

 

It reduces both the intrusion & the emission of RFI.

 

Could this change the noise profile reaching the receiving end of the USB cable?

 

Does it matter how the noise arises on the cable?

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34 minutes ago, Speed Racer said:

 

Intrusion from where? Outside the cable. Emission from where? The cable. That does not mean the cable reduces any noise the computer injects into the cable via the pins in the connector. It just means the cable reduces the noise RFI would otherwise add and does not emit as much RFI as (some) other cables.

Intrusion of high frequency noise from within the computer - usually from the many smps buck converters used in their PSD. 

By reducing emissions it could stop the RFI from polluting the USB shield, preventing SCIN - shield current induced noise?

These are all possibilities - I'm not saying they are set in stone but they counter your started view that USB cables cannot change the audible sound.

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35 minutes ago, fas42 said:

 

Okay, this looks like another case of using a semiconducting layer - the ferrous impregnated dielectric - perhaps to perform some 'magic'.

 

Perfect insulators are not always the best answer - a little bit of conductivity goes a looong way ... ^_^

I wouldn't call it magic - i know you are aware that it's a well known property of ferrous material, clamps, etc. so there's no unexplained mode of action. Just saying this in case readers think you are stating that this is some unknown audiophile 'magic'

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