thing is, I know damned well I can hear no differences
Nope, it's one's and zero's either way.
Those that stream from a PC which does not have IEC958 or optical S/PDIF output will output digital files via usb to a usb DAC and these DACs do indeed "deal" with computer noise.
explain why and how, thanks
OK, I have iTunes on my PC. If I want to play music in my iTunes Library to my Home Theatre System I have four routes: Airport Express, Creative Sound Blaster X-FI HD, Network Player (OPPO-205), or usb DAC (OPPO-205). Airport Express delivers optical S/PDIF at 16/44.1 to pre-pro, X-FI HD receives usb and converts to deliver optical S/PDIF up sampled to 24/96 to pre-pro, and PC delivers WLAN or usb to OPPO-205, which up samples to 24/192 and sends to analog preamplifier. The last methods present the best sound stage, tone, and detail from the AAC, ALAC, and AIFF files in iTunes; yet, the usb connection gives me direct control of iTunes from my cell phone used as a remote, or control from the computer eliminating monitor use for excessive scrolling on my TV.
OK, but this post does not answer my question. How does a USB connection transmit noise?
The usb DAC deals with jitter. In the case of my OPPO, it virtually eliminates it. Jitter is essentially noise produced when word clocks of digital sending and receiving units are not in sync.
what the truck, dude, so your saying there is jitter that is only eliminated in your DAC... come on man, answer the question.
Todays usb DACs are designed to reduce jitter. Jitter is noise produced when the word clock of the digital sending device is not in sync with digital receiving device. This is the noise the usb DAC is "dealing" with. My OPPO has a usb DAC that deals with jitter very effectively, in fact state-of-the-art effectively for HDMI transmission, but that's another subject.
OK, so then let me ask this... does a DAC nested in the same box as the streamer experience the same jitter?
Nothing yet completely eliminates jitter. The DAC in a streamer, dedicated or PC, sends an analog signal, so there's no jitter issue, since there's no transmission of digital from a sending unit to a receiving unit. At one time though, folks just output analog from their computer. However, computer DACs were seen as inferior so usb or S/PDIF transmission to external sound cards and now high quality DACs has become popular. Jitter was an issue then, but not now.
So then you are contradicting yourself...
That's what I've always said, read my first post. The usb DACs deal with noise (jitter); but, long ago, when digital transmission via usb or S/PDIF connection from PC to receiving units was a novel process, jitter was an issue.
USB can transmit noise over the ground plane. The DAC's job is to isolate itself. This is already done with modern off the shelf USB implementations however. Amir at ASR measured a Schiit DAC that didn't do Schiit to isolate and it showed up in measurements. Especially the 8Khz USB clock.
There is no audio clock on the USB packet stream. USB supplies it's own clocking for it's own packets. When packets hit the DAC the DAC decodes the packets, reads the clocking information for the audio it's receiving and reconstructs the audio clock and applies it. The more accurately it can do this the lower the Jitter.
And the reason for this is that the connection was synchronous and both end points had to establish a clock together. If one side was a bit off in it's timing you had jitter. USB is asynch in this regard. So is Ethernet.
Is this noise eliminated if the Dac and streamer are both in the same box?
It has to be. But there is still a databus feeding the dac portion. A streamer for all intents and purposes is a computer. It has a CPU, it has volatile and non-volatile storage, it has I/O and many are based on the Linux OS.
