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Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

RE: USB clocking for 44.1k?


The best way of thinking about the whole situation for USB Audio is that the USB System is a conveyor belt that sends one Bit-bucket every millisecond (USB 2.0 Full Speed) or 125uS (USB 2.0 High Speed).

Now that Bit Bucket comes with an address where it goes to and 80% of all buckets are reserved for "isochronous" transfers, in other words audio & video. A Bit Bucket may be empty, may be halve full or may ven only contain 1 Bit of "payload", whatever is needed to get the correct amount of bits across the conveyor.

The USB conveyor belt runs on it's own clocks and timing. When sending data to a USB Audio Device the buckets are filled with a bunch of bits every time the USB Audio devices requests a transfer. The USB Audio Device contains a data buffer and requests enough bits in enough buckets from the PC side to keep the buffer from running empty.

On the USB Audio Device the local audio clock will then be used to transfer the data from the buffer to the DAC. So in this case you have two clocks, asynchronous and (with perfect systems) without cross contamination or interference.

The USB clock matters to the USB system (too much jitter and data corruption may happen or the receiving system works harder), the audio clock matters to the Audio system (too much jitter and audio performance is degraded).

In practice beat notes between these two clocks may appear and power supply contamination may cause cross modulation, non-zero ground impedance may cause cross modulation and so on. So for audio devices there are plenty of chances for things to wrong, even if all bits arrive safely (which should not be taken for granted).

What all the various USB repeaters (I prefer this term over the marketing stuff) do is to take a USB signal that has been degraded, re-synchronise the clocks, restore the waveform and otherwise make the signal "like new" and all prior degradation is cancelled. As we are dealing with "digital" signals, such a process can be 100% lossless and can be repeated many times (in theory ad infinitum).

For a fun demo we take 4pcs of fully USB standard compliant USB A-B cables, plug an iPURIFIER 2 "A-Type" at the "B" end of each cable (and one iPURIFIER 2 "B-Type" at the DAC) and stream 768kHz/32Bit Audio perfectly with zero dropped packets...

Remove just one of the iPURIFIER 2 and the systems drops back to USB 2.0 full speed (12Mbps) and thus 96kHz maximum sample rate. Remove more and even at full speed we see dropped data. Remove all and the system does not connect even in USB 2.0 Full Speed mode.

Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

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