First… What is a “Transport”
Basically you have your music on the cloud or on your PC/NAS. If it’s on the cloud , you will use your smartphone to access your account and play. From your smartphone, in order to send the music to your DAC (DACs have USB and/or S/PDIF inputs) you need a transport that will be visible (to your smartphone / tablet /PC) as an Airplay device (iOS ) or UPNP/DLNA (Android/PC).
These transports have different outputs. USBridge is feeding your DAC through USB and DigiOne is using coaxial/bnc. Depending on your DAC inputs you need to make a choice.
Every DAC needs a source. For USB input there’s the USBridge. (DigiOne is for S/PDIF).
So why not use a cheap laptop, RPI or even Sparky as a USB transport? Well you can, but they all have some limitations.
First, laptops are noisy… very noisy. On USB we usually see spikes at 100mV of noise. That will have an impact on sound system.
How about RPI? RPIs are great and very energy efficient, but on USB they have 2 major flaws. First is that bandwidth is shared between the 2 USB ports (so in fact there is only one USB), and second is that ethernet is also shared on same bus! Of course, you also have the noise on USB at about 60mV.
Maybe Sparky? The 2 USB ports (next to the ethernet port) share one input to the CPU (split by a hub). Noise on USB bus is about 27mV (pretty good) and ethernet is completely apart (bus is not shared). So it’s ok, but not great.
So we developed a new board that is connected to the bottom of Sparky. Sparky has 2 independent USB controllers (2 highways), and we connect to the second one. Then we use a new USB IC that recloaks the stream using hi quality NDK Oscillators, and everything is powered by independent LDOs and hi frequency filters. The final noise of the USBridge is lower than the noise of a battery.
What Allo has designed and achieved is a true USB output for audio.