Hi All, I thought that I would post why a cheaper RPI home kit for pure streaming is not automatically better value-wise than a store-bought solution. In this case it's an argument for a Pro-ject S2 Ultra Streamer according to the Ultra's co-designer: John Westlake
Here's his case:
In answer to “why the extra cost” over the standard RPi3 – it’s required to give a basic explanation of the RPi3 weakness WRT high end audio applications (remember is designed as a lowest cost computer platform NOT a highend audio streamer). These weakness in no way are a criticism of the RPi3 – but rather its design limitation from my perspective as an audio designer. Basically its weakness are all based on Clock Phase Noise (Jitter) and local PSU noise (which directly impacts the Jitter performance) and also the resultant local RF emissions:-
1. The USB HUB / LAN IC is clocked by the CM3 SoC which has significantly higher jitter then an external Clock oscillator. More detrimental is that fact that clock phase noise is heavily correlated with the SoC software processing – resulting in Data correlated jitter.
2. The USB HUB / LAN IC PLL clock circuits are powered from the switching supply rail that also powers the SoC Memory, so as data is “processed” though the memory this causes modulation of the PSu Rail directly correlated with the Data patterns which then modulates the critical PLL in the USB HUB / LAN which is used as our USB “source” via this noisy PSU rail.
3. As one would expect little care has been taken with the quality of the 5V rail for USB DAC power – in fact its really pretty poor by “Audio” standards.
4. There are multiple “free running” switching regulators on the RPi PCB – whose clocks are not related to the USB clock Rate so there switching products are free to induced multiple / complex RF and Ground noise spurie in the system. I could list more weakness, but you can start to appreciate the “Deficiencies” (not unexpected) of the simple Rpi3.
1. The little ProJect streamer tackles these issues by making the USB 24MHZ clock the system “Clock master” and cleanly generating the LAN’s 25MHz clock and themultiple local switching supply’s operating from a divided down synchronised frequency of this 24MHz.
2. Great care has been taken with the internal PSU’s – I count 8 low noise PSU’s of the top of my head.
3. The USB host signal from the CM3 is RF filtered and USB “Detoxed” (the poor Phase noise of the CM3 USB Host controller) is attenuated by two Cascoded USB Hub bridges (and RF filter) resulting in a much attenuated USB Jitter on the “DAC” USB output.
4. The DAC 5V feed has a very low noise regulator – whose voltage and current can be monitored via the streamer software.
5. The DAC 5V USB voltage output can be disabled via software for DAC’s that don’t require 5V.
6. An external USB bypass to the DAC USB output is proved, this bypass “Detox’s” the external USB signal in the same manor as applied to the RPi CM3 USB Host output.
7. 16GB onboard eMMC is available for OS / software & local files. Sadly, all this makes for an expensive hardware solution – with “low volume” production costs are factored (ProJect will manufacture thousands Pcs – not millions) – the end selling price is just a fact of European low volume manufacturing – this is not China manufactured mass market pricing…
"Sorry for the errors in the published documentation / marketing – no text is run past me for proof reading…"
