Sound Quality in release 4.1

[rhizomusicosmos]

There may be a causal explanation for this difference in opinion between USB- and I2S-DAC users concerning LL- vs RT-kernels. 

I commend the series of blog entries at https://LeMaRiva.com about Preempt-RT built against kernel 4.14.y on RPi3B and RPi3B+ (and, yes, LeMaRiva gives instructions for building Preempt-RT on 4.14.y kernels; the patches are now a branch of the official Raspberry Pi kernel repo).

Here's a revealing quote from the end of #Raspberry Pi: Preempt-RT vs. Standard Kernel 4.14.y

Performance! (updated 26.02.2018)

I found a problem on the patched kernel, and I thing the problem is in every Preempt-RT Raspbian versions. The IRQ/39-dwc_otg process uses more than 30% of the CPU! That is too much! The interruption is related with the USB irq: A known problem of the Preempt-RT patches.

This should be a reminder that there is more in play here than simply substituting one kernel for another to lower kernel latency. It can affect a number of aspects of the system---temperature, USB, network, etc., and not always kindly.

Conclusions:

The Preempt-RT patched Raspbian kernel (4.14.y-rt) offers a solution to reduce the kernel latency (see results here). But, you lose a lot of CPU and communication performance. The data transfer over Ethernet is reduced to 34% and the CPU performance up to 12%.

LeMaRiva has some observations about temperature-throttling of cpu speeds which are relevant if one is packaging an RPi3B/B+ in close quarters.

His blog entries cover a variety of topics, are interesting, and well written. Give him a read.

Regards,
Kent

So according to that info, it appears that the RT kernel may be stressing the system and possibly affecting USB and ethernet performance. I'll check the system stats while running the tests and see what difference there is.

Thanks for that, Kent.

-Richard