Sound Quality in release 4.1

[TheOldPresbyope]

 . . .

What do people prefer: the RT or LL kernel?

-Richard

My experience was that LL was better.  From what I recall, people using external USB DACs tended to prefer the LL one, while people using internal i2s solutions overwhelmingly favored the RT Kernel.

Again, of course, just subjective appreciations and what I remember reading, so take it with a pinch of salt.

Best regards,
Rafa.

My experience on a Pi Zero W with I2S DAC (Pimoroni pHAT DAC) was that the RT kernel sounded best in 3.8.4.  LL sounded harsh to my ears.

In the early 4.0 betas, I found the RT kernel unusable, for whatever reason (kept getting audio stutters).

Phil

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