On Wed, Jun 08, 2016 at 12:27:48AM +0000, Kuninori Morimoto wrote:
Why would a user not want these clocks to be synchronous? A lot of CODECs will at least have better performance if their master clock is synchronous to the audio clocks so it'd be a better default, is there an advantage to not doing it?
I'm now confusing. We can set system clock on audio card, for example simple-card case, it is called as "system-clock-frequency". In my understanding, this "system clock" and above "master clock" are same clock. but "system clock" is fixed rate (= not related to audio clock in many cases). Because of this, some codec doesn't request synchronous between master clock <-> audio clock, but, some codec requests synchronous it. Am I wrong ??
A lot of CODECs can to varying degrees tolerate this but it will tend to show up in performance numbers, it's often not immediately obvious just from a listening test. Usually those that don't mind have a FLL or PLL they're using to generate the actual audio master clock at a useful rate for dividing down, or sometimes fancy digital logic to match things up.
It's not a problem to have this configurable, I'm just thinking it might be better to have the default be the other way around so that we default to synchronous but can turn that off if it's required.