Focusrite Scarlett 18i20

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Havoc
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Re: Focusrite Scarlett 18i20

Post by Havoc »

Came because I'm interested in swapping my FA-66 for such a scarlett. Read the whole thread but I still fail to see what the clock source would have to do with this problem or how external clock would solve this. There should not (and cannot) be a relation between the pc and scarlett with regard to the sampling clock. This isn't spdif of adat. If you don't get xruns, the sound should be free of clicks. Only issue might be that the clock isn't set to internal.

I know it is a silly idea, but have you tried setting the periods/buffer to 3 like you would do for a firewire interface? Could you try with a live cd of another music distro?
cowboystitching
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Re: Focusrite Scarlett 18i20

Post by cowboystitching »

Yes, I've most definitely tried setting the Buffer to 3. The clock has absolutely everything to do with the problem; the annoying clicks are clock jitter at 96k. Trying to run this thing at higher sample rates without the proprietary mixer app that Focusrite makes is the cause of the problem; though I would be lost to tell you exactly what the technical reasons for this would be. (Though I'm guessing all ALSA does is recognize inputs and outputs with the class compliant driver; if you run alsamixer with this unit you get a message saying that there are no controls for the device. If I remember correctly you aren't able to actually use this thing as a soundcard on it's own under Pulseaudio, but why would you want to? This thing is for recording, and for that we have Jack.) When you run it with MixControl on a Windows or a Mac you are able to change the sample rate through their application, and magically everything works. There was considerable discussion on the Focusritre Linux blog about adding in a driver that is there to specifically give more control over the unit in alsamixer (and the other interfaces in this series) with regards to things like routing audio inputs and outputs under ALSA, changing sample rates, etc. I guess the guys working on that haven't finished it, so that whole idea is kind of in limbo sadly....

I also mentioned trying this with a couple different distros (also various kernels, it seems that only 3.11 onwards even recognize this device in ALSA) and the problem persists in exactly the same way.

The interface is set to it's own internal clock; I know this because there's a little indicator light on the front of the unit that tells me so. :)

Like I said several times earlier, the only workaround I have personally discovered is to boot up into a Windows VM and manually change the clock source from "internal" to "spdif" and plug in an external time clock, and then set that device to your desired sample rate. Magically the problem disappears! This is definitely not a particularly efficient method of working, but if those high sample rates are absolutely necessary and you're dead set on using this unit, then it's the only way I've found to make it work. If somebody can figure out how to get rid of the clock jitter at 96k without having to do this I'd be very very happy to see how it is done.
Musicmaker
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Re: Focusrite Scarlett 18i20

Post by Musicmaker »

Running 18i20 with Ubuntu 15.10 out of the box. Set Sampling rate in windows at 96Khz - since in Linux there is no control for this (see amixer) which is a real pity. Booted Ubuntu Qjacktrl , with 96Khz. Pulse Audio Jack bridge to pump PC browser audio (e.g Youtube) to the 18i20 headphones.
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At 2048 I don't get XRUN's but Ido hear some very short, not that very loud, audible clicks. At 4096 everything seems fine. 88Khz works ok with 1024. Not sure about that high latency will be a problem for anything ?
tnovelli
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Re: Focusrite Scarlett 18i20

Post by tnovelli »

Musicmaker wrote:At 2048 I don't get XRUN's but Ido hear some very short, not that very loud, audible clicks. At 4096 everything seems fine. 88Khz works ok with 1024. Not sure about that high latency will be a problem for anything ?
Yeah, 85ms is awful. You want less than 5-10 ms for overdubbing or realtime FX stuff like Guitarix. I'm running my 18i8 at 128-256 frames, 1-3 ms latency, only a few xruns. This is with a lowlatency kernel, cpufreq governor on 'performance' mode, on pretty fast desktop. On my laptop I have to double those numbers.

P.S. check out qasmixer... (in the KXStudio PPA, IIRC)

P.P.S. make sure you're using a new enough kernel... 3.19.x for 18i8 mixer support; probably the same for 18i20.
cowboystitching
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Re: Focusrite Scarlett 18i20

Post by cowboystitching »

Those clicks you're hearing is the clock jitter I described earlier. The only real workaround for this is to do that witchcraft with an external clock and running the proprietary mixer app under a VM. Or just run it at a lower sampling rate. Mine still runs great at 48k.
Musicmaker
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Re: Focusrite Scarlett 18i20

Post by Musicmaker »

Does this mean thre18i20 cannot be set to a sampling rate and will default to 44.1 K under Linux even jack's irate is set differently ?
in lsusb of the 18i20 there is this:

AudioControl Interface Descriptor:
bLength 10
bDescriptorType 36
bDescriptorSubtype 11 (CLOCK_SELECTOR)
bUnitID 40
bNrInPins 3
baCSourceID( 0) 41
baCSourceID( 1) 42
baCSourceID( 2) 43
bmControls 0x03
Clock Selector Control (read/write)

If above description is indeed the sampling rate setting, All that needs to be done is making a change in the ALSA driver to let the clock rate setting show up in ALSAMIXER ? (Right now it is possible to select the source but not the clock rate in ALSAMIXER)
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