[LAU] Focusrite Scarlett 18i8

[James Stone]

On Fri, Jan 10, 2014 at 11:20 PM, Fons Adriaensen <fons at linuxaudio.org> wrote:
> On Fri, Jan 10, 2014 at 10:48:15PM +0000, James Stone wrote:
>
>> > Do you mean the spectrum analyser showed a trace at -90 dB ?
>> >
>>
>> Yep - the spectrum analyser showed a trace at around -93dB across all
>> bands. Is this meaningful?
>
> It means something... but what it means is another question.
>
> Assume
>
> * the noise is more or less white (if not: real trouble)
> * the spectrum analyser uses an 1024 point FFT (typical),
> * and a raised cosine window (also typical),
>
> Then the full bandwidth (1/2 Fs) is covered by 512 filters,
> and each of these has a bandwidth that is 1.5 times that of
> a rectangular filter.
>
> So each filter sees 1.5 / 512 of the total power.
>
> 10 * log10 (1.5 / 512) is -25.3 dB
>
> So the noise level would be around -93 + 25 = -68 dB.
>
> You could check this with jnoisemeter.

It's reported as -81dB on line-in with jnoisemeter.

The mic stage when turned right up (for my cheap SM57 copy) has
background noise of -65 dB (with mic still on, and quite a bit of
background noise, so not a great test), when the mic is switched off
(but still connected) it is -95dB. In practice, it doesn't seem that
noisy to my uneducated ears, but I guess others will have different
opinions.

The one thing I would say is that I am not that impressed with
realtime performance overall. It is really only stable down to about
128/2. If I use Mixbus, I have to use 256/2 on jack. According to
jack_iodelay this is around 27ms round trip. However, despite this, I
haven't really noticed any audible delay that makes recording
difficult. 128/3 seems better - around 15ms round trip according to
jack_iodelay.

With a realtime kernel - 3.6.4-bbq-rt-rt10, I can push jack right down
to 64/2 - (approx 10ms round trip according to jack_iodelay) but this
leads to xruns with minor stresses on the CPU.

James