On Fri, Jan 10, 2014 at 11:20 PM, Fons Adriaensen <fons@xxxxxxxxxxxxxx> 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
