Sergei Steshenko wrote: > On Sun, 21 Dec 2008 03:34:50 +0000 > Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote: > > > > Even 2x upsampling introduces distortion. > > No, it doesn't. > > It can be implemeted as a pure linear transform with constant coefficients, > thus it would be a simple FIR LPF. It depends what you are calling distortion. An upsampling FIR has energy in the alias band between input-Nyquist and output-Nyquist, because of the finite number of taps. To avoid those you need a sinc filter, which takes infinite taps; you can only approximate it. Since you can't perfectly filter the alias band with analogue filtering after the DAC, and you can't filter it digitally due to finite taps, that is audio energy you don't want. Thus the FIR output has a form of distortion from the desired continuous waveform, which has no energy in the alias band. (There is also quantisation error distortion - you can't implement the linear transform perfectly - but let's not go there.) Non-integer ratio upsampling is not a simple linear function, as you rightly say (it's a polyphase FIR LPF), and _can_ introduce more undesired energy all over the place, depending on the number of taps and filter design. But the point is that the increased output sample rate allows potentially better LPFs both in the polyphase FIR and in the analogue domain after the DAC, compared with the fixed 2x rate, which more than compensate at reducing alias band energy. -- Jamie ------------------------------------------------------------------------------ _______________________________________________ Alsa-user mailing list Alsa-user@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/alsa-user
