Sample rate conversion Options

[MannyCalavera]

Hello everyone.

I have a question regarding sample rate conversion algorightms.
I don't know how they work, but I guess you guys are more familiar with this

Let's say I have a 48 kHz wave file that I want to use in something I have to mix
with some 96 kHz material at certain places. This requires the 48 kHz file be resampled
and in the end back to 48.

So in short: does resampling 48 --> 96 --> 48 change the original or can the original
be restored 100% ?

I'm using Wavelab for this.

[Wombat]

after 2× band-limited interpolation, alternate output samples are exactly the same as the input samples.

I still don´t get how any sample can be exactly the same when a lowpass is applied to it in the output!? I hope i only need a small hint

Edit: Added pic of source 48k and upsampled. How can any sample be intact?





This post has been edited by Wombat: Apr 14 2011, 20:55

[bandpass]

I still don´t get how any sample can be exactly the same when a lowpass is applied to it in the output!? I hope i only need a small hint

Because (by definition) the lowpass doesn't affect frequencies below its cutoff frequency; if all the frequencies in the input signal are below the cutoff frequency, and the filter is implemented in a 'perfect' way, then the signal is unaffected. E.g. an input signal comprising a sine wave swept from 0-22kHz has no frequency component at or above the cutoff (~23kHz), so is unaffected.

Whilst half-band filters may leave samples unaffected, other FIRs may too—sox's filter is not half-band (a half-band upsampling filter always has some imaging remaining).

Is there any sample intact? And how to tell?

Using the 'code' I provided above; it uses a little trick to throw away the interpolated samples from the 96k file thus rendering it back to a 48k file (without filtering, but that's okay in this instance), which is then file-compared to the original. The files are identical barring a few samples at the beginning and end (which are the result of filtering discontinuties).

[Alexey Lukin]

Because (by definition) the lowpass doesn't affect frequencies below its cutoff frequency;

It does! — by definition of the cutoff frequency (-3 dB point).

Whilst half-band filters may leave samples unaffected, other FIRs may too—sox's filter is not half-band (a half-band upsampling filter always has some imaging remaining).

For other filters it's highly unlikely that they will preserve the original samples (which is not a flaw).
However SoX may well be the half-band filter (unless ordered a shifted cutoff frequency).