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]

I am getting slowly mad in here or lowpass myself

So many real experts and no simple words for an average human being like me.
May we take a snapshot and can someone please answer me 4 questions?

1. Is sox a resampler that upsamples with leaving samples intact when upsampling from 48k to 96k
2. If it isn´t, is there one you are aware of.
3. Is the reasoning that all apply a lowpass on upsampling and so change EVERY single sample correct?
4. Does it play a role at all then to upsample with an even or uneven number?

To my understanding atm it is No-No-Yes-No

[bandpass]

Okay, curiousity got the better of me: in the 48k --> 88.2k example below, every 80th sample of the input matches every 147th sample of the output.

CODE

sox -b 16 -n 1.wav synth 10 sin 0:12k
sox -D 1.wav 2.wav rate 88.2k
sox -c 80 -r 600 1.wav 1a.wav remix 1
sox -c 147 -r 600 2.wav 2a.wav remix 1
cmp -l 1a.wav 2a.wav

1. Is sox a resampler that upsamples with leaving samples intact when upsampling from 48k to 96k

Depending on 3 things: the input signal bandwidth, the selected filter characteristics, and the selected bit-depth, sometimes yes, sometimes no.

Not checked, but I suspect that if you select -q (cubic interpolation) then the answer is always yes (but the interpolated samples will be much noisier than with band-limited interpolation).

2. If it isn´t, is there one you are aware of.

See previous post for a brief mention of some other resamplers.

3. Is the reasoning that all apply a lowpass on upsampling and so change EVERY single sample correct?

No, every sample is processed by the filter but not necessarily changed by it.

4. Does it play a role at all then to upsample with an even or uneven number?

Not especially, as demonstrated by the 88.2 example above.

Just to bring things back into perspective a little, whilst output samples being the same as input samples might indicate that a certain care has been taken in the implementation, the converse is not necessarily true, and that resampler performance for a given application is generally measured in other terms, see e.g. the graphs and discussion at http://src.infinitewave.ca