Wombat Wrote: > I did a small experiment. I created a 25kHz sinus at 24bit/96kHz. > > Afterwards i resampled it to a 16bit/44.1kHz file with a fast > resampling routine. > > You can listen the attached file for what comes out. This is how a > cheap soundcard may do it. > > Edit: Strange! The simple resample routines always leave an amplitude > at ~19kHz. Shibatas SSRC does a perfect job and creates silence on the > whole spectrum.
Yep. That's called aliasing. Sampling theory (Nyquist) states that a 44.1 kHz sampling rate can only represent frequencies up to 44.1/2 = 22.05 kHz. Sampling a 25 kHz (22+3) sine wave at 44 kHz will cause the apparent sampled frequency to "wrap around" the max frequency of 22 kHz, and appear as if it was a 19 kHz (22-3 kHz) wave. Once you have sampled at 44 kHz, a 19 kHz input and a 25 kHz input are mathematically identical. Similarly, a 30 kHz (22+8) sinewave sampled at 44 kHz will appear identical to a 14 kHz sinewave (22-8). To extend the wraparound concept to an extreme, if you sample a 44 kHz sine wave at 44 kHz, the apparent sampled frequency wraps all the way around the 22 kHz max and back to 0 Hz or DC. Makes sense, since you are sampling the sine wave at exactly the same point each time -- the sampled signal looks like a flat line. The analogy some folks like to use is old western films where wagonwheels appear to be rotating slowly or backwards. The film frame rate is too slow to represent the rotational motion of the wheel: aliasing. The apparent rotation rate (sampled) is slower than the true rotation rate. Sounds like SSRC does the correct thing and lowpass filters the signal prior to resampling. That is why you get silence in that case. This is what is known as "anti-aliasing filtering". The simple resamplers don't do that. Hope this isn't remedial DSP for too many people. -- gbreit ------------------------------------------------------------------------ gbreit's Profile: http://forums.slimdevices.com/member.php?userid=4304 View this thread: http://forums.slimdevices.com/showthread.php?t=24693 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
