"When for instance a sine wave is used of say 5kHz which is stopped at the 0 crossing what would the ringing be?"
Interesting how I've never seen measurements of this... We always see just the worst case scenario with a single maximum amplitude impulse response when as I mentioned previously, this kind of thing does not exist in real music.
So then folks, let us have a look at what a single "pulse" looks like with the different filters at 5kHz and 10kHz. Again, remember that I am measuring the waveform back using the E-MU 0404USB at 24/192 so there will be some "ringing" imposed by the ADC itself when looking at the TEAC UD-501 NOS mode especially.
Note: Ignore the phase inversion between 5kHz and 10kHz - I accidentally caused this inversion and it's not due to the TEAC hardware.
I. SHARP (Linear Phase) Filter:
Reminder of the impulse measurement:
5kHz (16/44):
10kHz (16/44):
Ringing evident at 10kHz but lower amplitude and shorter duration than impulse (ie. worst case scenario). Notice that the frequency of the ringing is the same as the impulse so we're looking at around Nyquist (22kHz).
II. SLOW (Linear Phase) Filter:
Impulse response:
5kHz (16/44):
10kHz (16/44):
III. Digital filter OFF ("NOS-mode"):
Impulse response:
5kHz (16/44):
10kHz (16/44):
IV. Minimal Phase Upsampling to 24/192 with Digital Filter OFF:
Impulse response:
5kHz (16/44):
10kHz (16/44):
V. Summary...
Some comments about the linear phase pre-ringing which some folks obsess over:
1. Lower frequencies like 5kHz essentially is associated with no pre-ringing to worry about in linear phase filters... Since the ear is most sensitive to tones from 1-5kHz, this may be reassuring. As you can see, pre-ringing does show up higher like at 10kHz.
2. Ringing amplitude also correlates with the frequency. The lower the frequency, the less the pre-ringing amplitude. For example, the SHARP filter 10kHz pulse pre-ringing amplitude is ~1/2 of the impulse.
3. The pre-ringing frequency itself remains high at around Nyquist like the impulse response so I remain skeptical that it's even audible (seriously folks, real musical recordings have a noise floor as well as complex harmonics - how anyone can claim that it's low-amplitude, high frequency pre-ringing causing any defect in the sound is a mystery to me!).
Of course, there is an "easy" way to not have to worry about the pre-ringing phenomenon in the audible spectrum... Go download a hi-res copy of the music ;-).
SHARP filter, 10kHz sampled at 88kHz:
Presto! Pre-ringing gonzo!
Note: Actually, there will still be some very high frequency pre-ringing for 20kHz sampled at 88kHz but we're talking 40+kHz ringing at even lower amplitude... I'm pretty sure I can live with this! (Again, please ignore the phase inversion compared to the graph above.)
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