Noah Beck wrote:
[...]
I examined your "file65.mpg" and it appears to have no frequency content
above 16KHz, except for a handful of odd bursts at a few specific
frequencies > 16Khz during the first 5 seconds. The "tinny" sound would
appear to be due to frequency aliasing due caused by undersampling.
The aliasing would be the cause of the apparent "tinny" sound.
What is the special significance to the 16Khz-cutoff, being that it's
exactly 1/3 of the sampling frequency? Well, if you're sampling a
signal at 32KHz, but that signal has frequency content above 16KHz, it
"folds back" around the 16KHz frequency. There seems to be just too
much higher-frequency content to be accounted for in this way in your
file. I'm puzzled by this. There seems to be more going on, involving
oversampling as well, perhaps, to arrive at this result.
-Jeff
That's interesting. The card also supports a 32KHz sample rate,
doesn't it? Is there a chance that something is using a 48KHz
algorithm with a 32KHz sample rate?
Noah
Hey Noah,
I think you're probably right, though I can't draw any conclusions about
why it is, only that it appears to have happened. I wish I was familiar
with the code, because I need an answer, too!
I'm really glad you provided samples, because that lets me know that
we're talking about the same phenomenon. What I can say, with
reasonable confidence, is that the audio in your file has undergone
sampling at 32KHz somewhere during its life. I failed to mention before
that the file itself has a sample rate of 48KHz! Therefore it should
have measurable frequency components above 16KHz (if only due to noise
in the analog domain).
That means there was an upsampling to 48KHz as the later step, unless
somebody has a really darn good 16KHz lowpass filter ;-) Even if the
source signal was 16KHz band-limited, noise should show up above 16KHz.
What was really strange were the little bursts in the first 5 seconds.
They were at narrow frequencies, and there were just a handful of them.
If I had to guess, I'd say somebody made a 48KHz mp3 and threw out most
of the high frequency components, giving the impression of 32KHz
sampling. But... that doesn't really explain the 16KHz cutoff on the
remaining ("good sounding") audio after 5 sec mark.
Perhaps these observations will give rise to some driver-level ideas
about how this could even be possible.
-Jeff
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