Re: Frequency/Sample rate

Industrial One <industrial_one@xxxxxxxxxxx> wrote:

Most audio files on the net are recorded at a 44 KHz sampling rate,
but it's mainly referred as "frequency."

while 'frequency' is true of any periodic function
(as noted in other posts), 'sampling rate' is much
more precise. Essentially, the sample rate is the carrier
of the information in the waveform.

Now, humans can only hear up
to 20 KHz, so why would audio be recorded at 44 KHz (twice the audible
hearing range?)

technically, any waveform requires at least two samples per wave.
I say 'technically', because to fully replicate the wave, with
all of its harmonics, takes many more samples than two.

at two samples, for example, you really cannot tell
whether the waveform is sine, square, sawtooth, or whatever
variant the harmonics may imbue to the waveform. Even
at four samples, the original waveform is only approximated.
(i.e. sine can look square or sawtooth depending upon
at what degrees of arc the sample is taken.)

that said, few humans can detect the difference between
a sine, square, or sawtooth above 10k. But there is
a difference. How we as humans interpret that difference
is not easily quantifiable--some may speak in terms of
clarity, crispness, 'air', 'musicality', or what have
you--not very useful terms to the engineer.

As well, sampling at a fixed frequency any other
frequency pattern will result in sampling-induced
harmonics, non-musical, that are in fact lower
than the fundamental of the wave. the amplitude
of these harmonics reduces greatly with increased
number of samples per wave, so at under, say, 5K,
they are not noticable, when sampling at 44.1k.

finally, the lower the sampling frequency, the
increased number of artifacts created when resampling.
So, if one recorded some cuts at 44.1 and others at
48k, the movement back and forth to digitally (or analog)
mix will have the effect of altering the higher-frequency
waveforms in progressive generations of resampling.

the bottom line is that sampling at 44.1k has
a noticable and significant impact on all waveforms
over 11k, and some modest impact over 5.5k.
Impacts increase with successive generations
of resampling (any time bit rate changes, and
in particular, compression.
Measuring whether humans can detect such a
change is torturously difficult.

Sampling at 96k raises the affected frequencies
to the top of the hearing range, and 192k well beyond.

to me, the greatest benefit of higher sampling rate
is in the recording and mixing stages, where the higher
rate eliminates any detectable resampling distoration
(detectable=by a scope of by a computer image of the wave).

prior to the 96 and 192 rates, i found that recording
and mixing all within 44.1 resulted in less resampling
bias and artifacts than switching between 48 and 44.1.

Obviously, one can notice the difference if the song
was downsampled to 22, so why not coin the standard frequency at 22
KHz instead of 44, why is the number doubled? Also, just where the
hell did the number 44,100 emerge from? Why not 40,000?

again, sampling rate is NOT audio frequency.

Nowadays, DVD-audio songs are recorded at 96/192 KHz, is there a
point?

again, recording at 96/192 may be a good idea,
but it is definitely not required for playback.

And if this ain't the case, why would the sampling rate be called
"frequency?"

because the frequency of a sine waveform involves a
continuous function that goes both positive and negative
during one complete 'wave'. There are generally two
zero crossings during a wave. Let's say, for a moment,
that I sampled the audio energy at each zero
crossing--exactly two samples per waveform, or at exactly
22.05 KHz. Then, upon playback, i iterpolate a
straight line between the samples. What waveform
would result?

-steve
.

Relevant Pages

  • Re: Frequency/Sample rate
    ... of the information in the waveform. ... sampling at a fixed frequency any other ... than the fundamental of the wave. ... increased number of artifacts created when resampling. ...
    (rec.audio.tech)
  • Re: Analog Sampler
    ... A homebrew sampling oscilloscope would be an interesting project. ... I have been thinking that the ADCMP582 (fast latched comparator) would be an ... the waveform is repetitive, then sooner or later you can figure out exactly ... The other VCXO would be squared up and ...
    (sci.electronics.design)
  • Re: Simple sampling question
    ... I suggest you Google on the terms "bandpass sampling" and/or ... The sampling waveform can be viewed as ... When you do the frequency convolution for your case, ... When you slide 1Hz, there's an overlap and thus an "output" from the ...
    (comp.dsp)
  • Re: Analog Sampler
    ... I have been thinking that the ADCMP582 (fast latched comparator) would be ... if the waveform is repetitive, then sooner or later you can figure out ... the hard part as I saw it would be to generate the sampling pulse ... of a binary counter (also clocked by the comparator sampling clock VCXO) ...
    (sci.electronics.design)
  • Re: Analog Sampler
    ... A homebrew sampling oscilloscope would be an interesting project. ... the waveform is repetitive, then sooner or later you can figure out exactly ... One VCXO would be squared up ... the counter binary output would in turn drive the DAC that provides the ...
    (sci.electronics.design)