Hi John, Alberto and All,

Integer rate conversion is simple on the digital side.
One would typically sample at say 64x48kHz = 3.072 MHz
(with 12 bits)

A very simple anti-alias filter will suppress the signals
around 3.072 MHz that will alias down to audio frequencies.
A digital filter that cuts at about 24 kHz will ensure that
signals at say 34 kHz are well suppressed.
One can then down-sample by 64 and get a signal at 48 kHz
with 16 bits.

> >    If you have a sound card sampling at 48khz with no anti-aliasing
> > filter, and you have a tone spurious or otherwise coming into that card at
> > 38khz the sampled data will have a tone at 10khz in its output that did
> > NOT EXIST at its input.
The highest frequency is 24 kHz. Going all the way up to 34
would give an alias at 14 kHz. A digital filter can easily
suppress by 100 dB so this should not cause any problem.

> Yes, that's right, but are you sure they don't have an anti-alias filter at
> all ? Probably, given the fixed sampling
> frequency of 48 kHz, they have just a fixed, simple RC network. If you don't
> need to switch the filter along with the
> sampling frequency, you can build it simply and cheaply with just a few
> passive components.

With a very simple filter one can expect the 3 MHz signals to
be suppressed by only 30 dB. One can expect the anti-alias filter
to have 3 dB at say 100 kHz. HIFI enthusiasts want zero phase
shift all the way up to 24 kHz.....

In a direct conversion radio this could be disastrous but it
is very easy to correct with some filtering.

With music sources one can expect signals around 3 MHz to be
really weak so it is reasonable that manufacturers save
a couple of cents by not making the anti-alias filter better.

My concern is non-integer rate conversion. A soundcard
that actually samples at 3.072 MHz and sends 48 kHz to
the PC has to be down-sampled by 1.088435 times to
produce 44.1 kHz. If it were properly done in the PC it
would be perfectly OK, but judging from my experiment,
the Windows device driver is not very accurate.

My problem: How come that the computers do not ask
"do you really want a rate conversion ?" when the user
sets a speed that will cause a non-integer conversion?

Presumably a 48 kHz soundcard would be properly converted
to 24 kHz by the PC while conversion to 22.1 kHz would
be questionable.

73

Leif / SM5BSZ

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