Re: MP3 conversions disc for IIGS?

[supertimer@aol.com (Supertimer)]

mjmahon@aol.com (Michael J. Mahon) wrote:

>Supertimer wrote:
>
>>The file won't be an MP3 anymore.  Oversampler replaces the
>>MP3 compression with interpolation.  The file is first converted
>>to a standard wave file on the PC which is then reduced in size
>>by downsampling to 26360Hz.  On the IIGS side, Oversampler
>>then "oversamples" the file at either 2x or 4x to gain back
>>quality while maintaining a reduced file size.
>
>Interpolating to upsample adds exactly zero information to
>the output, so what exactly do you mean by "gain back
>quality"?

That is correct.  You don't gain back information you just
smooth out the result a bit.

>When downsampling, you must digital filter out everything
>above the Nyquist frequency, or half the sample frequency,
>to avoid power at those frequencies from aliasing back down
>below the Nyquist frequency and causing distortion.  Re-up-
>sampling does not undo this folding, and cannot restore any
>lost information.

Believe me, I know what you are talking about.  I had to
convince many folks right here on this newsgroup that
a 44.1kHz sample rate of a CD is designed to playback
22kHz high end frequencies.

>The only thing which upsampling can accomplish is to raise
>the frequency of the sampling noise, which, at 26KHz is
>already inaudible to 99.7% of the population--or 100% of
>the population that has attended a rock concert.  ;-)  So I
>can't see this as a viable reason to upsample in playback.
>
>>Another option would be to take the 44.1kHz wave file itself
>>and save that with the right attributes for the IIGS and use
>>Oversampler to play it at 1x sample rate.  However, because
>>the file no longer has MP3 compression, it will be very large
>>this way and as mentioned in my article, the IIGS power
>>supply has interference.  In my experimentation, I have found
>>that in order to make the 44.1kHz file sound right, you would
>>have to upsample the file to 52720Hz (2x 26360Hz).  This
>>will give you the best sound quality at the expense of even
>>more disk space.
>
>What do you mean by "sound right"?  Is there sufficient noise
>from the switching power supply (whose frequency is almost
>certainly not well controlled) that the beat with the sampling
>frequency is audible when the sampling frequency is 22.05KHz
>or 44.1KHz.  (The lowest beat frequency would be 4310Hz and
>8620Hz, respectively, but should be of relatively low amplitude
>if any care is taken in power supply filtering to the audio circuits.)
>Is ultrasonic power supply noise an issue on all IIgs's, or just
>some?

It is difficult to describe.  The effect can be noticed more readily
on a ROM 01 unit than on a ROM 03 unit, which correlates with
the electrical noise each generates.  The ROM 01 is notoriously
noisy while the ROM 03 is much quiter.  What I have noticed is
that everything sounds best at 52720Hz or 26360Hz.

>If power filtering _is_ the issue, then the addition of a 47mf capacitor
>from +5 to ground near the audio circuits should markedly decrease
>the problem in a way that would improve _all_ sampled sound on
>the IIgs in question.
>
>If you're a purist, then the fractional upsampling must be done
>very carefully (more carefully than linear interpolation) to avoid
>introducing some low-level distortion, but I suspect that most
>people playing back 8-bit sampled music on their IIgs's are not
>"purists".

This is true.  In fact, I suspect most people don't playback
digital music on their IIGS at all.  Once you start doing it,
however, it is pretty neat.  With today's hard drive space of
IIGS drives, it is certainly a worthwhile project.