Re: MP3 conversions disc for IIGS?

[mjmahon@aol.com (Michael J. Mahon)]

Supertimer wrote:

>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.

The "unsmoothness" would only be at the sampling frequency
of 26KHz, and I'd be surprised if the typical sound system had
much response there--essentially acting as a reconstruction
filter.  Perhaps headphones would reveal the problem at its
worst.

Ideally, any D/A is followed by a lowpass filter with a cutoff
of half the sampling frequency, thus optimally reconstructing
the original sampled waveform.  In my own case, my ears
are a low-pass with a cutoff around 12.5KHz, so I wouldn't
notice it at all!  ;-)  I salute your ears!

Upsampling is effective at compensating for the lack of a
suitably low lowpass, as is perhaps the case in the IIgs.
(But I thought that the IIgs had a 6db/octave lowpass--not
much, but helpful.)

>>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.

Interesting.  I'm glad that the ROM 3 mostly cleared it up.

>>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.

I agree.  You probably know that a few years ago I made a PWM
software DAC for the 8-bit Apples that oversampled to 22.05KHz
precisely to eliminate the 11.025KHz "carrier" which dominates
the output without oversampling.  Perhaps the 22KHz noise in my
version would be a problem for some, if they do not "turn down the
treble".

I've thought about ways to stream audio from a hard disk for a
//e, but since the SoftDAC consumes 100% of the processor,
it isn't possible.  It isn't even easy to bank switch auxram without
getting an audible pop (as IISOUND demonstrates, since it also
uses my player).

(BTW, my Sound Editor is available for download from my web page.)

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/