Re: RAM Applesoft found!

["Michael J. Mahon" <mjmahon@aol.com>]

Andy McFadden wrote:
> Michael J. Mahon <mjmahon@aol.com> wrote:
>
> > > Maybe. It's good enough for music tapes though. And I noticed that the 
> > > Apple tape played much less loud than the new copy I produced with the 
> > > Commodore Datasette in an early attempt to get this working. Do tapes 
> > > get softer over time as the magnetization averages itself out maybe? Or 
> > > maybe Apple tapes are just by nature less loud than Commodore tapes.
>
> [...]
>
> > The alignment doesn't have to be off by much to degrade the high
> > frequency response enough to bother the Apple read routine.  I have
> > also encountered "professionally" duplicated cassettes that were
> > recorded with bad alignment--I had to misalign a deck to recover
> > the data from that one!
>
>
> You can see some screen shots of a WAV file on http://www.fadden.com/fft/
> from a piece of an Apple II tape.  The page is actually about a buggy
> FFT filter implementation, but it shows the sort of distortion that
> accumulates over time.  The Apple II relies on zero crossings, and in
> some cases low-frequency noise pushes the signal completely above or
> below zero.
>
> I don't actually know *where* the noise comes from.  It could be
> related to my decidedly non-studio-grade equipment, but it doesn't show
> up on all tapes.

It looks like ~60Hz noise, very likely from the cassette deck being
near a monitor, or from ground or ground loop noise.

I'd be willing to bet that it is not actually on the cassette, but was
picked up in transferring the cassette to a .wav file.  (It *could*
be on the cassette because of a noise problem when it was recorded,
but that seems unlikely.)  There is no mechanism for creating an
additive noise like this during storage of a cassette.

If such a signal *were* recorded on the cassette, then adding an in-line
highpass filter would remove it (as you did using a digital filter).

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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