Re: [stella] Supercharger tape specs...

Subject: Re: [stella] Supercharger tape specs...
From: Jim Nitchals <jimn8@xxxxxxxxxx>
Date: Wed, 3 Sep 1997 06:43:24 -0700 (PDT)
> Low level tape format:
> The optimal widths that maximize signal strength through
> the filter are:
> 0 bit: 227 microseconds (range from 158-317)
> 1 bit: 340 microseconds (range from 900-2450)
Yes, probably derived from my Supercharger documentation.

> Is the 227 microseconds correct?  That works out to about
> 4405 Hz, right?  The C64 can only handle up to about 
> 4000 Hz... seems funny to me - can a regular tape deck
> handle that well?

Cassette tapes can actually handle extremely high frequencies
better than you'd think.  Any roll-off below 8000-9000 Hz is
easily heard by aging, feeble-eared males with disposable
income :)

>  Anyway, assuming it's right, I can't
> use these optimal frequencies.  Another point of
> clarification, is this pulse width of 227 microseconds
> the whole cycle?  From 0 to + to 0 to - and back to +,
> if you get what I mean... basically, does "pulse
> widths" = "cycle length"?

These tests were performed using sine waves, not square waves,
i.e. a full cycle from 0 to max, down to -max, back to 0.

> When the C64 generates a pulse waveform, you choose how
> much of the cycle is spent at the "top" of the waveform...
> I've currently got that at 50%, I guess this works best...

> I've arbitrarily chosen new widths of:
> 0 bit : 286 ms = 3500 Hz
> 1 bit : 428 ms = 2337 Hz
> Should these be fine?

Should be wonderful, but you can definitely use lower frequencies and
improve reliability against jitter, if you have imprecise timing caused
by other sources.  Have you accounted for screen DMA and interrupts?
Vblank interrupts will definitely ruin the timing; screen DMA may do
harm to frequencies as high as you're pushing.

The "optimal" frequencies were determined only by their sensitivity
through the audio filters.  Lower frequencies pass through OK, to
allow for junky cassette players.  Dropouts and frequency distortion
were very common in those days - the format is quite robust and able to
handle a wide range of timings, as long as the 0:1 bit time ratio is 
high enough (50% higher for "1" is good, but tune for performance.)

The Supercharger CD was designed to optimize load speed without having
problems with the playback equipment.  That's why we analyzed the
audio system: to make the CD more reliable.  Square waves were not
used because the impulse response of CD player analog filters can
vary quite a bit.

Here are some figures to work with (22.68 microseconds per clock at
44.100 KHz):
                Zero    One
                7       14
                8       12-19
                9       13-23
                10      14-24   lowest amplitude needed is using widths 10, 15
                11      14-25
                12      16-27
                13      17-28
                20      23-54
                30      33-87
                40      43-107

- Jim Nitchals
Supercharger CD technical lead

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