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) ^300 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 -- Archives updated once/day at http://www.biglist.com/lists/stella/archives/ Unsubscribing and other info at http://www.biglist.com/lists/stella/stella.html
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