Forbidden64 wrote:Now I am by no means a hardware whiz here...so correct me if I am wrong/help me on this one. I checked the hardware schematic for the c64. The CASS RD line goes from pin 4 which is the edge connector...gets a quick boost from a 5v rail with a 3.3k resistor. Then there is a decoupling cap, then it just goes straight into the flag pin of the 6526. To my rudimentary understanding of what happens after that, the 6526 initiates an interrupt at that point. I guess my question would be HOW the donkey's ass does it know where the trailing edge of the signal would be??? Does the flag revert that accurately and then he just kept the interrupt going repeatedly pinging for the flag to be off? I just don't know...sounds most likely though.
The 3.3k resistor is a standard pull-up which ensures the !SRQ IN is a logic high when no datasette is connected so the IEC port works the way it should.
The 470pF capacitor just acts as a noise canceler and ensures a bit of current flows every time the logic value of the RD line changes. However, it's impact is at frequencies considerably higher than the range used by the data rate of the datasette. The overall impact of the 2 components in the digital world (at datasette bit rates) is almost nothing. This is normal since Cassette RD is tied to !SRQ IN which must be usable at IEC speeds.
There is one obvious clue though...which is that he elected to go with that particular speed, and not a faster one. From what I have read it seems to be quite reliable though. If the author is anything to go by. Rabbit stores 300k on 30 mins of tape, or 10k/min or ~166 bytes per second. Not bad for an old datasette!
I am not exactly impressed by the figures you're providing here. I think a normal Commodore save/load is around 120 bytes per second. I think Turbotape (Compute's Gazette) provides about 300 Bytes per second.
BTW I too have been fixated by the idea of making an "ultra-sette". I think the easiest way to modify the speed is just to replace the pulley wheels to be a different ratio. Just have the motor pulley use a [larger] pulley, then shrink the other pulley size down. You could even configure the pulleys so as to use the same size belt perhaps. If it comes to the amp being the problem...perhaps the ones used in floppy drives might be more useful than the ones in the stock datasette. Since they are reading data from a much smaller azimuth head.
Well if you understand my previous posts, this cannot work.
The signal the VIC-20 or C64 receives from the datasette is an analog signal processed through a Schmitt trigger. The Schmitt-trigger effectively prevents you from having any idea of the amplitude of the analog signal being turned into a square wave. As the frequency increases, the amplitude of the signal diminishes and it's reliability decreases accordingly because of the high order filter/integrator built into the datasette's electronics.
If you want to create an "ultra-sette", the first step is to alter the filter to ensure it can handle the higher frequency range required for a higher bit rate. The integrator effect of the filter boosts the amplitude of some specific frequencies and is therefore essential to the ability of the Schmitt trigger to provide a reliable signal. An "ultra-sette" equipped with a filter designed for operation at a higher frequency would therefore become incompatible with the normal Commodore protocol, probably with Rabbit, Turbotape and others as well.
I am not very familiar with the way a floppy disk RW head encodes data on this different type of magnetic surface but it seems to be a whole different ball game.
Be normal.