Data rates and capacity - cassette vs disk

[fibrewire]

Can someone get me some accurate specs regarding the 8-bit program recorder? I see 600bits/sec which is about 4.5KB/min? Is there any way to boost more data per minute of tape without hardware modification (other than the obvious Side B)?

 

I can't believe that 30 minute of tape holds the same amount of data as a 1050 enhanced density single sided disk... If it weren't for the audio feature they would surely go in the trash...

[Rybags]

600 bps = 60 bytes (start/stop bits mean 10 bits per byte)

60 * 1800 seconds = 108,000 bytes.

 

That's assuming one huge block of data, no IRGs which generally doesn't happen. I did a custom SIO mod that allowed big block saves at a higher data rate. You can generally push it to a higher bitrate, from memory maybe a touch over 800 bps without mods or reliability issues.

 

IRG wastes about 1/4 second - the benefit of getting rid of them is somewhere around 13% Also, getting rid of IRG means the other overhead of sync bytes (2), control byte and checksum (2) is reduced.

 

Traditionally tapes have usually held more than disk. The old mainframe computer tapes were around 350 Meg for a 6250 BPI tape of 2400 feet.

When they went to single reel cartridges the capacity was slightly less but hardware compression bridged and exceeded the gap.

 

Not sure what todays tapes hold but I've seen the single reel ones that are essentially slightly smaller volume than a VHS tape that can cram somewhere in the order of 40 Gig, today is't probably even more.

 

Atari OS supports a range of bitrates (for reading), although the top end is probably very optimistic. In normal operation though the anticipation is that only motor speed and tape stretch were to be the reason for drifting off spec.

[fibrewire]

Looks like there are modifications to increase the baud rate. The Rambit mentioned here boosts the baud rate to 3600bits/sec, an increase in storage capacity per minute of tape from 4.5KB to 27KB. Are these more susceptible to errors? Can the baud rate be increased even further?

[fibrewire]

Found the Cassette Handler section on AtariArchives here

 

thats a long 1800 seconds to wait for 108KB, but with IRG its about the same storage space as one single sided single density disk?

 

Also, if the Atari 800 Pac-Man were loaded from tape, it would take approximately 120 seconds to load 8K from cassette?

Edited June 12, 2013 by fibrewire

[Rybags]

A slowdown factor that can't be avoided on standard OS - the tape handler ignores about 5 seconds of leader regardless of how you've saved the program.

 

I had Assember/Editor on tape with a special loader + 8K long single block. From memory it was under a minute to load.

 

FSK encoding means there's a fixed limit on what can be done - you need sufficient part of the waveform to be able to be read so the electronics can differentiate between a 0 and 1 tone.

 

With a more direct way to read tapes possibly better results could be had. Problem is the electronics in the tape unit are used for FSK decode so there's fixed standards which can't be avoided.

 

Something like Manchester encoding might be one idea. It works by using a standard length overall waveform and the delay between 1->0 transition determines if a bit is a 0 or 1.

In a sense it's FSK anyway - it's just that Manchester doesn't repeat the waveform.

The other thing is start/stop bits - stop bits are sort of redundant anyway. Start bits could be reduced to only 1 per 2 or more bytes for some saving.

 

But all this means hardware mods. The other factor is that a bootblock or two is needed as well. So you get stuck with that standard OS delay of 5 seconds or so, plus the extra 2 or so seconds per record needed for the boot.

Edited June 12, 2013 by Rybags

[fibrewire]

Cassettes were a great medium for homebrew, the only real drawback is the time to load anything, and the severe limitation on space. Computer tapes were often limited to 10 or 15 minutes of tape in the early 80's

 

eBay Auction -- Item Number: 140995008136

 

Does it make sense to release new games on cassette, or are most new software titles for the 8-bit too big?

[Rybags]

IMO it's a pointless media type in the modern day especially. Anyone calling themselves an enthusiast will have at least a disk drive and possibly SIO2PC/APE, SIO2SD or equivalent and possibly an IDE class device.

[thorfdbg]

<p>

Is there any way to boost more data per minute of tape without hardware modification (other than the obvious Side B)?

Only within tight limits. The Os++ system disk (not yet available for public) will include an extended tape handler that offers a slightly faster mode by doubling the size of the tape records and slightly increasing the Baud rate. The major limitations are that the tape drive uses a very primitive form of FM ("two tone modulation"), and the decoding was done in the program recorder, hardware wise, by two narrow band-pass filters. This is quite unusal compared to state of the art (back then) where software decoding based on zero-passes were common (C64). Thus, you could not change the overall design much because the limits are given by the filters set in hardware and their response time.The second problem is that the playback speed of the tape is not precisely constant (due to mechanical wear of the tape, for example) and thus the Os measures the actual tape speed every record. You thus shouldn't make the records arbitrarily long either.In either case, it's pretty much outdated technology, even back then, and thus the ROM resident tape handler in Os++ was removed and replaced by a disk-based solution (there was better use for the ROM space). That said, if anyone is interested in extending it or helping me to test the extended (disk based) version, you would be welcome.

[fibrewire]

Only within tight limits. The Os++ system disk (not yet available for public) will include an extended tape handler that offers a slightly faster mode by doubling the size of the tape records and slightly increasing the Baud rate.

 

Can the computer boot from this new format?

[ilaskey]

I can't believe that 30 minute of tape holds the same amount of data as a 1050 enhanced density single sided disk... If it weren't for the audio feature they would surely go in the trash...

You have to remember that back then, a 410 cost half a month's wages and an 810 3 and a half month's wages for me. Iworked in a bank in the UK and my takeohome was just over £100 a month. A 410 was £50 and the 800 £350. That;'s why most people used tape. Even when you ended up spending an hour trying to load a tape that was baling out right at the end of it's 15min load with 'BOOT ERROR'. Once you had that game loaded, you sure made sure you got your time/money's worth playing it!

[snicklin]

You have to remember that back then, a 410 cost half a month's wages and an 810 3 and a half month's wages for me. Iworked in a bank in the UK and my takeohome was just over £100 a month. A 410 was £50 and the 800 £350. That;'s why most people used tape. Even when you ended up spending an hour trying to load a tape that was baling out right at the end of it's 15min load with 'BOOT ERROR'. Once you had that game loaded, you sure made sure you got your time/money's worth playing it!

 

I didn't get a 1050 till around 1994 or so. My Atari came with an XC12 in 1988 and I was a child and couldn't afford a disk drive. Then I got one from Derek Fern I believe for £100 as at 16 I was allowed to dip into my bank account. It was a lot of money then. Still is in fact. For around double that now (which is probably about the same money in real terms), I could buy a modern gaming system.