Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!emory!gatech!mcnc!taco!hobbes!kdarling From: kdarling@hobbes.ncsu.edu (Kevin Darling) Newsgroups: comp.graphics Subject: Re: IFF, 24-bit color, displays, Amiga Message-ID: <1991Mar13.053548.18866@ncsu.edu> Date: 13 Mar 91 05:35:48 GMT References: <1991Mar11.223827.21988@ncsu.edu> <1991Mar12.014550.25284@mintaka.lcs.mit.edu> <1991Mar12.082113.16351@ncsu.edu> <1991Mar12.193951.634@mintaka.lcs.mit.edu> Sender: news@ncsu.edu (USENET News System) Followup-To: comp.sys.amiga.graphics Organization: North Carolina State University Lines: 42 In <1991Mar12.193951.634@mintaka.lcs.mit.edu> rjc@wookumz.ai.mit.edu (Ray Cromwell) writes: >>In article <1991Mar12.082113.16351@ncsu.edu> kdarling@hobbes.ncsu.edu (Kevin Darling) writes: >>Another thing to consider: I read that its RAM is loaded by sending the >>R,G,B components as separate images from the Amiga video output lines. >>In the highest res mode, that would take 6 Amiga display frames to do. > > The Colorburst transfers data at 5.5 megabytes/sec through the db23 port. > A lo-res screen is 40bytes widex200 lines deep x 24 bits. That's 192k, > at 5.5mb/sec gives up 30 frames per second. Yes and No. Here's the bottleneck: CPU --> DISPLAY RAM --> COLORBURST slow--^ fast--^ Sure, you can easily get 5.5 megabytes/sec coming out the video connector, or around 100K per 60Hz field. Assuming the data is sent out continously, the Amiga could output the 192K needed for a static 320x200x24-bit picture in 1/30th second, just as you said. The question is: _Where_ does all that data come from? :-) It comes from data being placed into the Amiga display RAM. A friend and I sat down this afternoon and did some animation figuring: At the res needed to output 100K/second, the cpu access to Amiga CHIP ram (display ram to others) is tied up by video DMA around 80% of the time. Nothing unusual; this shut-out is well known when using the Amiga display in higher res modes. But in blunt terms, that means only around 3,000 16-bit transfers into the CHIP ram can be done (obviously fewer if the cpu is doing it), per field, at the same time as the data is being sent to the Colorburst. At 24-bits/pixel, that's roughly a 50x40 pixel area every 1/60th second. Maybe double that on an Amiga 3000 with 32-bit CHIP access. Now sure, you can fill up the CHIP ram with a limited amount of preset data, use lower bits/pixel, slow down fps, and so on. You can bet that demos do just that. But any way you cut it, their 5.5MB/s figure is akin to those common hard disk xfr specs which ignore seeks, etc.... it's a best-case, limited duration transfer rate... dependent upon source data availability. It's a useful board, but at the same time, reality must be observed :-). Followups to c.s.a.graphics, where I'll also be happy to answer your HAM-E questions, if you still wish. best - kev