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From: p554mve@mpirbn.mpifr-bonn.mpg.de (Michael van Elst)
Newsgroups: comp.sys.amiga.hardware
Subject: Re:  GVP Trade-in
Keywords: SCSI, GVP
Message-ID: <1151@mpirbn.mpifr-bonn.mpg.de>
Date: 31 Aug 90 21:38:04 GMT
References: <02048.002057@thiger.UUCP> <552@DIALix.UUCP> <14069@cbmvax.commodore.com> <1990Aug30.121102.11096@hod.uit.no>
Reply-To: p554mve@mpirbn.UUCP (Michael van Elst)
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn
Lines: 22

In article <1990Aug30.121102.11096@hod.uit.no> borgen@stud.cs.uit.no writes:
>So where does the available bandwith go? I mean, looking in the HW-ref.man
>there is three cycles for disk-DMA every scanline (4 for audio), which
>makes me believe there is 3*28k = 84k/s theoretical transfer speed(after
>MFM that should be 42k/s), but disks spin 5 tracks/s = ~12k*5 = 60k.

Well, 5 revolutions per second, 5.5k per track --> 27.5k/second.
To handle arbitrary start points, you'll need to read an additional sector
per revolution, this reduces the (user-)data rate to about 25.2k/second.

Scanline frequency is about 15.7KHz. If there are three free slots per
scanline, you can transfer 3*15.7K words per second -> 94.2Kb/sec.
MFM data comes with 500Kbit/sec == 62.5KB/sec. Therefore the disk-DMA
does not use every slot but if only two slots were available you would
need extra buffers and a slow genlock signal might induce disk-DMA failures.

Regards,
-- 
Michael van Elst
UUCP:     universe!local-cluster!milky-way!sol!earth!uunet!unido!mpirbn!p554mve
Internet: p554mve@mpirbn.mpifr-bonn.mpg.de
                                "A potential Snark may lurk in every tree."