🚀 go-pugleaf

ajb92@ecs.soton.ac.uk (A J Bryant) writes:

>A REAL PROJECT !!

Ohh, mummy, look! 8)

>I am attempting to design and build a video capture card for a PC.  I want
>to transfer 8bit b&w video data from a standard ISA card directly to the
>hard disk.  The data rate will be approx 500kbytes/s which I believe is the
>maximum capability of most old IDE HDDs.  (I don't have the budget for a
>fast IDE or SCSI)

That's also pretty much beyond the sort of sustained data rate that you can
expect to move around inside an ISA PC.  How do you arrive at this number?

>I know that it is possible to setup a DMA transfer from a port to a chunk
>of memory, but what I really need is to setup a transfer directly from a
>port to disk.  Is this possible?  If I have to DMA from the port to memory,
>and then out again to disk, I suspect that the system will not be fast
>enough.

You're absolutely correct.

>I have heard the rumour that standard IDE does not support DMA transfers.
>This seems a little strange as the ATA interface does support DMA.  Where
>does the inconsistency arise?  And what is the best way to transfer large
>amounts of data, quickly to the HDD.

The ATA spec offers DMA as an option.  Most "old IDE harddisks" are actually
emulations of the WD1003 (which is where IDE comes from, says it all really)
and don't support DMA.  The fastest way to transfer data to the HD is via
programmed IO. (ie. copying it through yourself.)

>To give some background on the hardware, I plan to use dual port RAM on the
>card to buffer the data.  Control electronics on the card would control the 
>capture of the data, to one 'page' of ram, while the computer is pulling
>data from the other 'page' of ram via DMA.  At the end of a frame, the
>'pages' are swapped.  I plan to capture about 60 sec video data to a 40M
>HDD file.  The specification disallows lossy compression, making MPEG a bit
>of a non winner.  I could use compression, but that would probably need to
>be implemented in hardware on the card.

You would also need to implement a DMA busmaster on the card to do what you 
want to do, and then look for a new and expensive IDE harddisk that will
understand when you try to DMA to it.  What's the need for DP ram if
you're only talking to it once?

>I would like to dump the data to a standard file, but I could try just
>dropping the data onto a particular chunk of disk.  (very carefully!)

That's even worse 8) - are you planning on implementing a minimalist FAT
filesystem in firmware on your card?

>If anyone is interested in following this project, email me and I will
>start a distribution list, otherwise please comment.

I'd be interested to hear what other people have to say, but my experience
says you're short about 50% of the bandwidth you need.

>Thanks, Andy.

--
# mike smith : miff@apanix.apana.org.au - Silicon grease monkey        #
# "The question 'why are the fundamental laws of nature mathematical'  #
# then invites the trivial response 'because we define as fundamental  #
# those laws which are mathematical'". Paul Davies, _The_Mind_of_God_. #