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Subject: DMA Operation???
From: George Styles (george_at_ripnet.co.uk)
Date: 2002-10-18


Hi,

Im still trying to make the Jukebox 'beep' so I can try and implement audio
feedback of remote control keypresses. This is so that I can try out an idea
I have had which is to use key combos on the remote to turn on/off shuffle,
and so on, and I believe that some kind of audio feedback would be required
to confirm an action.

My idea is to use (say) vol-up then vol-down then vol-up then vol-down as a
kind of 'escape' sequence, which can then be followed by other keys for
other operations (say prev for shuffle on, next for shuffle off etc).

I am having trouble with the 'beep' side of things, and want to run an idea
past the gurus before beginning my hacking ...

I am looking at mpeg.c and can see that the MP3 data is DMAed using dma
channel 3 from the mp3 buffer to the MAS chip - it seems to go to address
0x5FFFEC3 which I assume is where the MAS's mp3 buffer is located

question1: is this buffer on the MAS chip circular? by this I mean does the
chip start playing from the beginning to the end then back to the start
again?

I was thinking of putting the 'beep' code within the DEI3 function, as it
looks to me like that is what is called when the DMA controller has finished
one transfer, and wants more data -

question2: How often does that interupt get fired? is it every 64k (about 8s
of audio at 64kbs), in which case its not often enough for me to slip a beep
in (what I was thinking was to point the DMA controller to an alternative
MP3 buffer containing a short beep, setting a flag to tell it to return to
the 'real' buffer next time DEI3 gets fired off.
Is this feasable?

has neone documented the disc-buffer-mas transfer situation?

thanks
George



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