Re: MISC-d Digest V96 #51

[jfox@xxxxxxxx (Jeff Fox)]

Dear Misc Readers:

>Date: Sun, 5 Jan 1997 15:02:40 -0500 (EST)
>From: Penio Penev <penev@venezia>
>To: MISC
>Subject: NTSC/PAL encoding
>Message-ID: <Pine.SGI.3.94.970105145527.24153G-100000@venezia.rockefeller.edu>
>Content-Type: TEXT/PLAIN; charset=US-ASCII
>
>I'm trying to underestand whether I can use the video out of P21 for a 
>general analog-out signal.  For example, can I program it go give 1 bit
>(ON/OFF) signal?  2-bit?  n-bit?

Yes.  You will still need to insert some Refresh instructions into the
stream of instructions that the coprocessor will execute if you are
going to be using DRAM and you program does not self refresh a small
area of memory as it runs.
If you use a slow clock input then you will need a higher proportion of
refresh instructions in the sequence to keep DRAM happy.  Different
brands of DRAM will have different refresh requirements.  The current
P21 software uses 1 to 3 refresh instructions per horizontal video line.

B/W gives you two levels and all the colors gives you 15 levels.  There
is no difference between dark-black and bright-black.  A jump instruction
will be get pipelined so that it will not effect the timing of the
pixel data instructions so you can construct wave patterns in memory
and execute them with the video coprocessor.  This is all the video
instructions do, but they also use some other instructions to construct
the video signal.

For audio frequencies you might want to make half the instructions refresh
and the signal levels from the chip expect an 80ohm video cable

>Does anybody know of a good description of the NTSC signal?  Availbale
>through the web? 

I will see what I can find.

>The whole point is to use the external clock as a reference of a signal
>that comes out.  Otherwise i would have to connect an external timer, and
>read it in polling mode, which would result in temporal jitter in the
>analog signal that I output. 

Yes.  The video coprocessor on P21 is really a programmable analog
output coprocessor that clocked by an external signal so you can pick
a clock, setup the instructions with the cpu (or even put them in rom),
and just turn the coprocessor on and off.

F21's analog and video coprocessors include a cpu interrupt instruction.
This allows the cpu to service an i/o coprocessor when it reaches an
interrupt instruction in the i/o instruction sequence.  Since P21 does
not have this feature you can turn on the output coprocessor but then
the cpu has no way to determine where it is after that.

>If I could use the video out as a 1-bit high frequency ADC, I could do
>audio out with the correct oversampling and filtering. 

Yes, just remember that it also generates refresh and you will have
to figure out a good say to figure out when to turn it off.

You could just put it into a tight loop at the end of your sequence
and then restart it again at some time in the future.

You might be able to add some external circuitry to detect one of
the signals and read it with the cpu.  this would allow the cpu to
poll to see when the output coprocessor reaches this testable
instruction.

Jeff

Jeff Fox
jfox@dnai.com        Ultra Technology Inc.
jeff@itvcorp.com     the iTV Corporation
http://www.dnai.com/~jfox