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Choosing an Input Jack

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By Chris Pirazzi and Micheal Portuesi.

This document tells you what you need to know to select a particular input jack for a VL_VIDEO source node. Unfortunately, the VL provides no simple device-independent scheme for doing this, but instead exposes hardware architectures directly.

We will tell you how to pick a single jack on a single device. Each piece of hardware offers totally different and unusual constraints on which jacks can be used by different VL_VIDEO source nodes simultaneously, and this lore is currently beyond the scope of this document. A hint: some of this complexity is directly exposed in the VL controls you use to choose jacks, and some of it is not. None of it is documented.

At first, we tried to organize this document by presenting the relevant VL controls, and then trying to come up with some sensible definition for each which make it obvious how to select any particular jack. This rapidly became futile.

Instead, we offer a more pragmatic approach:

  • ij.h --the hack--click here to see what it's about
  • ij.c --source code for the hack
  • testij.c --sample code that uses IJ hack
  • Makefile (see below) --sample Makefile for ij and testij


default: testij

DEFS=-g -I -I$(ROOT)/usr/include

testij: ij.o testij.c
	cc -o testij testij.c ij.o $(DEFS) -lvl

ij.o: ij.c ij.h
	cc -c ij.c $(DEFS)

To learn about input jack selection, first read the comments in ij.h,

and then if you dare, check out the code in ij.c.

Warning: this version of the code has not been tested for all jacks for all devices. So far, we have verified that VL path setup succeeds for all jacks for all devices, but we haven't done a complete test with an input signal for all of those cases. This code will be fully tested and any corrections posted in a month or two. But even now, you should find the general information quite useful and most of the specifics functional.