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Document the VIDC audio format.

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bjh21 2001-01-12 00:12:38 +00:00
parent 531e3e2075
commit 0bb3b087a6
1 changed files with 25 additions and 2 deletions
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27
sys/arch/arm26/vidc/vidcreg.h
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@ -1,6 +1,7 @@
/* $NetBSD: vidcreg.h,v 1.1 2000/05/09 21:56:04 bjh21 Exp $ */
/* $NetBSD: vidcreg.h,v 1.2 2001/01/12 00:12:38 bjh21 Exp $ */
/*-
* Copyright (c) 1998 Ben Harris
* Copyright (c) 1998, 2001 Ben Harris
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
@ -128,4 +129,26 @@
#define VIDC_WRITE(value) *(volatile u_int32_t *)MEMC_VIDC_BASE = value
/*
* VIDC audio format is mu-law, but with the bits in a strange order.
*
* VIDC1 has:
* D[7] sign
* D[6:4] chord select
* D[3:0] point on chord
* so 0x00 -> +0, 0x7f -> +inf, 0x80 -> -0, 0xff -> -inf
*
* VIDC2 has:
* D[7:5] chord select
* D[4:1] point on chord
* D[0] sign
* so 0x00 -> +0, 0xfe -> +inf, 0x01 -> -0, 0xff -> -inf
*
* Normal mu-law appears to have:
* 0x00 -> -inf, 0x7f -> -0, 0x80 -> +inf, 0xff -> +0
* Thus VIDC1 is NOT(mu-law), while VIDC2 is NOT(mu-law)<<1 | NOT(mu-law)>>7.
*
* I think the A500 uses VIDC1 and the Archimedes uses VIDC2.
*/
#endif