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[aprssig] Re: *** TNC Test CD Update - Mirror Server Now Available ***

Stephen H. Smith wa8lmf2 at aol.com
Tue Oct 31 18:11:43 UTC 2006

mwrobertson at comcast.net wrote:
> With the raw ISO images, most burners that I am familiar with, take 
> right off and you're there. Our problem, (mine), is not RTFM and I 
> apologize.. I had not see this method or heard of it, the dual CD I 
> mean, so I learned something again yesterday! Thanks...
> Robbie

I would have FAR FAR preferred to use the .ISO format which is supported 
by just about ALL CD recording applications,  but unfortunately it won't 
work with mixed-mode CDs.  

As I explained on the web site (and in the readme file), I used CD audio 
tracks rather than regular CD-ROM data .WAV audio tracks to avoid the 
timing errors present in many motherboard-based sound systems on 
low-cost PCs. 

I first discovered this problem several years ago when I was trying to 
make test recordings for SSTV slant correction.   I found that many 
low-cost sound systems integrated into PC motherboards have substituted 
interrupt-driven software for the timing functions traditionally done by 
dedicated crystal-controlled hardware in conventional sound cards.  The 
main clock osc of a PC motherboard (from which the interrupt timing is 
derived) is normally controlled by a dirt-cheap crystal that is not very 
accurately calibrated and not very temperature-stable.  Further, the 
rate actually varies depending on how many other software processes are 
running (and competing for interrupt services) at the same time.   Many 
of these "brain-dead" no-hardware sound systems supposedly running at 
the standard 11025 Hz sampling rate were actually off by 5-7% or more.     

This problem is steadily getting worse as more and more traditionally 
hardware-based functions are offloaded onto software models multitasking 
on today's super-fast CPUs. Dirt-cheap color printers that use the 
Windows GDI (graphics device interface) rather than dedicated processors 
inside the printer, and nearly zero-parts-count 56K modems that are 
basically just an audio transformer and interface are two of the biggest 
offenders.   The characteristic nature of these  "brain dead" devices 
is  that they have HUGE  (multi-megabyte) drivers loaded from CD -ROM  
that then suck up  massive amount of system resources and hard disk 
space compared to their  hardware-based  predecessors.

With these cheap sound systems, it is virtually impossible to keep SSTV 
programs like mmSSTV, MixW, ChromaPix, etc properly slant-corrected, 
UNLESS you make a point of shutting down ALL other programs including 
background utilities like virus scanners, firewalls, anti-spyware, 
printer status monitors, etc.

Even inexpensive audio players (boomboxes, DiskMans, car stereos or even 
the headphone output on a CD-ROM drive) etc) have dedicated 
crystal-oscillator timing chains that are far more accurate AND STABLE 
than most of todays cheap PC sound systems. Consequently, I opted to 
make the test disk play CD audio rather than CD-ROM data. 


Stephen H. Smith    wa8lmf (at) aol.com
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