[aprssig] Clock/timing accuracy when generating an APRS signal
Stephen H. Smith
wa8lmf2 at aol.com
Wed Aug 31 11:20:23 CDT 2011
On 8/31/2011 8:28 AM, Richard Corfield wrote:
> I've found in experiments the opposite. I need an accurate clock source in my
> generator for it to be received - at least by the AGWTracker software.
Are you using a soundcard softmodem like AGW Packet Engine with Tracker?
If so, there will be wild variations in timing if you are running it on the
"brain-dead" parts-reduced basic sound system embedded in the motherboard of
most modern PCs.
Classic add-on sound cards did most of the heavy lifting of decoding and
synthesizing audio with dedicated hardware controlled by an accurate
The cheap motherboard-based systems have replaced most of this with a simple
A/D converter and massive software stacks running on the machine's main CPU
instead. The sound card sampling processes derive their timing from
interrupts asserted on the CPU. Depending on how many other processes are
running at the same time competing for attention by the CPU and the number of
pending interrupts, the latency in acknowledging interrupts can vary wildly.
This contributes to random phase jitter in the decoding/tone generation process.
Further, depending on how many other programs (including background apps like
firewalls, virus scanners, instant messengers, printer status monitors, etc)
are running at a given time, the sample rate timing in sound card apps can
actually change by hundreds of parts per million. If you go through the
sample rate calibration drill on a sound card app like mmSSTV or MixW, and then
start up power-hog programs like MS Outlook, Photoshop, Firefox, etc, you will
actually see the sample rate in the sound card app, as shown by their
calibration utilities, change by 50-100 ppm.
[This el-cheapo approach has become nearly universal on PCs since CPU
throughput went through the stratosphere with the first Pentium One MMX cpus.
The CPU has so much throughput, most unused, it's cheaper to burn up CPU clock
cycles and interrupts simulating a sound card in software than spending $5 (and
the necessary PC board real-estate) on dedicated sound card chips. This same
approach is used with modern cost-reduced Windows GDI printers, and the final
generation of dial-up 56K "WinModems". ]
Substituting a "classic" hardware-based sound system, either internal like a
Soundblaster PCI card or an external USB-connected one, will make a MAJOR
difference in the performance and reliability of sound-card softmodems. Note
that you don't need a fancy card with Dolby 5.1 surround, "SRS 3-D audio",
etc. Just a basic $20 two-channel PCI sound card.
External sound systems (including the Tigertronics SignalLink USB which is
actually an external sound system combined with a ham interface in a single
box) have another advantage: The analog part of the sound system is removed
from the firestorm of digital noise, switching power-supply hash and AC
ground-loop hum present on most motherboards. The noise floor of the analog
input on an external device is often 10-20 dB lower, which makes a noticeable
difference in real-world weak-signal copy, by not adding noise to a signal
already degraded by less than full quieting.
Stephen H. Smith wa8lmf (at) aol.com
=== Now relocated from Pasadena, CA back to 8-land (East Lansing, MI) ===
Home Page: http://wa8lmf.net
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