Dear Brian, Stewart, LF group,
Thank you Brian for the report - the 30mW ERP signal was
generated with abt 120W into my inverted L at 8m height. I will put
the beacon on again tonight, starting at 2000utc, this time with
30mW ERP for an hour, then going down to 10mW ERP.
I checked the sample rate errors of my soundcard - on
8000samples/sec this is about -5600ppm and on
11025samples/sec it is +2000ppm. So a lot bigger than likely
errors due to crystal tolerance, for some reason. I have been
looking at the clock crystal frequencies in all the soundcards I
could find - the older ones had all sorts of different frequencies,
most of which had no obvious integer relationship with popular
sample rates. Quite a few used 14.31818 MHz, 46.61512 was
popular too. Some had 2 or 3 crystals, one had none at all. The
newer ones have mostly 24.576MHz crystals - this seems to be
the norm for the AC '97 standard based soundcards which Alberto
mentioned recently, which seem to be universal now. I found some
data on 3 types of "audio codec" A/D and D/A which I saw, which
conform to this standard:
Sigmatel STAC9721T
http://www.sigmatel.com/technical_docs.htm#9721
Crystal CS4201
http://207.87.22.21/design/products/overview/index.cfm?ProductID
=138
Analog Devices AD1818
http://www.analog.com/techsupt/application_notes/EE_notes/pdf_fil
es/ee_54.pdf
Although 48k seems to be the basic sample rate of the standard,
they all have hardware support for the popular sampling rates such
as 11.025k. However they do it in different ways, eg. the Sigmatel
A/D converter always runs at 48k and converts the sample rate
digitally, while the AD1818 seems to have a synthesiser that can
tune between 7k and 48k samples/s in 1 sample/sec steps. This is
all very interesting, but probably means that frequency oddities will
continue to be a feature of soundcards.
For VE2IQ's "Coherent" software, I have one of G4JNT's PIC-
based interfaces - this performs the same function as the original
VE2IQ interface, but is rather simpler, having just the PIC, a 5V
regulator, and an audio preamp. It certainly seems to work
perfectly well, and the crystal can easily be tweaked.
Cheers, Jim Moritz
73 de M0BMU
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