LF: Spectran accuracy
Thu, 04 May 2000 13:41:17 -0400
Talbot Andrew wrote:
> Another problem with soundcards is that of timebase accuracy, and in
> some cases even stability. Many cases of severe errors were identified
> by G3PLX during the development of PSK31
> In the better cards (real Soundblaster compatibles) the frequencies are
> derived from dedicated correct frequency crystals so the CD sampling
> rates derived from 44100 Hz are exact (well, within crystal accuracy
> anyway). The 8 kHz rate is a different matter though. This cannot be
> derived exactly from the same xtal as the 44k1 rates, and an
> approximation is often taken by selecting the nearest integer divide
> ratio. Errors seen here amount to several Hz at 8 kHz depending on the
> individual chip set in use. The best cards have a separate xtal for
> this rate and the exact frequency is generated.
> Some SB hardware built onto PC motherboards uses whatever crystal is in
> the PC (often the 14.318 MHz one) to generate the SB sampling rates.
> Again with integer dividers errors of several Hz can be seen.
> The worst, according to Peter, is a CR controlled system. I have never
> seen anything this bad and the story may be apocryphal, but he has had
> errors of many 10s of Hz at 8kHz reported.
> I have checked the following :
> 1) Older 16 bit true Soundblaster - all sampling rates exact
> (meaning within about 10 parts per million of the true values)
> 2) Newer PCI bus compatible true Soundblaster - ditto.
> 3) Onboard SB hardware on a wide range of Dell pentiums (Pentia ?
> :-) - 44k1 derivatives 'exact'. Errors from 0 to 6 Hz on the 8 kHz
> rate depending on age / type of machine.
> 4) Toshiba satellite laptop - 44k1 derivatives 'exact' , 8 kHz
> rate - 7 Hz error
> PSK31 has a calibration routine built in to take out this sampling rate
> error and store the calibration constant. But with the temperature
> ranges that PCs go through from turn on to several hours of operation,
> the crude packaged oscillators they use can easily move a few 10s of
> PPM. At 8 kHz this drift alone is several times the minimum Sprectran
> resolution and a few Hz initial error is off the scale without
> calibration !
> Andy G4JNT
> > >>Further I noticed that launching another application while running
> > Spectran
> > >>caused a kind of 'frequency shift' (see screenshot at my webpage).
> > >>
> > >Alberto found this same problem, but he says that debugging will be
> > difficult,
> > >and will probably take some time. Incidentally, he is releasing a new
> > "beta"
> > >version (beta3).
> > Just to straigthen things a bit up : we are talking of two different
> > problems :
> > 1) The frequency error Marco is talking about, is known to me since
> > some time,
> > and it is caused by a wrong initial design assumption of Spectran,
> > i.e. that the
> > frequency ruler scale must start at an integral multiple of 1 Hz.
> > This because at
> > that time Spectran was meant for EME use, where sub-Hertz
> > resolutions are not
> > needed. Now that with beta 3 we can do down to 21 milliHz, this
> > must revised.
> > 2) The frequency shift observed and reported by Rik and Alan, is a
> > completely new
> > thing, for which, at the moment, I have no explanation. I saw, one
> > time only, a shift
> > of 0.5 Hz, which I attributed to my RX. I wasn't launching or
> > using any other
> > application at the moment. As this shift has been signalled by
> > more than one person,
> > and on signals locked to some standards, then I must investigated
> > into it.
> > Thanks for those reports, and don't be shy to send others. The very
> > first reason for
> > putting out a beta before the final release was just to receive this
> > kind of reports.
> > 73s
> > Alberto I2PHD
> > -----------------
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