Message-ID: <3A6C390C.FA0B5286@usa.net>
Date: Mon, 22 Jan 2001 14:43:40 +0100
From: "Alberto di Bene" <dibene@usa.net>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Argo, capture timing
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Alan Melia wrote:

> [snip]
> But that is maybe a  useful thought , you could set the default dependent
> upon the mode setting, as that defines the screen width. A value slightly
> less than a screen width would allow accurate 'pasting' of successive
> windows. This would be useful as it is not all that obvious, when you begin
> a slow scan, how much time it will take to fill a screen width. Anyone
> wanting odd periods between saves to align with transmissions like Jacks
> could over type it.
>

Hi Alan,
     suggestion accepted, I will add an 'Auto' choice for the capture interval,
that will change accordingly with the mode setting.

> I am interested to see it suggested that the '2 Hz LF' is a common error on
> all the cards. Maybe this is a divide error like Bill had to deal with on
> Africa.
>

I think it is higly dependent on the brand, the model and the batch of the sound card.
I use an old ISA AWE32 SB card, and, as far as I can tell, this error is not present.
I haven't checked on my laptop, a Thinkpad with a Crystal Sound chip, yet.
While we are at  this, I take this opportunity to answer to a private message of
David, G0MRF, who signalled me a frequency error on the 3 and 10 sec/dot modes,
but not on the full band view mode. I think that the problem is that the 5512 S/s sampling
rate is not a standard one, and is implemented differently by the various manufacturers.
In Argo  all modes, except the full band view, use this sampling rate. On my sound card,
I see no frequency errors when using this sampling rate, but maybe this is not true for
every card model. It would be useful to have reports on tests done on many sound cards.

>
> I used the Magnitude Squared estimator and left the AGC on. Calibration by a
> sig-gen coupling into the aerial and being measured to 0.1Hz resolution on a
> counter referenced to Droitwich.  The interesting thing is that there is
> little sign of drift over two nights. The RX has a TCXO and has been ON
> continuously for four months, but the soundcard seems to perform adequately
> considering that the usual 'computer grade' crystals are not usually very
> good quality..... and the computer does get switched on an off.
>

Assuming an overly optimistic stability of 10 ppm over the whole temperature excursion range
for that crystal, this means a deviation of 0.01  Hertz for an audio signal of 1 kHz.
At this frequencies, even cheap crystals can prove adequate.

Thanks for your tests Alan,

73   Alberto    I2PHD