[hfsig] Performance of 24/192 soundcards? USB Soundcards

Lyle Johnson wa7gxd at fidalgo.net
Tue Sep 21 13:34:07 EDT 2004

> I haven't fully understood why LF should be an issue for 
> G3RUH 9600 packet. 9600 NRZI HDLC has a max run length 
> without transitions of 6 bits (0.625ms), so lowest frequency 
> component would be 800Hz. With a 5x margin, 160Hz cutoff 
> should be enough.
> Even with 300Hz performance might suffer in poor SNR but 
> shouldn't fall over. I don't see anything in G3RUH that 
> breaks properties of HDLC so it may be the G3RUH 
> implementation is at fault. I don't think anyone has looped 
> round a G3RUH via an adjustable HPF to get any hard facts anyhow.

There is a 17-stage scrambler that could give you a run of 16 or 17 bits
without transitions.  Now we're down to 50 Hz or so.

James is a very careful engineer, and he did some pretty extensive work with
all this when he designed the modem and specified the required passband
response.  The BER goes up pretty quickly as the LF response of the system
degrades.  I don't have the details at my fingertips, but I recall that he
published some information about this.  Might be on the AMSAT website
archive of his various articles.

Lyle KK7P

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