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

[Anthony.N.Martin at marconiselenia.com]

> We try to find if a card can work on 9600b, (On 9600b I
> am using the standard 44.100khz sampling rate), and
> imagine there are Soundcards that cannot work even on
> 44.100khz right.

Members on the AGW list suggested LF response was also an issue.
No-one specs LF response, but all PC audio codecs use DC
blocking caps and possibly also some IIR digital HPF in the
sigma-delta ADCs. For digital radio, LF response is probably
important to you, particularly I&Q matching of the LF response,
and if it's determined by a pair of -20% +50% electrolytics...

Consumer cards are often 5.1 surround these days, which means
a dedicated bass subwoofer output and who cares about the LF
on the other channels.

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.

Ant M1FDE

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