[RTTY] A Question for Chen... Soundcards?

[Kok Chen]

On Mar 24, 2016, at 3:06 PM, G3YYD wrote:

> But avoid the $5 "bargain".

At one time, I started writing a review of various sound cards, but I switched to writing demodulators right into an SDR, so I never bothered to finish the white paper.  

With an SDR, you don't need a sound card.

However, I still have all the data that I collected.

The five representative soundcards that I imeasured were the microKeyer II, SignaLink USB, Griffin iMic, a FireStudio mobile (best soundcard I ever owned; I could pass a 3 kHz passband to it and it never buckled) and a sub-$10 Amazon special (Syba SD-CM-UAUD).

The Amazon special wasn't too terrible in terms of measurements.  The second harmonic (important if you want to use it as a skimmer) only breaks the -80 dBc mark when the signal is within 2 dB of clipping.  The noise floor was only 2 dB worse than the much more expensive MicroKeyer II, but the 5th order IMD was high enough that I made a note of it -- again, IMD  is not nice if you want to demodulate multiple RTTY signals.

However, the real deal breaker is that the copy of the cheap sound card I got from Amazon was unstable (breaks into oscillation by itself) when the source impedance is higher than about 200 ohms!  I ended up finishing my measurements using a source impedance of 91 ohms.

In general, if you don't do multiple signal demodulation over a 3 kHz passband (really the only way to operate PSK31; 3 kHz is not quite wide enough for RTTY -- you need an SDR), cheap soundcards should work with narrowband RTTY.

73
Chen, W7AY