Performance Advantages of Sound Card Interface?

[AK3Z]

I am trying to quantify the performance differences between using a sound card interface with the sound card in a computer vs. a sound card interface with built-in sound card.  I know that signal to noise ratio of the sound card affects performance--higher is better.  And the best sound cards on the market are usually about 20 DB better than the cheap ones.  But I also know that having the sound card located within the interface and at the rig usually means less noise and less chance of RF feedback, etc due to using less cabling between computer and interface.  But I see no SNR specifications or noise specifications listed for the major sound card interfaces with built in sound card.  So what equipment yields the best performance and ability to copy the weakest signals?  A PC with a top end sound card or an interface with sound card built in?  Also, what don't I get with a $100 interface that I do get with a $300 interface?  Any comments are appreciated.  Thanks.  John 

[N4CR]

Here's the deal. Almost every sound card out there is quieter than the output of your radio with the volume at 0. And when you start turning up the volume, the sound card noise floor becomes less and less important because your RF noise floor coming through your audio chain is so much greater in comparison.

Let's say your audio card has a crappy noise floor of -80. Once you turn on your receiver and crank up the volume until you are getting a decent signal to decode, the RF noise floor is going to be around -50. (or -40)

Yes, the 30 dB lower noise floor adds in, but at a rate of 1/1000 of the audio from the RF noise floor. That ain't much even for a crappy audio card.

I have literally tried dozens of combinations of cheap to expensive sound cards and unless you are chasing signals WAY under your RF noise floor there just isn't enough difference to make a hill of beans. I did find some really cheap sound cards that had so much distortion that they refused to decode almost everything, but that had nothing to do with the noise floor.

Personally, I settled on the SignaLink USB because of it's price, size and convenience. I put toroids on the USB and the audio signal line, I did the mods listed below and I just use it. It's noise floor is WAY under where I work with any signal I'm trying to decode. In fact, the software decoders work better when you crank up the gain to just below where the decoder overloads. At that point, the sound card noise floor is about 70 dB down and absolutely irrelevant.

SignaLink USB Mods: http://www.frenning.dk/OZ1PIF_HOMEPAGE/SignaLinkUSB-mods.html

I believe if you did the AB8JV mod and swapped out the 600:600 transformers you'd be as good as doing all the mods. I don't know because the AB8JV mod showed up recently, but on top of all the other mods it managed to make a tremendous difference for the noise floor.

So, a $99 device, an hour of modding and $20 in parts puts it up to about as good as you can get at any price as far as decoding even weak signal modes.

If I wasn't full time employed, I'd make a cottage industry of modding SignaLink USB interfaces for other hams.

[VA7CPC]

QST ran a review of several different sound cards, a few years ago.

They had thoroughly different specs, and at least one was a "serious" card with 24-bit, 96 kHz sampling (and maybe a mic preamp).

. . . There was no difference in performance when decoding PSK31 signals.   

The reviewer was disappointed -- he was expecting something to show up.

.         Charles

[N4CR]

QST ran a review of several different sound cards, a few years ago.

They had thoroughly different specs, and at least one was a "serious" card with 24-bit, 96 kHz sampling (and maybe a mic preamp).

. . . There was no difference in performance when decoding PSK31 signals.   

The reviewer was disappointed -- he was expecting something to show up.

.         Charles

I tried a 24 bit 192 kHz card. It made no difference at all but performed as good as any. It's in my HTPC now. Sounds great with fiberoptic output/input.

[AA4PB]

The primary benefit of an interface (or a radio) with a sound card built in is simplified interconnection and convenience of operation rather than improved quality or ability to copy weaker signals. They allow you to continue to use the computer sound card, its settings, and speakers for normal Windows sounds, players, etc. without interfering with the digital mode interface.

In any case, the sound card in a digital application is simply the analog to digital (and digital to analog) converter. All of the encoding and decoding (the hard part) is done in software runnng on the main CPU.

[W5DQ]

I've used everything from an expensive Soundblaster EMU soundcard system (tinkering with SDR) to the cheap $5 all-in-one soundcard (spkr/mic) on a USB thumbdrive style device and found no difference in 99% of the uses. Like was said, unless you are chasing extremely low level signals like EME or WSPR, you probably won't see any difference either. Get a cheap $20 Soundblaster compatible card and go for it. I have 3 of these in my shack PC for doing multiple bands at once on multiple rigs. Works great and cheap to replace if needed. I use a Soundblaster X51 device for JT65-HF and it works flawlessly. I have logged QSOs where I could not even hear the other station's audio yet the soundcard/PC decoded it at -25dB.

Gene W5DQ

[W5LZ]

For what it's worth
I don't think it makes a big difference which sound card you happen to have unless it's a really terrible one.  I'm using a very small 'Sabrent' USB plug-in sound card, I think it was something like $20 (got two of them).  It performs as well as the 'built-in' sound card in the desk top and laptop computers (generic?).  Of course, that doesn't say if those built-in sound cards are any 'good', and I have no way of 'measuring' how well any of them works, other than just using them.
Why the 'plug-in' cards?  Cuz I use the built-in card for something else.  (Also a little curiosity because I'd never seen a sound card that tiny, and they were cheap(!) of course.)  
 - 'Doc

(Just 'googled' that Sabrent card and the price has gone down.   Dang!  I should'a waited. )