[RTTY] Sound Cards

[Kok Chen]

Hi Rick,

> I'm wondering if there is any advantage or performance improvement  
> to be had with MMTTY and other RTTY programs by using a 24-bit PCI  
> card ? Tests done? Charts etc?

IMHO, it is all about (blocking) dynamic range.  Roughly speaking...

If you are using a narrow I.F. filter where the demodulator is only  
seeing a single RTTY signal, you really need only about 70 dB to 80 dB  
of dynamic range (since the rig's AGC is handling the rest) and almost  
any of today's 16 bit sound card will do that for you.  How much you  
need depends on the dynamic range of the audio from the receiver to  
the sound card.  From what I'd measured, the microKeyer II's 16-bit  
codec has a 96 dB of blocking dynamic range and also has no IMD above  
the noise floor for the full range.  Even cheap 16-bit sound cards  
that I have measured have dynamic range of 80 dB before any IMD sets  
in (and for single signal RTTY, that is not a bit problem, in any case).

On top of that, if you have some means of riding the gain of your  
receiver, and are willing to keep adjusting knobs, a software  
demodulator probably won't even need more than 30 or 40 dB of dynamic  
range.

Now, if you are using a wide filter, i.e., you are using 2.4 kHz  
passband and the sound card is doing all the work to reject adjacent  
channel interference, etc, for the sound card not to be the limiting  
factor of your receiving chain, you need perhaps 10 dB more of sound  
card blocking dynamic range than your receiver's BDR.  That extra  
headroom is needed because you don't want to work with just a single  
bit to decode a weak signal.

A weak signal under clean propagation that only moves a single bit of  
the sound card will suffer some 2.9 dB of SNR loss, which can mean the  
difference between perfect copy or horrible copy. (Anyone really  
interested can look up the Van Vleck Noise Linearization theory, also  
called the "Arcsine Law" in Probability books.)

Again, if you are willing to ride the gain in this case, the dynamic  
range of your sound card only needs to be as large as the dynamic  
range of all the signals in the audio passband (plus the 10 dB or so  
headroom in case the signal you are trying to copy is one of the  
weaker ones).

On the other end of the spectrum [sic], the sound card is almost  
always the limiting factor of the dynamic range of an SDR that uses a  
Tayloe sampling mixer.  You will want as much dynamic range from a  
sound card as you can muster.  The Flex-5000, for example uses the  
Asahi AK5394 codec, which is spec'ed for a dynamic range of 123 dB.  
(That is why you will see the blocking dynamic range of the Flex-5000  
quoted at around that number).

In addition, if you really want to take advantage of SDRs, you will  
want to use very wide passbands, and you will want your sound card to  
also handle 192 ks/s sampling rate.  Many of them don't.

An affordable and good sound card for this application is something  
like the E-MU 0404 if you use USB 2.0, or the Edirol FA-66 if you use  
FireWire.  There are better "sound cards," but you can be talking  
prices near or over the list price of the original ST-8000 (:-) and  
they often come with 16 or more channels, which you don't need unless  
you are experimenting with phased arrays.

The "direct-sampling" SDRs such as the Perseus are a different breed  
than the Flex and SoftRocks, they use "process gain" to achieve a high  
dynamic range. (I.e., if done properly, each time you reduce the  
sampling rate by 2, your dynamic range increases by 3 dB.)  However,  
since they start off in the region of 100 Ms/s sampling rate, there  
aren't any codec available with the kind of dynamic range as the Asahi  
chips have at 192 ks/s.  When decimated down to the passband of a CW  
signal, they can be better than the Flex-5000.  (Take a look at the  
measured dynamic range numbers between the Perseus and the Flex-5000  
at Sherwood's list, for example.)

That being said, you can use process gain to achieve more blocking  
dynamic range not just with the direct sampling SDRs but with  
SoftRocks and even "regular" modems.  Decimation and process gain is  
just another weapon that DSP designers have at their disposal.

In conclusion, the "short" answer is: if you are using a narrow I.F.  
filter that only lets through a single RTTY signal, don't even sweat  
it with today's sound cards (just pick the cheapest :-).  If you are  
using a wide waterfall with a standard superhet type receiver, then  
you need a good sound card since the software is the one that is  
filtering away the strong QRM.  If you are using the SoftRock, the  
quality of your receiving system is going to be almost completely be  
determined by the sound card.

Just MHO, of course.

73
Chen, W7AY