[RTTY] DOS computer for improved RTTY reception?

[Kok Chen]

On Jan 19, 2011, at 10:29 AM, Bill, W6WRT wrote:

> MMTTY has a spectrum display. Can't that be used?

I don't know.  I don't have a computer that can run MMTTY to know what it has.  Does it have a spectrum display calibrated in dB so you can eyeball a 3 dB or 10 dB change?  If so, it will definitely work. 

The main thing is to get the receiver noise floor above the sound card's noise floor.  If the noise floor only rises by 3 dB when you turn the rig on, it means that the demodulation algorithm will have to also deal with a significant amount of noise from the sound card itself.  

Too much rise in the noise floor is never a bad thing for weak signal reception :-).  However, when you take it to the extreme, it is just a waste of dynamic range and you may have to "ride the gain" more often when you do that.

You can choose a better sound card to avoid having to "ride the gain" (I do that, personally).  You eventually have to "ride the gain" though, when your receiver's front end folds from a loud signal :-).

The noise floor of a microHAM sound card is around -96 dB from a full scale signal.  The best theoretical 16 bit sound card has a dynamic range that is a fraction under 98 dB.  An E-MU 0202/0204/0404 (based on the second tier 24-bit Asahi codec) has a dynamic range of about 115 dB and the Presonus FireStudio Mobile has a couple dB more even.

The best commercial sound codec that I have come across is the top tier Asahi codec that used in the Flex-5000; with a dynamic range of 123 dB.

In general, the dynamic range is a combination of the chip used and just as importantly, how much care the designer takes with analog preamps, isolating the analog noise from the digital noise, etc, etc. 

Once you get into the 100 dB range, you probably also need to wire things up using the sound cards' balanced input, and then ground the sound card to the radio using a short ground strap (that is what I do with the E-MUs).

Incidentally, and for good reason, the ST-8000 also allows you to strap it as balanced input.

73
Chen, W7AY