[RTTY] RTTY spectrum analysis article

[Kok Chen]

On Jan 15, 2013, at 11:46 AM, Joe Subich, W4TV wrote:

> It is just as important with the built-in sound cards as
> with the external interfaces to use good proactive to keep drive
> levels to the proper level and avoid external sources of common mode
> RF/hum/etc.

Joe,

I think Andy's point (assuming I am interpreting him correctly) is that you cannot overdrive a sound card past its full scale digital numbers (+,- 32767 if 16 bits converter).  The full scale drives the codec to a known audio level, often to 0 dBu or +6 dBu range (I have see high end codecs go a bit higher).

Since the rig manufacturers know this value, they can design their analog chain so that the full scale digital value can never clip their other stuff.    

All the junk we see in opposite sidebands and such are probably coming from way overdriven balanced modulators, or there is a DC component in the AF going into the balanced modulator.  If the audio sine wave is not symmetric between positive and negative voltages (which I can see happen when you are not careful with your audio chain), you can indeed produce a small DC component.

With a built-in sound card, with the manufacturer doing the right things, the only way you can overdrive or create a DC component, is that if the AFSK generator of the software modem is horribly ill designed.  I guess it can happen by accident, but any error can also be corrected very quickly by the developer.

As to hum and all that, the consumer simply cannot produce it with a built in sound card short of opening up the radio and doing malicious things to the hardware.  If the radio manufacturer did the right things, their noise floor can be as low as any external sound card, and probably better -- just look at the codec in the Flex-5000; the noise floor is better than -120 dB from full scale.  Find me a ham quality digital interface sound card that can get within 20 dB of that!

73
Chen, W7AY