[RTTY] 300hz or 500hz IF filter? [oops, bad typo]

[Kok Chen]

On Aug 22, 2013, at 3:45 PM, Kok Chen wrote:

> it should never clip on the largest signal and the sound card's own noise floor should be at least 10 dB above the noise floor of the receiver 

Oops, sorry, but terrible typo in the above.  

The sound cards noise floor should be 10 dB *lower* than the sky noise.  

I.e., if you are watching the noise spectrum (e.g., on a software spectrum analyzer that is fed from the sound card), you should see the noise floor on the display rise by at least 10 dB when you connect the sound card to a real live receiver that is connected to an antenna.

If the receiver/sky noise has the same magnitude as the sound card noise, you lose 3 dB of SNR (which is a *lot* in terms of RTTY error rate).  This is why you need to operate the sound card so that its own noise floor is vanishingly small.  

There is not much we can do about sky noise, but there is something we can do about sound card noise.

73
Chen, W7AY