[RTTY] 24 bit sound card

[Kok Chen]

On Mar 18, 2005, at 10:17 AM, rwmcgwier at comcast.net wrote:
> The dynamic range of the audio coming out of the modern  
> transceiver  from
> ANY port,  headphone, speaker, line level output,  is less than  
> that afforded you
> by an 8 bit audio card.

I will try to find time to record the audio amplitude variations  
across the band in some future contest and also study how many bits  
an RTTY signal (plus noise) needs to be above the LSB of an A/D  
converter for it to be "close enough" to the optimal case.

I will also try to record the strength of the really weak signals  
that come across the Pacific when the 20m band is "dead."

On top of the audio variation, hard clipping a signal before passing  
it through a matched filter is obviously not optimal, so you will  
need a few more bits than just letting a weak signal tickle the least  
significant bit of the A/D. The only analyses I have seen are based  
on the AWGN case, so it is worth looking at more real world cases.

The case of quantized FSK can be readily simulated with the sound  
files from VE3NEA (especially Alex' multipath and selective fading  
files) and are probably worth doing, before going to actual recorded  
signals off the band.

BTW, here is another vote for the SDR-1000.

With the SDR-1000, even good two channel 24 bits A/D converter do not  
have enough I and Q channel balance across the passband to deliver  
the kinds of dynamic range that the SDR-1000 is capable of, but the  
"radio" does have a pulse generator in the front end to give you the  
capability of equalizing the A/D converters.  Macintosh users will be  
happy to learn that there will soon be an alternative to using a  
parallel port to control the SDR-1000, which means you can run it on  
any modern Macs.  I don't think that it is listed at Flex-Radio's web  
page yet but I have been playing with a USB (as in Universal Serial  
Bus and not upper sideband) adapter from them (you do have to write  
code yourself to download the adapter's firmware -- but it is just  
the standard Anchor chip set).

73
Chen, W7AY