[RTTY] Fldigi Question

[Kok Chen]

On Jan 29, 2009, at 1/29    7:56 AM, Lee Buller wrote:

> I've tried to glean through the documentation, but does Fldigi  
> support devices like SignaLink USB?

It should work.  There is really no issue in the SignaLink USB a  
software modem has to specifically address.

All the SignaLink has is a 16-bit sound card (using a Burr Brown chip  
from the PCM2902 chip set), a couple of transformers (one in, one  
out), pots for level control, and a VOX circuit to generate PTT (what  
microHAM calls "auto PTT," but only available on the microKeyer II as  
far as I can tell).

The only connection you need to make from the computer is to the USB  
sound card.  And you don't need a special drivers when used with  
Windows and Mac OS X since the Burr-Brown is USB audio class  
compliant.  I suspect that is true with Linux, also.

You didn't say which operating system you are using fldigi on.  But  
there could be issues that are not fldigi specific, but specific to  
the PCM2902 and the underlying operating system (you didn't say if  
you are running it on Windows, Linux or Mac OS X).

The thing you need to be careful of when using the Burr-Brown, ergo  
the SignaLink USB, in Windows is that you need to select sampling  
rates that evenly divides 96000000.  I.e., don't choose 11025 s/s or  
44100 s/s sampling rates, or your transmit frequency might not zero  
beat to your receive signal (no kidding!)  Choose something like  
12000 or 16000 or 32000 s/s instead.

Mac OS X modems that uses Core Audio does not have this frequency  
discrepancy since Core Audio resamples audio to the internal clock of  
the computer based upon time stamps.

However, on the Intel version of Mac OS X, has a much worse problem  
if you choose 11025 or 44100 s/s with the Burr-Brown PCM2902.  So you  
also need to use 16000 or 32000 s/s if you are on an Intel based Core  
Audio modem software.

fldigi does not use Core Audio, so it does not have that problem on  
Mac OS -- but to be safe, use 16000 or 32000 s/s when you use a Burr- 
Brown codec sound card such as the SignaLink.  You can identify them  
by the PCM2902's USB PID/VID.  You should be able to Google for the  
PID/VID of that codec.

73
Chen, W7AY