Hi Mark, I think G3PLX recently mentioned the same thing, with regard to USB sound cards, as he found they have the biggest offset problems. However, in dealing with the masses, if we can find a way to nullify the offset's effect long enough to complete the ARQ transfer, it will work with anyone, and I think using DEX is the answer, but MFSK16 still appears to be the best mode when there is no static problem, and there is QSB taking the signal below the noise threshold. DominoEX can be mistuned up to 200 Hz and has a 200 Hz drift tolerance, without AFC, so that should take care of most situations. I know that DominoEx16 can be mistuned +/- 100 Hz and still keep printing, because I have tested that multiple times on our 2 meter net. BTW, that brings up another issue - drift tolderance. NBEMS has to deal with transceiver at 2 meters with no TCXO, and transceivers with a TCXO. The drift of the FT-897, for example, is about 50 Hz from start of transmit to about 5 seconds after, and with PSK63, I always lose the initial text from the station without a TCXO, but with DominoEx16, I never lose one character - same as the station with a TCXO, so we think we will replace PSK63 with DominoEX16 for our 2 meter net, after two more weeks of tests to see how it performs with multipath reflections.
I'll include mention of the sample rate to use in the Help file. Thanks for the tip! 73, Skip KH6TY ----- Original Message ----- From: Mark Miller To: digitalradio@yahoogroups.com Sent: Wednesday, May 14, 2008 1:23 PM Subject: Re: [digitalradio] Signals on 3584 + audio At 08:49 AM 5/14/2008, kh6ty wrote: >The problem with MFSK16, as you found, is the mistuning tolerance. >For messaging, when there is a fast series of ARQ exchanges, if one >station has uncompensated offset between RX and TX (NBEMS must work >with untrained and inexperienced operators to be successful for >emcomm), and if the offset exceeds 4 Hz, which is not so unusual, >eventually it will not be possible to decode MFSK16, and therefore >the ARQ requests or confirmations may be missed. Skip, One thing I have found is that when the sound card can be configured for a 12000 Hz sampling rate, the offsets are not present in most sound cards. It seems that when 11025 is used that the offsets are noticeable in many sound cards. I am not sure how an 8000 Hz sampling rate performs, but just thought I would mention this observation. 73, Mark N5RFX ------------------------------------------------------------------------------ No virus found in this incoming message. Checked by AVG. Version: 7.5.524 / Virus Database: 269.23.16/1432 - Release Date: 5/14/2008 7:49 AM
