--- In digitalradio@yahoogroups.com, Tony <[EMAIL PROTECTED]> wrote: > > Ed wrote: > > > Are there any communications engineers in this group that can > > give us some idea whether a useable quality digitized voice can be > > sent over a 2.5 KHz wide HF channel with SNR comparable to or less > than > > what is required for analog voice? > > I was thinking about this today Ed. I'd sacrifice a bit of voice qaulity for > better SNR performance. > > WinDRM defaults to the MELP codec and I was wondering if the SPEEX or LP-10 > offer an improvement in SNR performance? > > Tony KT2Q
Tony, The key to better weak signal performance will be primarily in how the data is sent over the air. Observing the signal for digital SSTV on 20M has been very interesting. Often a hole in the spectrum can be observed as it moves across the spectrum. It takes out about 20% of the spectrum and has a time span at one frequency of around a second. The packaging of the data must be done with enough redundancy spread throughout the spectrum so it can be recovered in spite of these spectrum holes. On 80M atmospheric noise tends to have short impulses that take out the entire signal for much shorter periods of time - much less than a second. To mitigate this, the data redundancy must be spread over time so the data can be recovered in spite of a complete loss of signal for a few milliseconds. Of course adding redundancy means reducing the data rate for the encoded voice data. This would probably require some adjustments to the voice coding algorithm. With cellular signals, although the maximum data rate may be 8K bps, the effective rate is usually less than 1/2 that due to the fact that speech is not a constant signal - there are holes in it for short periods of time during which no data needs to be sent. Can someone tell where to find the specification for the modulation format used by WinDRM? Ed WB6YTE Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
