I will try to fill in a few details.
--- In [email protected], kd4e <[EMAIL PROTECTED]> wrote: > > > --- In [email protected], "John Champa" <k8ocl@> wrote: > >> Is what I am hearing true? That BPL has no affect on digital voice! > >> > > I'm too new at DV to know, but why would this be true? Interference > > is interference, right ? > > > > Andy K3UK > > I am going to attempt an explanation though I > may be broadcasting ignorance and generating > more smoke than light! > > Analog audio is converted to digital data. For specific types of audio such as speech, the conversion to digital data is done using techinques that minimize the amount of data required to replicate the audio. A modem designed for speech would not work for transmitting audio for SSTV or music because speech coding (vocoding) techniques take advantage of the fact that speech does not occupy the entire spectrum from 300 to 3000 Hz. > The digital data is chunked into packets. > > Each packet has a header and footer that ID's > the content. > > If the packet arrives damaged it is automatically > resent until it arrives intact. Not necessarily true - it depends on the application. There are 2 basic categories of techniques for maintaining with data integrity - forward error correction (FEC) and automatic repeat request (ARQ). In the ARQ mode, some information is added to the data to allow the receiving to determine if there are errors. If the receiving station determines there are errors in the data, a request is sent to the transmitting station to retransmit the data. In the FEC mode more redundant information is sent to allow the receiving station to not only determine whether there are errors in the data but also to possibly correct those errors. For example, AMTOR (and PACTOR) have two modes of operation with the names FEC and ARQ. When sending CQ, the FEC mode is used but during a QSO the ARQ mode is normally used. The FEC mode allows anyone to decode the transmission with tolerable errors. The ARQ mode requires the receiving station to send a report periodically to indicate whether the data was received correctly. PSK31 uses only FEC so it does not require an error report from the receiving station. Digital SSTV (such as HamPal) creates a Bad Segment Report (BSR) that can be sent to the transmitting station to request a retransmission of the missing data. Since the reception errors are different at each receiving station, the FEC mode is more suitable for CQs or roundtable discussions where it is impracticable for each receiving station to send a repeat request for the missing data. For most of the modes used by ham radio, such as text, speech or SSTV pictures, some errors in the data reception are tolerable and do not degrade the content significantly. The attractiveness of the PSK31 mode for many people is its enhancement to weak signal communications. A QSO can be successful even with a 20% data loss rate. I have been able to receive some pictures using HamPal with remarkable detail and quality but the success rate without the BSR is very low even with strong signals. It does not appear to me to be well designed for HF ham radio applications. Analog SSTV is very popular even though the picture quality is considerably less than digital SSTV. Digital voice and SSTV will become much more attractive when modes are developed that deliver acceptable quality at lower signal to noise ratios than current analog modes. > In theory what would be interference to an analog > modulated signal (we'd try to filter out with analog > and/or dsp filters) would only slow the successful > and clean transmission and reception of a digital > "packeted" one, not stop it. This is true for the ARQ mode although increasing interference would eventually cause data loss. In general, the more that is known about the characteristics of the interference the more one can do to eliminate, avoid or mitigate it. Ed WB6YTE > Do I have that correct -- in my simplistic layman's > terminology? :-) > > -- > > Thanks! & 73, > doc, KD4E > ... somewhere in FL > URL: bibleseven (dot) com > 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/
