These numbers seem very much what others have reported as well. A difference of 3 or 4 dB lower is highly significant, even with AWGN tests.
When we were testing SCAMP, which used the RDFT protocol, there was nothing so frustrating as to watch the mode time itself out even though signals were more than adequate to carry on a solid SSB voice contact. Which, of course we could not do because we were in the data portion of the band. But just a few dB improvement in the protocol would have meant the difference between success and no success at all, even if it had to run a bit slower. But it was either good speed or no speed. And this is the difference between digital data and digital voice. With voice you have a threshold that you really can not go below or else the quality becomes unusable. With data, you can have fall back positions, albeit at a slower speed. 73, Rick, KV9U KT2Q wrote: > Rick... > > >> I got the impression in talking to the WinDRM >> users on 7173 SSTV group, >> that it worked with lower than +10 dB S/N. Maybe >> around 7 dB? >> > > For what it's worth, I did some path simulator > tests with WinDRM and the SNR decode threshold > seemed to be around 8db. It was about 3 to 4db > lower with DVDRM. These were AGWN tests without > any simulated ionospheric disturbance added in. > > Keep in mind that the modes might start to decode > at these levels, but being right at the threshold, > any QSB or selective fading would cause the signal > to drop out. I think 10db is a more realistic > figure for reliable copy. > > >> maybe it has a similar modulation scheme >> to OFDM? >> > > I think it does. > > >> The audio quality is that "internet" sound >> > > Yes, it does sound digitized to some extent, but I > think the near zero noise floor makes the user > forget about the robot-like characteristics! It's > fun to use... > > Tony KT2Q > > > > > > > > > > ----- Original Message ----- > From: "kv9u" <[EMAIL PROTECTED]> > To: <digitalradio@yahoogroups.com> > Sent: Friday, March 23, 2007 5:51 PM > Subject: Re: [digitalradio] Re: DVDRM KV9U > > > >> Hi Tony, >> >> I got the impression in talking to the WinDRM >> users on 7173 SSTV group, >> that it worked with lower than +10 dB S/N. Maybe >> around 7 dB? >> >> The older programs used the RDFT protocol which >> did require around +10, >> and that is at least part of the reason for so >> rapidly abandoning RDFT >> based software and moving toward the OFDM type >> as found in WinDRM. I am >> not sure how RDFT works either, maybe it has a >> similar modulation scheme >> to OFDM? >> >> The audio quality is that "internet" sound that >> we used to get with low >> quality dial up speeds and is not unlike some >> cell phone connections. I >> am assuming this has a lot to do with the number >> of dropped packets. >> >> 73, >> >> Rick, KV9U >> >> >> >> >> >> KT2Q wrote: >> >>> Rick, >>> >>> WinDRM does need a fairly good SNR. The >>> threshold >>> seems to be around 10db. Of course it's much >>> easier to achieve that on the upper HF bands so >>> it's usually not an issue there. On 40 meters >>> and >>> below it seems that DVDRM mode does a better >>> job >>> coping with QRN. >>> >>> It's not exactly hi-fi as you say, but it's >>> interesting to note that the decoded audio has >>> a >>> range of about 4khz (see attached). The lows >>> dip >>> way down and the high-end is slightly above >>> 4000 >>> hertz. >>> >>> I guess you could say audio response is pretty >>> good when you consider the RF bandwidth is the >>> same as used for SSB! You'd need 4khz to >>> duplicate >>> this with analog. >>> >>> Mel and I have fooled around with EQ a bit and >>> you >>> can enhance the DV audio to sound >>> terrific, but the problem is getting software >>> EQ's >>> to work simultaniously with WinDRM. An outboard >>> unit would work fine. >>> >>> Check with Mel about the DVDRM mode info... >>> >>> 73, Tony KT2Q >>> >>> >>> >>> >> > > > > > > Announce your digital presence via our DX Cluster > telnet://cluster.dynalias.org > > Our other groups: > > http://groups.yahoo.com/group/dxlist/ > http://groups.yahoo.com/group/themixwgroup > http://groups.yahoo.com/group/contesting > http://groups.yahoo.com/group/wnyar > http://groups.yahoo.com/group/Omnibus97 > > > Yahoo! Groups Links > > > > > >
