For many years now on the short wave bands broadcast stations have been using CCM (Controlled Carrier Modulation, also being called other names) to reduce the carrier as much as 6db. That along with multipath can cause some strange distortion problems on the receive end. That has as of a few weeks ago been approved by the FCC for AM broadcast. Add to that stations can do 125 (or more) positive peak modulation and it makes clean decoding a bit more of a challenge.
Dave KU4B On Thu, 2011-12-15 at 03:51 -0500, ke...@3950.net wrote: > On 12/14/2011 10:47 PM, Brett Gazdzinski wrote: > [...] > > What as the problem with early versions of psdr and the agc working > > off the audio? > [...] > > > > For those who want the best fidelity from AM, the problem is this: > The carrier gives an almost perfect reference for receiver AGC, > without having to depend on the transmitted audio level for AGC > detection as one necessarily does in suppressed carrier SSB. If you > allow the audio level to be a factor in AM AGC detection, then you > lose some of that fidelity (dynamic range preservation) advantage -- > the AGC becomes a _de facto_ audio compressor, and those who care > about fidelity cringe at that. They complained about it and got it > changed. But people with less critical ears might never have noticed > the problem. > > I do wonder if there is now clipping happening somewhere in the > receiver / audio system since the change, when you're receiving one > of those amateur AM stations that likes to run 150% or 200% positive > peak modulation (AM broadcast stations usually limit themselves to > 125% positive). > > If so, maybe the ability to switch in lower post-detector gain, or > peak AGC, on such stations should be added. When extremely high AM > positive peaks are present, a light could flash, too. It could be > labeled "W3DUQ." > > > 73, > > Kevin, WB4AIO. > > _______________________________________________ > FlexRadio Systems Mailing List > FlexRadio@flex-radio.biz > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/ _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
