First of all... I’d like to thank TOLL_FREE for starting this thread. Technically, this is the BEST thread to appear on this forum in a long, long time !!!!
Secondly, I’d like to recommend that all of the posters to this thread look at this paper published by the National Association of Broadcasters. It concerns Modulation, Overmodulation, and Bandwidth on AM broadcast Stations. There is much space devoted to negative peak clipping and other audio enhancements. It is technically strong, but if I can understand it, you guys won’t have any problem.
http://www.nrscstandards.org/SG%20ref%20docs/AM_Mod_Overmod_1986.pdf
What I got out of this paper is that negative peak clipping (in broadcast stations, anyway) generates more splatter than regular overmodulation (carrier pinchoff). Also, if NPC is used, it should be implemented in the audio chain, and be followed by a sharp rolloff low-pass filter (the type of circuit that MrSuburban described and uses on his Johnson Valiant). Finally, a "protection clipper" should be installed in the rf output circuit to limit any negative peak overshoot that might cause carrier pinchoff. The circuit that TOLL_FREE posted fits this requirement completely.
Lastly, speech clipping is an effective method for increasing AVERAGE modulation and making a radio sound louder on-the-air. Some early tube rigs (Browning Eagles and Sonar CBs, in particular), utilized speech clipping, and because of this, are still great performers, even today. In solid-state CBs, only the Uniden "858" rigs had speech clipping circuits, and they worked so well that they were banned by the FCC in 1979. These radios included the Cobra 21, 25, 29, 89, 138, and 139 in the XLR series. Identical chassis were marketed by Courier, Realistic, President, Robyn, Sears, Teaberry, and others. The problem is that speech clipping introduces distortion that removes much of the "NATURAL" sound of the audio. Supposedly, NPC also raises the average modulation, but has audio with a much more natural sound.
I run 858 SSB radios on my base exclusively, and after reading MrSuburban’s post, I decided to try his mod on my TRC-457. I jumpered the positive peak limiting diode (D21) with an spst relay so I could switch the mod in and out. On a ‘scope, the positive mod peaks showed a definite increase, but since there are no other local stations around here, I couldn’t get an audio check to see if I sounded louder (and clearer) with the NPC mod or not.
Maybe LOOSECANNON can try this on his 139XLR and post the results.
This type of clipping circuit can be easily added to any radio. If you want specific info, let me know.
Sorry that this was so long-winded. Hope it made sense.
- 399
