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#8
July 16th 03, 07:51 AM
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Frank Gilliland wrote in message ...
In , (Richard Cranium) wrote: I'd like to be there when you tell that to Avanti, makers of the PDL-II and Moonraker series of antennas, which feature selectable horizontal and vertical polarization! I think they would laugh at you, Frankie, as will the many owners of those antennas. I would like you to be there, too. When one of you has a time machine, let me know. I was under the impression that Avanti was no more. :~) the Digital Speech Processor kit. I have the skills required to construct either, but not the knowledge to know if either project is worth the effort. A speech processor is an excellent idea, but I have no idea if Lou's even works. I certainly have some misgivings about 90% average modulation! What? A "commercial broadcast engineer" that can't look at a schematic and know whether the circuit works as advertised or not? Not when you have four high-gain, high input-Z OP-amps on the same chip, all inverting, all DC isolated on both the inputs and outputs, and all outputs are unloaded. Digital speech processor? I have details of his "DSP" - Dynamic Speech Processor, which makes claims about a 90% average... so seems to be what is being referring to above. Looking at the diagram, the first amp is a buffer stage, the second has a gain of about 45 and the third a gain of about 21. From his figures for a standard mic output, a loud whistle should produce about 6 volts into the clipper... which is down to 500mV from the clipper into the output amp. IOW, any one of them could start oscillating at any time. I assume that you do not have a copy of Lou's guide to the DSP... quote you may get self oscillation (squealing) due to the overall high gain of the combined DSP, power mike (if used), and radio circuits /quote And... quote The squeal problem is very common in newer Uniden SSB rigs /quote His "cop-out" is that with "correct adjustment", this should not happen. But, he is putting this adjustment in the hands of people who he has said cannot even solder correctly. And how can anyone adjust it correctly with, as he suggests, an SWR/power meter or someone listening to your signal? Or, like I said, the thing may not even work at all because it might lock up on power up. More likely, it will fail due to poor building of the circuit. I had someone ask me to fit one for him, which he supplied as supplied by the shop, ready made. There were all kinds of problems, including the screened wires not being correctly connected - they were shorted between screen and inner wire. Also, the version I have the details for had the LM324 fitted in place of the LM3900. It should be noted that the LM324 is a "domestic" spec chip... and may fail in temperatures below Zero or above +70° centigrade. The LM2902 (-40° to +85°) or LM124 (-55 to +125°) are better. "90% average modulation" would be great on AM (ask a real commercial radio engineer, Frankie; they use 'em all the time), Considering that the circuit is nothing more than a preamp and clipper, I have no idea where he gets the idea that it will achieve 90% average modulation. Does he claim that *the DSP* will manage that magical 90%, or just that the figure is possible? quote With correct speech processing, it is possible to increase the average modulation to about 90%! /quote Unless someone has seen a statement claiming that his product does that "90%", or that the "90%" can be done without unacceptable distortion or harmonic content... then it is nothing but sales bumf- careful wording. Also, it says, "about 90%". However, here is another of quote from Lou: quote Up to 15dB of clipping is quite acceptable to the ear /quote The average peak to average ratio figure for the human voice is 14dB, although some people may be higher or lower. The 90% quoted may be for some people with a lower than average ratio. Even then, to get such a high average, without an unacceptable level of distortion, would be pretty tight... and certainly not possible without using proper test equipment to adjust the device exactly. in case you missed class, any form of compression and/or clipping is a form of distortion; i.e, more compression equals more distortion. Certainly with clipping, but how about a VOGAD? If it is set to reasonable attack and decay times, there should be no problem. However, the time delay means that it would not work the same as clipping. Years ago, some AM broadcast stations would crank up the compression to the point where the sound quality was offensive, but that's rarely done anymore. In the UK, Radio1 manages to create "offensive" audio without adding distortion... they just play crap "music". ;~) but when the operator isn't saying anything there'd be a rather obnoxious noise transmitted as the circuit searches for anything to keep the modulation at 90%. As witness the BBC some years ago, until they learned to turn down the processing. The "searching for noise" will probably be associated with something more complicated than the simple amp and clipper used in the DSP... possibly a combination of VOGAD and clipping. Brainbuster. |
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