Hi, Jeff You bring up an interesting point. First of all, I am still attempting to understand why it is absolutely necessary to provide substantial pre-emphasis to AM audio above the broadcast band (usually a 7-12DB peak at 2.5 kHz). A microphone, transmitter and receiver, all when used together will sound beautiful on the commercial AM band will sound muddy and muffled if the same equipment is operated at 160 meters. Signal-to-noise might be a factor, unless the signal is local and fully quieting, and even then pre-emphasis appears to be necessary. Commercial shortwave stations pre-emphasize their signals. The popularity of the D-104 which has this very pre-emphasis built-in, are popular even on the CB band.
When I purchased my Kenwood TS-440 (well before interest in AM), my first wish was to optimize the set for weak-signal operation and I set about to find a 1.8 or 2 KHz filter to fill in the unused M1 Selectivity switch position. Much to my surprise and disbelief, I was introduced to the notion that a filter of this bandwidth would be next to useless for long distance audio recovery because it would chop off so much of the audio bandpass that very little audio information would be available for noggin processing. This has indeed turned out to be the case, and in fact instead of filling in the unused selectivity position with a narrower filter, I actually obtained a wider filter, think 2.8 KHz (one supplied with the Kenwood TS-830) and literally widened the receiver. The result has been excellent as the same filter is used for transmit and audio reports have been very complimentary. I almost never use the original 2.3 filter that was supplied with the receiver and only in special circumstances have I found it any more effective than the wider filter. It is usually worse. Further, I have listened to AM signals from across the country, and usually not with the TS-440. I will agree a few have been so bassy that they can be difficult to understand, but overall signals with "50 to 5KHz" modulation are overall so much easier to hear through noise that one wonders where the notion of "communications quality" came from. There are some signals that are so well processed (and frequency limited), and where the noise just happens to be strong in a different part of the audio spectrum, or the voice characteristic of the operator happens to overcome noise that such weak signals cut through. Overall, in my experience, a full-fidelity signal will be heard in most cases where a "communications quality" signal is torn up by noise and fading. This goes for the receiver as well. The SX-62 is very wide, and the audio capture from an am signal is obvious when compared with the TS-440. I don't dislike the '440 at all, and use it all the time, but have observed that even the wider positions of the Collins R-390 will allow better understanding of a voice buried in noise. Maybe it is just my noggin, but Mike Dorrough, K06NM would talk here about "power bandwidth," and Rich Measures, AG6K would talk about "information bandwidth." Another interesting experiment is to adjust the variable bandwidth control of a SB-200/BC-779 series Hammerlund receiver and listen as the received signal jumps out of the noise when the bandwidth control has been adjusted to an optimum. The adjustment varies greatly with every station, which indicates to me that the signal to noise ratio varies with more than simple amplitude with every station. No doubt received noise rarely has a flat power bandwidth across the audio spectrum. In short, audio needs to be pre-emphasized on short wave (don't know why), and the transmitter and receiver should be matched to the same information bandwidth. Then, within reason, the more information transmitted means more information is available to the receiver and can offer an advantage to sustain a communications path. In my experience, presence of lows has made the difference of whether or not a station can be understood. Of course if communications is the first priority in a weak signal environment, then use of the SSB mode offers a tremendous advantage. Use of AM involves other factors such as audio quality, ease of receiver tuning, application of and restoration of vintage equipment, and use of homebrew equipment, all of which go way beyond simply establishing a radio contact. Often on-air correspondence differs in character from SSB contacts, both in duration and depth of information. I just slipped on the soap. 73 de Bill, AB6MT [EMAIL PROTECTED] ----- Original Message ----- From: "Jeff Edmonson" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, July 24, 2002 4:24 PM Subject: [AMRadio] Audio response and Long distant QSO's (was: Suggestions Please) > > > : If you have to ask, then you are probably using too much. If you are > : listening, you probably aren't using enough. If all you think about is > : comparing AM to SSB, then you are just looking for a silly argument. > > Trust me, Bill, this is not someone "trolling" for a fight in a group. > > I'm serious when I ask, though - what's the purpose of trying to run > audio, starting at 50Hz, on a noisy HF band like 160, 75 or 40m? > > honestly, from across the nation, those transmitters who have that much > low-end response are the same people who want that low-end response > enhanced, and in periods of QRM/QRN and selective fade, it's more of > a determent to be able to copy, than an enjoyable, enhanced audio > performance. I'm absolutely sure that the later is the goal intended, but > for talking cross-country on noisy bands, SORRY!, it's just not gonna > happen. > > Now, it's possible to have GREAT audio and still filter and tailor the > audio so that it's not out of the passband, such as that of Don Chester, > K4KYV. I'm hoping that Don can come in here, and remind me how it is/was > he was peaking the audio at around 3kc, and then rolling the whole thing > off at 3.5kc (or slightly higher). And, what's the bottom end? > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
