|Joined:||Sat, Apr 4th 1998, 00:00||Roles:||N/A||Moderates:||N/A|
|Hi Fi AM||kc2ifr||on 26/11/13|
The FCC intended it that way. The Part 97 limits on bandwidth were left deliberately vague, defined in terms of "good engineering and amateur practice", in order to give amateurs the maximum flexibility for experimentation and self-instruction in the radio art. A few years ago, Riley reiterated that at the FCC forum at Dayton when someone posed a question regarding so-called ESSB, although for whatever reason, he went on to raise the issue of whether ESSB has any place in amateur radio at all.
Just as variable selectivity is a desirable feature in a receiver, some variability in audio bandwidth is equally desirable in a transmitter. There are many ways of accomplishing this; in the audio chain that drives my transmitters I have incorporated switchable, passive, low-pass audio filters, something I picked up decades ago at a surplus store and at a hamfest. One filter has a very sharp brick-wall cutoff at 3400 Hz, while the other has a more gradual cutoff that begins just above 5 kHz with everything gone at 7.5 kHz. The third option is no filter at all, with the highs limited only by the frequency response of the audio transformers. Normally, the 3400~ filter is used under congested band condition or when there is a nearby adjacent QSO, and the 5000~ filter is used when the band is less congested. The no-filter option is rarely used except for testing purposes.
Passive filters like mine are hard to find these days, and usually the ones that show up are expensive, but current technology allows one to easily build effective filters at very low cost using active circuitry. I have seen circuits published on websites, using nothing more than a handful of components, usually a couple of transistors and IC chips, plus a few resistors and capacitors. More sophisticated filter circuits can be found using digital technology, for those so inclined.
There is little use in transmitting audio frequency response that would produce 20 kHz wide signals, since very few amateur receivers would be set to wide enough selectivity to receive such a signal in its entirety, and it would be poor engineering and amateur practice to deliberately transmit such a broad signal merely to keep the adjacent channels clear. But, OTOH, "wide" signals that cause harmful interference to adjacent channels are more often the result of spurious distortion products than the frequency response of the audio used to modulate.
I would recommend first and foremost, a "clean" transmitter that is not pushed beyond its modulation capability, and then an appropriate low-pass audio filter in line to limit bandwidth as needed. Merely lopping off the higher frequencies beyond a certain point doesn't cut it; the overall audio response curve needs to be adjusted for balance to produce pleasant sounding audio that is still readable under adverse conditions.
I often get reports that my audio is "broadcast quality" when using the 3400~ filter. A "presence rise" of some 9 dB in the upper mid-range seems to allow the articulation of consonant sounds to still pass through, and compensate for the limited high frequency response. Nevertheless, when running an A/B comparison under less congested band conditions, most reports tell me that the signal sounds better with the 5000~ filter. But they usually tell me there is little or no difference in audio quality between the 5000~ filter and no filter at all.
An interesting tutorial on AM and audio frequency response can be found at http://www.amwindow.org/tech/pdf/eam.pdf
|AM on HF - Are you with us ?||WA3VJB||on 30/7/11|
A far cry from the League's policy of "benign neglect" (don't ban AM but let it die a natural death) as expressed by the young lady (I don't remember her call sign) who moderated the Forum for the League at the ARRL convention I attended at the Statler-Hilton in Boston some time around 1974 or 1975.
I also appreciate the support the League extended to the AM community regarding the ill-conceived AM power reduction that was included in the FCC's revision of the power limit rules, which took effect in 1990. The League filed a Petition for Reconsideration to permanently extend the seven-year grandfather clause to allow AM to unquestionably continue using the same historic power levels that had existed dating back to the very first voice transmissions ever attempted by amateur radio operators.
Unfortunately, the FCC dodged the issue with its own deceptive spin. At the Dayton FCC Forum, speaker Johnny Johnston began the session by complaining how the amateur community was wasting the Commission's time with frivolous rulemaking petitions. His first example was ARRL's Petition for Reconsideration (which asked for the permanent extension of the seven-year Grandfather Clause to continue the historic AM power limit). "Here, we have the ARRL petitioning to CHANGE THE RULES to allow AM to run twice as much power as everyone else. And then we have this other petition to eliminate AM altogether... (he then pulls out a petition that had been submitted years earlier, with no action taken, to outlaw AM below 28 mHz, but which inexplicably was suddenly and conveniently given an RM-number). Of course his tactic was to demonstrate to the public that the FCC would maintain a "fair and balanced" approach and do nothing, allowing the Grandfather Clause to expire and the AM power reduction to go in effect.
|General Comments||AA6E||on 26/7/11|
|I logged in to the ARRL website and received the welcome message, but the forum page displayed the error message "Login failed. Invalid username or password." Let's see if this post goes through.|