Same thing downriver where 1230 is noise with hints of voice far away, 1240 same description, 1250 the IBOC clatter buzz hash trash from a Radio Disney mind-crusher at 1260. It's about 2-hours after sundown.
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The edit timer says I have 21-minutes remaining, so I want to continue the discussion on bandwidth vs. antenna Q.
Today I toyed with audio bandwidths and observed the results on several AM radios.
The ear detects little change because the audio bandwidth is already narrowed by the receiver, as we've seen from Rich's graph, the audio top-end rolls off fast after about 3kHz, but listening to the sidebands it IS possible to hear the slashy sounding overshoot on vocal sibelance, when feeding audio with energy between 10 and 20kHz.
In any case, reducing the audio bandwidth DOES NOT INCREASE THE ANTENNA Q.
Antenna Q has to be designed at the transmitter/antenna system.
Once Q is significantly sharpened by appropriate antenna design, it self-filters the audio bandwidth, I believe, which I'm guessing makes it unnecessary to also reduce the audio bandwidth at the audio input to the transmitter.
Am I as right as I am acting I am?
Enabling NRSC pre-emphasis by any means available is pretty much a requirement to match the de-emphsis that is supposed to be in place in the consumer radios. Depending on whether the peak limiter is before or after the pre-emphasis is important to know. In the AMT5000, the internal pre-emphasis is located after the peak limiter. This means when you enable pre-emphasis, the limiter (MODULATION control) must be adjusted downward somewhat to ensure the high-frequency peaks (after pre-emphasis) don't exceed 100% modulation (by much anyway) to prevent over modulation distortion on the boosted high frequencies.
The Q of the AMT5000 output is such that the 3 dB audio bandwidth is about 11 kHz assuming a "typical" antenna ground resistance of 30 ohms. A lower ground resistance will increase the Q and a higher ground resistance will decrease the Q.
The pre-emphasized high audio frequencies are filtered out by the output Q, so there is no direct need to reduce the bandwidth of the audio to match the RF output bandwidth, but it is important to not over modulate the transmitter at high audio frequencies. High frequency over modulation is not easily perceptible. It usually manifests itself as distortion of the "S" sounds.
It may seem counter productive to have to reduce the modulation level of lower frequencies to achieve the NRSC boost curve. But, that's the price paid for good high frequency fidelity. When a broadcast station decides to reduce its audio source bandwidth to say 5 kHz, you can see on Rich's NRSC curve in post #21 above that the pre-emphasis only needs to be 7 dB instead of 10 dB, so the lower frequencies are brought up 3 dB relative to the 100% modulation limit.
Step 1. Manufacturers reduce the high-frequency audio.response at the outputs of AM radio receivers to eliminate or reduce nighttime interference by channels adjacent to the one the radio is receiving -- the purpose being to eliminate buyer complaints and (probably) to reduce the cost of manufacturing those radios.
Step 2. Broadcast stations and industry groups recognize the loss of fidelity in AM reception as a result of step 1, and develop NRSC pre-emphasis in an attempt to overcome it.
But narrowband receivers now have more interference to their audio in the band from 20 Hz to 5 kHz from AM stations on adjacent channels, when all of them are using the NRSC curve. The (freehand) illustration below shows why this is true.
NRSC is useful in improving the fidelity of AM stations in narrowband receivers tuned to stations having relatively no interference from adjacent channels. However that improvement is limited to areas close to the transmitter where field strength is very high compared to the signals of adjacent channel stations.
Slightly reduced modulation level of the lower audio frequencies to avoid overmodulation of the pre-emphasized high-end is less of a problem because the lower frequency region is less susceptable to noise.
But we wonder whether there is reason to chop off or roll-off part of the low end.
For example, according to Dave Moulton's audio spectrum chart (TV Technology 4/1/10) the 1st octave of sound, 31.25 to 62.5Hz, contains little musical content, zero voice content, and is not reproduced by most loudspeakers. Perhaps this octave should be removed so it cannot waste unheard energy.
Note in the graphic I posted in Reply 33 that it is the audio producing the highest modulation percentages near ~9.5 kHz on the adjacent channels that produces low-frequency audio interference on the desired station.
The receiver demodulates interfering signals with respect to the desired carrier. Interfering signals produced by 9.5 kHz modulation of the adjacent channel signals are located 500 Hz from the desired carrier in the desired channel, and therefore produce an interfering audio frequency of 500 Hz in the receiver output -- which is very audible even in inexpensive AM receivers.
This is all irrespective of whatever low-end cutoff is used to modulate the desired station.
The fine arts of AM radio hold many secrets.
I'm glad you repeated the business about LF artifacts being produced by way of signal mixing under the stated circumstances. I'd missed that point on first reading.
Given that situation, it appears that low frequency audio below the range of reproduction (by the radio) does not cause any problem in itself.
It's probably not very scientific/accurate but there was an idea advanced on Page 15 of The Low Power AM Handbook about using the WinAmp and other equalizers to approach boosting the highs.
channels.
I pretty much go in expecting nothing.
Every now and then something pops up.
On the 6 local channels all together, I've
gotten 32 stations. My best distance so
far is 399 miles.
If something REALLY interesting happens,
I'll let you guys know.
Bruce, DOGRADIO
Using the EQ in Winamp is very handy, and indeed could provide a quick trick for AM pre-emphasis.
I have been using the DSP Winamp Plugin called StereoTool, which has a low and high pass set of filters, and can be spectrum shaped with EQ settings.
My new Spring project will be designing a High Q AM antenna that is self filtering for around 6 or 7kHz, which is an overshoot just half-way into an adjacent channel.
My new Spring project will be designing a High Q AM antenna that is self filtering for around 6 or 7kHz, which is an overshoot just half-way into an adjacent channel
Make it narrow in diameter.
Yes, Dade City, but how narrow?
I am going to start with a hair thin wire antenna. What kind of bandwidth will that be?
Wow, that's some good reading, to keep us busy for the next month.
See you sometime in March!
I'm a bit late to this thread, but have some comments regarding bandwidth/frequency response.
As has already been noted, there is nothing inherent in the process of amplitude modulation to limit bandwidth. Current FCC regulations set the upper frequency limit for audio content on the AM broadcast band at 10KHz. But, and this is a big BUT, that has not always been the case, and in fact that rule does not apply to us under Part 15.
In days of yore, when radio began, the state of technology was the limiting factor in fidelity. By the 30's or so, microphones and speakers were the limiting factor in frequency response. By the mid-late 50's true HiFi was coming of age, and 50-10K response was common, and 30-15K was attainable. By the time the 70's had rolled around, the 'gold standard' for frequency response in the audiophile world had settled as 20Hz-20KHz, which is also assumed to be the extreme limits of human hearing.
Anyhow.... at some point in all this, probably in the 50's or early 60's, the FCC designated 15KHz as the limit for AM Broadcast. A tad short of 'audiophile" quality, but nonetheless still excellent. By the late 60's or early 70's, stations had begun to discover the "benefits" of simple compression and equalization in attempts to make their station sound "louder" or "better" than the competition. That practice has, of course, continued to this day, with ever-more-complex audio processing being applied for virtually all broadcast audio (am, fm, digital, analog, tv, internet, you-name-it).
At some point, and without trying to research it I'll again take an educated guess and say perhaps late 80's, and maybe coincident with the appearance of C-QUAM stereo, the FCC cut the bandwidth on AM BCB down to its present 10KHz.
But we, operating under Part 15, are not subject to that, and therefore can offer (to those listeners who can receive and appreciate it) a wideband 15K or even 20K signal, provided our transmitters are capable. And most transmitters sold for Part 15 use are indeed capable of at least 15K or better, as they do not employ any steep or "brick wall" filtering in the audio path.
One exception to that would be Radio Systems and LPB transmitters that may have been used in as TIS stations: If you repurpose one for Part 15 use (carrier-current, most likely) that was previously used for TIS, you'll want to bypass the "TIS Filter" which limits bandwidth down to 3KHz. I'm not sure if this filter is there for practicality or if there is an FCC rule limiting TIS stations to 3K audio bandwidth, but you'll definitely want to bypass that filter (if present) unless your station is to be talk-only.
OK... 'nuff rambling on my part.
At the present time the rules for TIS (Traffic Information Stations) do indeed specify an audio bandpass limit of 3kHz.
No reason is officially stated in the rules, but insiders believe the NAB (National Association of Broadcasters), in one of their smaller efforts to protect their corporate members, used their lobbying power to restrict the TIS frequency response as a means of making such stations "hard on the ear" and thus of no possible competition to the Membership, who in anycase sabotage themselves by their poor programming, loud and clear as it may sound.
MRAM 1500 streams the TIS station from his Ohio town, and it's actually better than 95% of the stations on my home dial.