Anyone ever think of designing a LP Tx that uses digital techniques to modulate the final RF stage? or even generate the entire C-QuAM Signal digitally and then feeding that to a RF output section?
Anyone ever think of designing a LP Tx that uses digital techniques to modulate the final RF stage? or even generate the entire C-QuAM Signal digitally and then feeding that to a RF output section?
I know a lot of FM exciters and FM Stereo generators now do this.
I tapped into this thread because it's about digital radio transmission, and here is my question at this time:
We all know that AM & FM in the U.S. is using a system known variously as i-BAH, i-BUZZ, and a few other names, including HD radio.
The present system puts double digital signals below and above the analog carrier, and both the lower digital side-band and the upper digital side-band can carry separate channels, for a total of 3-channels per station.
And yet, many forecasters have promoted the idea of an all-digital future, at least for the AM band. But wouldn't that reduce the number of discrete channels made possible to each station? The analog carrier would be gone, for one.
Stations that cannot afford to make one channel work might not want to give up additional channels, because one of them might do better than the ones that didn't.
Just noting that in the present state of HD broadcasting in the US, all three of the digital programs are carried (with various amounts of data compression) on the digital carriers that are present mostly above and below the carrier frequency and AM sidebands of the conventional, analog main channel. This is a technique used to reduce the amount of audio distortion present when the digital signals arrive at receivers using multiple transmission paths.
The best quality digital channel carries the same program as the analog AM/FM station, and the receiver will switch back to the analog version if the digital signal falls below a certain quality level. Getting the timing/synch correct between the two can be a problem at times.
So in reality, a fully-loaded HD station transmits four program channels, but two of them (one analog and one digital) must have the same program information.
That is impressive technology, the 4-channel digital analog systems descibed by Rich, even though many complain about it.
What, then would be the channel capacity of an all-digital AM service that's been suggested by some, including spokespersons from the FCC?
I'll take a simpler approach to this question. By "digital techniques" k8gpd may be referring to the now common practice of creating amplitude modulation by driving the RF output stage of high power AM transmitters with a high level series modulator that operates in highly efficient "class D" mode.
Classic high level amplitude modulators were simply Class A or Class B audio amplifiers in series with the power supply feeding the RF amplifier. Going way back to tubes, this was called "plate modulation" which was the best of the various AM schemes. This method requires the modulator amplifier to provide a maximum power level equal to half of the RF final input power. So, if you had a 1000 watt transmitter, the modulator had to provide 500 watts of audio power for 100% modulation. A 500 watt class A or even class B audio amplifier will produce a lot of heat due to relative inefficiency.
A class D audio amplifier operates as a binary switch. It's either fully on or fully off and spends very little time in the transition region, so it's very efficient. The varying power delivered to the load is determined by the ratio of on to off time at any instant. The input signal is sampled at a high frequency rate and converted to a PWM (pulse width modulation) single bit digital on/off waveform to drive the audio transistor. A simple LC filter on the output of the amplifier smoothes the steps to re-create the original analog waveform.
This would work fine in a part 15 AM transmitter, but all said and done, there is no advantage to this for Part 15 AM transmitters because of the very low transmitter power limit. The heat generated by a simple, albeit inefficient, class A audio amplifier supplying 50 mW of audio is not a concern. The Part 15.219 100 mW rule applies to the input power to the final RF stage. The power dissipated in the modulator stage is not part of the equation.
Not sure what kc8gpd was asking but it could be about a digitally encoded audio modulation scheme. It appears that a digitally encoded scheme could be used but the receiver would need to be able to decode same.
To add to the comments about digital techniques used in AM transmitters which produce conventional AM as the output here's a <a href="" http://hawkins.pair.com/wabcnow.shtml#digmodsect" ;" LINK to an article about the Harris DX-50 AM transmitter which uses digital techniques. The technical part is about half way down on the page.
Neil
There is a request posted with "Forums" asking that we tap into existing threads rather than starting new ones. But I have just proven that this doesn't always work.
The question I've been trying to ask was partially answered by Rich, but after that the responses went to the opening post from Robert kc8gpd, and my ultimate question never got addressed.
From now on I'm starting new threads to avoid the inter-confusional mis-matchery brought on by nearly unlike dis-postings.
By all means, start a new thread. This will help us to focus on the questions or information contained therein.
The "forum advice" is intended to collect posts on a topic in a single thread but this is not always practical as illustrated by Carl's experience. There is no need to search through months or years old postings for a "fit".
Neil