A recent commentary article in Radio World addresses the issue of AM bandwidth. I believe there are hidden agendas.

In the North America, AM stations are assigned to frequencies from 530 KHz to 1700 KHz in 10 KHz increments (other parts of the world use 9 KHz increments). When a radio signal is amplitude modulated, sidebands are generated at the carrier frequency+modulating frequency and the carrier frequency-modulating frequency. For example, if a station at 1000 KHz modulates the transmitter with a 1 KHz tone, it will create sideband frequencies at 999 KHz and 1001 KHz.

Since stations are spaced 10 KHz apart, any modulation frequency in excess of 5 KHz results in sideband products spilling over into the adjacent channels spectrum. Years ago, the audio modulating AM transmitters was rolled off after 5 KHz to prevent interference with adjacent channels.

FM, with it’s capability of reproducing audio frequencies up to 15 KHz, began to compete with AM. The fidelity of AM became more important. Stations were allowed to modulate up to 7.5 KHz improving the fidelity. This did cause some interference to adjacent channels but normal wideband receivers were not selective enough to separate adjacent channels. Limiting modulation to 7.5 KHz still meant the adjacent channels spectrum was clean out to 2.5 KHz from it’s carrier allowing narrow bandwidth receivers to receive an adjacent channel without interference.

FM continued to gain market share. AM stations were allowed to go up to 10 KHz which means they’d splatter over an entire sideband of an adjacent channel resulting in objectionable interference even for a listener with a narrow bandwidth receiver. AM stations even boosted high frequencies to compensate for high frequency roll-off in receivers.

Recently, the NRSC performed limited listening test using only three receivers and thirty subjects. Adjacent channel interference used simulated noise instead of real program material. Simulated noise made it possible to bias the tests towards a listener preference for constrained bandwidth by exaggerating the resulting interference. These tests determined that there was no advantage to a 10 KHz bandwidth. I guess that’s why broadcasters pushed to go to 10 KHz from 7.5 KHz.

HD Radio, iBiquity’s digital radio system for AM, utilizes spectrum from either 5 or 7.5 KHz to 15 KHz from the carrier, depending upon mode. To use this system you must constrain the audio bandwidth of analog audio. Ibiquity’s system only achieves a data rate of 36 Kb/s with 7.5 KHz – 15 Khz and 56 Kb/s with 5 KHz – 15 KHz. This is very poor use of precious AM broadcast spectrum, the total bandwidth utilized by an hybrid iBiquity digital AM signal is 30 KHz.

The HDC codec is a multi-streaming version of HEACC, which is AAC+ with SBR, the same Codec used for XM satellite. I’ve read in the XM blogs, 48Kb/s provides high quality encoding, at 40KB/s the definition is lost, at 32Kb/s quality is seriously impaired.

SBR, spectral band replication, reduces encoding bandwidth by discarding high frequency audio detail. At the decoding end it replicates the lower frequency spectrum to replace the lost high frequencies. Most high frequency program material is harmonically related to lower frequencies. The problem is that the exact nature of the harmonic content varies greatly in music from instrument to instrument. Flute producing almost a pure sine wave with almost no harmonic content. A brass horn or stringed instrument is rich in harmonics. The decay envelope for instruments is not the same for harmonics as for the fundamental.

Ogg Vorbis encodes good quality stereo at 45 Kb/s, the whole spectrum, no need for SBR. It is an open format and iBiquity can not extract licensing fees.

DRM, Digital Radio Mondale, is able to achieve a 30 Kb/s in a 9 Khz spectral space using using QAM64 modulation of the CODFM carriers. 15 KHz of spectrum would yield 50 Kb/s which would suffice for full spectrum Ogg Vorbis stereo encoding. The DRM format is used for LW, MW, and SW digitial broadcasts in the rest of the world.

I am all for digital radio, just not 30 KHz wide inefficient hybrid proprietary digital radio with garbage audio . We can have high quality audio in half the bandwidth with a non-proprietary encoding scheme. The F.C.C. was mandated with the task of regulating the public airwaves in the publics interest. The AM version of HD radio is not in the public interest. A DRM digital stream and Ogg Vorbis encoding would utilize bandwidth more efficiently and provide higher quality audio. Receivers and transmitting hardware would be less expensive since no royalties would need be paid to iBiquity.

I’ve added an RSS Feed to this blog so those of you who wish to follow it with an RSS News reader or add it to your website or headlines to your blog can do so.

Perry Lind made the comment that the old Westinghouse Electric transmitter sounded better than the modern transmitters, I’ve got mixed feelings. The old high-level plate modulated transmitters did sound good if they were well engineered and maintained within the limits of their capabilities. Modulation transformers and reactors resulted in bounce and tilt and limited the modulation that could be obtained to lower levels than it otherwise could be without overmodulating negative peaks.

Some of the other systems like the RCA AmpliPhase or CCA’s Doherty modulation sounded ok if everything was perfectly tuned and tweaked and in good order but were difficult to maintain in that state. The ampliphase system was kind of icky in that the relationship between phase angle and power output wasn’t linear and I don’t think predistorting the audio to compensate did so adequately.

One problem with any of the old tube transmitters is that tubes don’t just run perfect until they fail, they gradually lose their ability to emit current from the cathodes efficiently and high power tubes are expensive so there was a reluctance to replace them until they were really sick. This would cause the RF finals to be incapable of handling positive peaks linearly or the modulator tubes would get weak and become incapable of creating enough power to modulate the transmitter completely.

I know the CCA’s were cable of high modulation. I don’t know what KHHO (formerly KTAC) is using these days, but back in the last 70’s they had a CCA beast of some sort and I remember being down there and looking at the modulation monitor, a unit with a couple of LEDS when peaks exceeded the threshold set on a couple of calibrated dials. The positive dial was calibrated up to 133% but the knob actually turned physically past that point (and was), and the LED was for the most part continuously on. And actually it didn’t sound that bad given the degree of audio crunching that was taking place.

The Harris pulse width modulation (class D) transmitters and later the digital modulation transmitters (which essentially is equivalent to pulse with modulation using the RF stage as the pulse width modulator stage at the carrier frequency) also sounded good and both were capable of heavy modulation.

The tendency of transistors to work until they don’t and the fact that they’re operating in a switched mode making linearity a non-issue, means that the audio quality and modulation capability doesn’t gradually degrade as the tubes age. The efficiencies are also high, which perhaps if you’re in a cold region of the country in the winter and not paying the power bills is not such a good thing.

I think what sounds not so good with many of todays stations is the audio processing and audio sources that are used. Take bad audio, run it through an audio chain that stomps it entirely flat, and no transmitter is going to sound good.

I added a link to blatherWatch blog on the sidebar because they’re willing to, “listen to talk radio so I don’t have to”. With the content of today’s talk radio, at least in this region, that’s some real self-sacrificing going on over there.

I used to enjoy talk radio many decades ago. I loved to stay up late and listen to KGO 810 from San Francisco. That was back when there were clear channels and before KGNW 820 was allowed to broadcast at night.

Talk radio today isn’t like talk radio then. Now it’s talk propaganda. No longer are all sides of a topic discussed. Now you have your choice of about a dozen right-wing wacko stations and one left-wing wacko station. Neither camp will allow an issue to be examined from all sides or in depth. They no longer encourage people to think, instead, they encourage you to allow them to do your thinking for you.

My thinking may not be the best but at least it’s mine and I prefer that it stay that way. For the most part then I’ll allow blatherWatch to listen to talk radio so I don’t have to.

These KOL photographs were sent by Perry Lind. The cool stuff I missed out on by not being born yet!

KOL’s Early Westinghouse Electric Transmitter

KOL’s Early Transmitter Control Room

Frank’s Electron tube pages is a great resource for those who are still enjoy the warmth and glow of vacuum tube technology.

On this site you will find PDF data sheets for just about every tube known to man including hi-power transmitter tubes. There are also links to sites with foreign tube data sheets and substitution guides.

I’ve added a link to the sidebar so that it will remain available even when this post is long buried.

I was pleasantly surprised by e-mail from Perry Lind. He and his father were engineers at KOL from the early 40’s until the early 60’s which was prior to the time I got to visit there (I was born in 1958), but still good to hear from someone who worked there.

Perry is looking for a vintage AM broadcast transmitter that he can restore and convert to operation on the 180 meter ham band, particularly Westinghouse. If anyone knows of such an item potentially for sale, contact me and I will forward the info to him.

One of the things Perry Lind mentioned is that he was involved with the installation of an RCA BTA-5F transmitter. (There are some exceedingly cool pictures of this transmitter at http://www.coutant.org/transmit/). I suspect that would be a bit overkill for his application.

I am still looking for old pictures of KOL, particularly I’d like with the call signs on the tower.

It seems like I run into this term with just about every new digital transmission technology. What the heck does “orthogonal” mean when applied to frequencies? What are the advantages of this modulation scheme?

It was this paper written back in 1994, by Phil Karn, an amateur radio operator, that made it make sense for me. He explains the concept in great detail along with the math behind it and also proposes one particular coding scheme (but there are many possible).

After gaining an understanding, I am curious if this isn’t what the proprietary Telebit protocol PEP is. They didn’t have the high end digital signal processors to work with but they had the Motorola 68000 CPU.

Now that I have a reasonable understanding of the protocol and the math behind it, I understand the reason it is being so widely implemented and why folks are pursing ultra-wideband transmission schemes. The potential benefits of this protocol are so great that I wonder if any other modulation schemes will still be in use in a couple of decades.

My question with respect to how “Orthogonal”, a geometrical term, applies to “frequency”. Imagine an old linear radio dial, the type with the numbers from one end to the other with a little pointer that moves when you twist a knob to select a station. Now imagine that dial is linear. Now on this linear dial you have lines marking off frequency divisions equally spaced apart. You’ve got a bunch of parallel equally spaced lines on the dial, they are orthogonal. Now that you can visualize that you can see how orthogonal can apply to frequencies.

Orthogonal frequency division multiplexing divides up bandwidth into a bunch of narrow channels using equally spaced carriers each modulated at a slow symbol rate. This is realized by using fast Fourier transforms rather than attempting to create hardware to modulate and demodulate each carrier. It is the advent of fast digital signal processors that has made it practical to encode high bit rate streams suitable for digital video or local area network applications.

There are a number of very interesting ramifications. If you use a large number of channels, this modulation technique can approach the Shannon limit which defines the theoretical maximum data transmission rate possible with a given bandwidth and signal to noise ratio. Another way of putting it is that OFDM utilizes the available spectrum with near perfect efficiency.

OFDM can transmit data over a channel in which the noise is much higher than the signal. That makes it possible for multiple devices to share the same spectrum without interfering with each other.

Each channel operates at a very low rate relative to the total speed. Each symbol transmitted is smeared out over a long time frame and in the receiver integrated over a long time frame. Ordinary modulation would suffer severe interference with even brief interruptions, but because of the way OFDM smears the transmitted symbols out over time, a brief interruption in the received signal will not cause a loss of data. Those of you who’ve been annoying by the swish-swish on FM while driving down the road can appreciate this.

By itself this scheme is still sensitive to selective channel interference. Additional coding can be done to reduce this. When this is done, OFDM becomes CODFM. Again the article references above gives one example.

In addition to making efficient use of bandwidth OFDM also makes efficient use of transmitted power since it can allow the maximum transmission rate for a given signal to noise ratio.

As the cost of digital signal processors continue to drop and the power of digital signal processors continues to increase, I expect that it’s a matter of time before CODFM pretty much displaces conventional modulation schemes altogether.

I just added a little feature in the right border that gives a quick synopsis of space weather conditions and resulting high frequency propagation.

Check it out. It’s free to add to your website if you so desire.

Digital is the world today, analog is on it’s way out. NTSC (Not The Same Color) television is being replaced by HDTV, and now AM and FM radio stations are being digitized. There are big advantages. Digital transmission allows for error correction and redundancy eliminating many of the annoying reception problems of analog service.

We’ve all experienced the swish-swish-swish of a marginal FM station signal or the buzz on an AM station as we drive under power lines. Digital promises to eliminate these things.

Digital also promises to improve sound quality giving us digital AM signals that approximate the quality of good FM reception, and FM signals that approximate CD quality.

On FM, digital will make additional channels available allowing the targeting of niche broadcasting markets.

There is a dark side. In the United States, AM channels are spaced 10 KHz apart allowing for 5 Khz upper and lower side bands without splattering into adjacent channels. However, current FCC regulations actually allow modulation up to 10 KHz, but sidebands past 10.2 KHz must be attenuated at least 25 DB from the main carrier. The reality is that there isn’t much energy content above that in most program material anyway.

Digital transmission is allowed to go out to 15 KHz. The analog audio occupies 0-5 KHz, and the digital portion of the signal occupies 5 KHz – 15 KHz. The ramification of this is that adjacent channels are going to be totally clobbered. Under the old rules, the sideband distant from the adjacent channel was reasonably protected, so with a narrow receiver you could still receive a station next to a stronger adjacent station. With digital, both sidebands will be clobbered and the adjacent channel will no longer be receivable.

Analog signals decode gracefully, as the signal gets weaker, the signal to noise ratio gets worse, but the signal may be receivable over thousands of miles at night. With digital either there is enough signal to decode, or there isn’t. This means that signals will either be receivable cleanly or not at all. The effect of night time fading may be to have signals come and go entirely.

Ah, but you still have the analog signal! Yep, except it will probably be buried by the digital transmission of a nearby digital station on an adjacent channel. I’m afraid they’re trying to kill one of my hobbies (DXing). It’s hard to say how it will play out. Surprisingly, TV DXers have had a fair degree of success with digital television.

To prevent adjacent channel interference, AM stations are presently only allowed to transmit digital during the day. I’m sure that this restriction will be lifted if digital AM proves to be popular.

Digital FM is less problematic because FM sidebands are wide enough to allow the digital signal to fit within the existing spectral footprint. With frequency modulation, sidebands are created not only at the sum and difference of the modulating frequency but also at harmonics of the modulating frequency. However, it is possible to choose a modulation index that limits significant sideband energy content to a desired bandwidth. Digital transmission may require discontinuing the use of certain subcarrier frequencies that would overlap the digital signal but the digital signal provides additional channels that can replace the functionality of the subcarriers.

The amount energy that goes into the digital subcarriers is very small so digital FM should not substantially detract from the analog signal and therefore should not negatively impact long distance reception.

A trend that seems to be accompanying digital conversion is power increases. I feel the FCC’s rules regarding FM broadcast really need to be revised to accommodate the realities of modern receivers. The FCC generally considers the 1mv contour an FM stations service area and does not protect beyond that contour. But many modern receivers have sensitivities below 5uV (mv = millivolt 1/1000th of a volt, uV = microvolt, 1/1,000,000 of a volt). I found one receiver with an advertised sensitivity of .25 uV. Even the cheapies tend to be better than 50uv these days.

In this region, what tends to be the reception limiting factor is not signal strength but multi path interference. The frequency of an FM station is being continuously modulated by the program content. This means that a 100.1 Mhz FM station might be 100.099 Mhz one moment and then a millisecond later 100.101 Mhz. The first signal arrives directly at the antenna, and then a signal reflected off a conductive object arrives at the antenna a millisecond later, and the two signals create a “beat” signal of 2 Khz. The end result of this is that you here this static distortion of the original signal that renders it very unpleasant to listen to.

More power doesn’t help with this problem, if often seems to make it worse. Modern FM signals which have a stereo pilot, stereo sidebands, and a variety of SCA subcarriers make the problem worse because all of those signals are modulating the frequency as well. Digital should improve this situation.

It is my belief that current FM stations are in large part overpowered. If you aren’t able to receive a distant station it is only rarely because that station is underpowered. More often it’s because a closer adjacent station is overpowered and overmodulated.

Given the reality of energy supplies being tight, this would seem to be an area where a lot of energy could be saved. I believe the FCC should update the signal level requirement to reflect modern receiver sensitivities, replace 1mv with 50uv or so, reducing transmitter power accordingly. There are 9000 FM stations in the United States, their total power output approximates that of a medium sized nuclear reactor. If contours were reduced from 1mv to 50uv, that would reduce

While increasing power generally does not help with multi-path interference, but does create more interference problems for people listening to other services, I believe the rules regarding antenna height above terrain verses power should be re-worked to favor higher towers over higher power. If the rules were re-worked to give higher towers an advantage over higher power then more broadcasters would opt for big towers and lower power.

This would be good for everyone concerned. It would be good for the broadcasters because they would get increased functional coverage area for only a capital expense while their operational expenses would be reduced. It would be good for the consumer because they would get a cleaner more multipath free signal. It would be good for the planet because less energy would be wasted.

In areas where the land is flat, not mountainous and hilly, and the antenna is not located at an extreme height relative to the terrain, a combination of more bays providing a higher gain and vertical directivity, that is to say power is concentrated in the horizontal plane, and lower transmitter power (to compensate for the higher antenna gain) can result in equivalent signal levels at a distance but lower signal levels near the transmitter resulting in less interference and more economical operation.

On the AM bands I am also seeing power increases left and right, KJR from 5 Kw to 50 Kw, same for KOL, KRKO also is getting a power increase later this year. With AM, if we still had clear channels, these power levels would be useful. Under present circumstances I’m not sure it really accomplishes anything except to better overload receivers in the local vicinity and cause more co-channel interference.