As a longtime part 15er it has gotten to feel like there is nothing new to think about, but that all changed with the March issue of Radio which on page 16, RFENGINEERING by Jeremy Ruck, PE, presents the topic: Synchronous AM Boosters Could Help Revive Senior Band.
True, we have had the Hamilton Rangemaster1000 with a synchronous option, but I think I heard recently that that feature has been dropped.
While the article discusses the long history and modern advancements in synchronizing AM transmitters for operation on a single frequency... to de-tune the carrier beat-frequency and lock the audio...what got my attention was a very brief description as part of the textual caption underneath a photo of the original towers for WBZ in Boston:
"WBZ and WBZA were synchronized: the carrier was generated at WBZ, divided downward to an audio frequency, fed over a phone line, then remultiplied at WBZA to create a precisely locked carrier."
That's the only information disclosed in the article about what sounds like a wild and amazing way of achieving synchronization that could probably be accomplished as a part 15 do-it-yourself project!
So far I've tried to reverse-engineer this idea using my limited knowledge of a few things...
An AM carrier has a bandwidth of 10 kHz, so we need that much space in the audio band for our down-divided signal;
The audio base-band is 20 kHz, so we need half of it for our encoded carrier;
Special equalized audio phone lines are normally good for 15 kHz bandwidth, so our down-converted carrier needs to be centered within this space.
What I cannot guess is whether the carrier is pre-modulated with program audio or is sent as a pure signal with modulation injected at both transmitter sites and also synchronized (somehow).
Sound like fun?
if i were to take a stab at it i bet it involves a 25Hz or so sub audible tone used as a refernce on the audio stream generated by downconversion from the main transmitters oscillator be it pll, dds, or crystal. my guess is for high stability they are using crystal locked transmitter.
i will try and get in touch with the engineer who designed it, he may be on one of my broadcasters email listerv's.
keep in mind neither Hamiltons RS485 nor this audio system will work over internet due to the high latency involved it may (YMMV) work over a private point to point IP microwave link.
Rangemaster didn't really drop the capability exactly, what happend is the crystal sources are dissapearing and the new adapters replacing them don't work with sycronation.. The original models can still be syncronized, but even that wasn't so cut an dry to do, it involved additional expense and expertise or I would have gave it a go.
Regarding the Rangemasters, what sync information is on the rs485 line from the master to slave link ?
Is it a 10khz reference, and does the slave require any crystal to be fitted ?
Paul.
If that question was directed to me, I don't know, it's been years since I even considered the idea. But this should be of assistance:
http://www.am1000rangemaster.com/zerobeat.html
and: Multiple Transmitters to cover more area Section 1 - RangeMaster AM ...
Part15 Engineer said: "If i were to take a stab at it i bet it involves a 25Hz or so sub audible tone used as a refernce on the audio stream generated by downconversion from the main transmitters oscillator be it pll, dds, or crystal. my guess is for high stability they are using crystal locked transmitter."
That makes sense to me in a way that I cannot fully explain... it reminds me of using SMPTE Time Code to synchronize audio with video in an analog video production.
SMPTE Time Code is, to the best of my knowledge, an analog signal generator that produces timed pulses which are kind of like digital on-off bursts, but are in fact analog.
Two licensed AM stations being synchronized would be miles apart, so there would probably be a delay caused by the length of the phone line and this would need to be compensated for by slowing the originating transmitter down so the far away transmitter could catch up. What the method for delaying such a signal without losing its purity is a mystery, since that was all done before the digital age.
Gotcha, End80, on the Hamilton Rangemaster situation with the crystal problem.
How far apart would part 15 AM transmitters be when synchronized in such a way that they'd combine to cover a larger area?
The caption quoted by Carl is very interesting and I would think that the only signal on the phone line would be a tone derived from the original station's carrier without modulation for the reason Carl mentioned.
Not only is the frequency sync important but the phase sync is critical. Two synchronized stations radiating at separate locations will produce peaks and nulls, in other words, a pattern which will change if the phase relationship changes.
The SSTRAN, for example, uses an internal 10 kHz signal to lock the carrier onto the desired RF frequency but the phase is not controlled since it needn't be but the point is that an audio frequency reference could be used. Maintaining the desired and specifiec phase relationship is more involved than just locking the frequencies but it is doable.
(refer to "Einstein's Clocks, Poincare's Maps, ....Carl will know about this)
Neil
KKDA in Dallas had a booster station for a very long time, I think they just recently were ordered to shut it off by the FCC after decades of "experimental" licensing.
Korean?
an external gps reference on each synced transmitter would probably be best. everything locked to the same reference so keeping a phase lock shouldn't be all that hard.
i have seen GPS modules as cheap as $15.00 or so bucks!!!
It is possible to link the RS485 over a 6MHZ video link, I make an adapter for doing it.
I also have made a GPS based link in the past, but discontinued it.
It is a standard RS485 signal.
I'm not sure if anyone was ever successful with linking multiple Rangemasters which are distanced from each other before.. I do recall discussions of multiple transmitters serving a town (I forget where at the moment), utlizing Barix boxes, but the specifc details were not revealed about how/if they were actually perfectly synced, how far apart they were, or they were simply spaced far enough apart so not to interfer with each other, meaning dead air spaces between them (I continue to consider this idea, but then there's also the problem of how on some days the signals seem to travel farther than on other days.. meaning occasionally they would overlap and cause havoc on each other)..
Anyway, about a month ago I had detoured on some research which had steered me to some TIS/HAR study documention from 2011 titled Preliminary Investigation Caltrans Division of Research and Innovation Produced by CTC & Associates LLC , Improving Highway Advisory Radio Predictability and Performance..
On page 16, it described the option of utilizing as many as a hundred syncronized Part 15 transmitters to cover an area.. and although syncronization is specified their proceedure also involves a degree of dead air space between each transmitter..
From page 16:
5.8.1.3 Low-Power AM Transmission (No FCC License Required)
“Low-power HAR has been developed as a means of tightly controlling the broadcast zone and thereby limiting interference from adjacent zones. Low-power HAR differs from the previously discussed 10-watt HAR in that its broadcast radius (per transmitter) is generally limited to 500 feet to 1500 feet. By FCC regulation, each transmitter is limited to a maximum 0.1 watt power input to the final frequency stage, and the total length of the transmission line, antenna, and ground lead can not exceed 3 meters. Whereas this limits its broadcast range, it also provides for a reasonably welldefined area of influence, which, through an interconnection and synchronizing process, permits upwards of 100 transmitters to be coordinated into larger and well-defined saturation zones. Once a car leaves this broadcast area, the signal quality becomes too weak to be heard. This permits a second zonal configuration to be established nearby, transmitting a different message on the same frequency.
“By using this concept, a series of zones all operating on the same frequency, may be established whereby unique site-specific messages may be transmitted to provide condition updates in advance of decision points. Aside from the flexibility provided in establishing multiple message zones, lowpower HAR may also broadcast over any available AM radio frequency without the need to obtain additional FCC licensing approval. Though the ability to install a system without FCC approval provides the user with great flexibility in installing a system wherever desired, there is no guarantee that once installed, it will not be interfered with by some future more powerful transmission.
“The relatively low signal strength must compete with a variety of obstacles, including overpowering commercial broadcasts, signal skip (particularly at night), and poor signal propagation. These difficulties can be overcome by saturating an area (zone) with multiple transmitters and synchronizing their broadcasts. However, this concept is relatively new and, very expensive as the number of transmitters required is large.”
Preliminary Investigation Caltrans Division of Research and Innovation (PDF) Download: http://www.google.com/url?sa=t&rct=j&q=&esrc ="s&source=web&cd=11&ved=0ahUKEwjSvuCuvLrSAhVBTSYKHRYHBgo4ChAWCBswAA&url=http%3A%2F%2Fwww.dot.ca.gov%2Fresearch%2Fresearchreports%2Fpreliminary_investigations%2Fdocs%2Fhar_preliminary_investigation_8-5-11.pdf&usg=AFQjCNHA-p34fwskNvKExqbCnwUatuVTXA&sig2=HR_gbmJDMuVoaF-bpKl2WA"
EDIT.. And in case you didn't notice; the study also states the part15AM transmitters are generally limitted to 500 to 1500 feet, which by the way, numerous other highway dept documentation going back as far as 40+ years concerning use of Part 15 transmitters also quote similiar range capabilties.. A far cry from the "Public Notice" of 200 feet.
Also, the study suggest this method is a new concept, however it is not, the same method was being described in the early 1970's during the proceedings in front of the highway commision (before licenced TIS came into existence), when Diversivide Tecnologies Inc. first proposed the idea of using their transmitters on the highway.. So it's by no means a new idea.. however, they had quoted using up to 4 transmitters per mile depending on the particular circumstances of a given area. One such example involed a 9 mile stretch of highway.
Good Info Here