jdobbs2001
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- Apr 20, 2005
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8992 every time they transmit those sky king messages it sounds like they are using one of those cb echo mikes. Is there a reason for the poor echo audio?
Why does the air force use cb echo mikes?
- Thread starter jdobbs2001
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I believe thats the signal travelling around the world and back again. Not an echo mic.
The reason you hear the echo is because you are hearing multiple sites transmitting the same message.
Uh. No.
You are hearing the direct skywave signal and then 1/7 th of a second later you hear the signal that has traveled 22,500 miles around the globe back to your antenna if the conditions are correct.
n0nhp
Member
If you are referring to the HF-GCS system and EAM messages, the operator is transmitting on multiple sites world wide.
The poor audio is that the "technicians" trained in the military fashion do not know how to balance the audio going to multiple transmitters per site and to those multiple sites.
Simulcast transmissions require very time consuming and exacting adjustments that change every time anything in the system changes. When running the audio through copper, fiber, satellite and who knows what other methods, each step has to be balanced and adjusted then the previous step has to be re-adjusted and the following adjusted etc. etc. etc.
Just one of the fun things to get used to listening to utility HF.
Bruce
The poor audio is that the "technicians" trained in the military fashion do not know how to balance the audio going to multiple transmitters per site and to those multiple sites.
Simulcast transmissions require very time consuming and exacting adjustments that change every time anything in the system changes. When running the audio through copper, fiber, satellite and who knows what other methods, each step has to be balanced and adjusted then the previous step has to be re-adjusted and the following adjusted etc. etc. etc.
Just one of the fun things to get used to listening to utility HF.
Bruce
There is another option, assuming the are using a setup similar to what we had for HICOM:
The operator did not turn down their monitor speaker. Since the transmitters are separate from the receivers, the microphone could be picking up its own signal. (There is enough inherent delay that there is no squealing feedback.)
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The operator did not turn down their monitor speaker. Since the transmitters are separate from the receivers, the microphone could be picking up its own signal. (There is enough inherent delay that there is no squealing feedback.)
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prcguy
Member
That depends the specific techs, but I would agree that most operators are not given that training. My training included such tasks, although I could only control the equipment at the facility I was working in.
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If this was VHF+, I'd be more inclined to agree, but I wonder being HF, if making a set of adjustments that lasts is even possible. Given propagation delays, is it realistic to be able to adjust the timing as such? We are talking about hundreds+ miles between listeners, day vs night, maybe short path vs long path, etc.
Hooligan
Member
Sez you!
Token
Member
Let me apologize right now for this being a long post, there really is not a short way to discuss it.
As you have seen in this thread, there are several beliefs as to why this echo on HF-GCS exist. The people who know for sure are not talking, and data in the open source does not clearly define this. I have talked with personnel who work with the system, but again, they refuse to talk much about it, as it should be. In my military past I used the system myself, but that does not mean I understand how it is structured today.
In my opinion the most likely reason for this echo is network latency.
This system uses 12 transmitter sites distributed around the World. Ground based operators in the network can select any or all of the transmitter sites for transmission. This means the operator can select multiple transmitter sites that you might be able to hear, depending on conditions. Each transmitter site will receive audio from the source via a network.
The first example is based on the audio in the network arriving at all transmitter sites simultaneously, I mean exactly at the same time.
If you can hear two different transmitter sites from your location, and are closer to one than the other, then the signal from the closest will arrive before the signal from the furthest. The actual propagation times will be slightly over the numbers I quote because of reflections and multipath, but I will use the simple path times in my examples. Lets say that one transmitter site is 300 km from you, and the other is 3000 km from you. The signal from the closest site will take about 1 millisecond to get from the transmitter to your receiver. The signal from the second will take about 10 milliseconds to arrive. The difference between these is 9 msec, and this will be the depth of the echo you hear.
Echo times and detectability are somewhat variable, so the following are just approximations. Echos below about 50 msec are difficult to detect by ear as they tend to stretch the sound, giving it depth or shaping it, instead of showing up as separate sounds. Echos over 100 msec tend to be more distinct. With these guidelines you can see that you would not really hear the difference between a station 300 and 3000 km away.
Still assuming the audio is transmitted by all transmitter sites at exactly the same time, what about the possibility that you are hearing both long path and short path at the same time? This happens when a station is heard via the shortest route (short path) and also heard when the transmitted signal goes all the way around the World to get to you (long path). So the signal from the station 300 km away will arrive at your receiver in about 1 millisecond traveling the 300 km and then arrive again 132 msec later after traveling all the way around the World the long way, or about 39775 km (40075 km – 300 km). This makes a very detectable echo, no doubt, but such conditions on all HF-GCS freqs are very uncommon, and it is not uncommon to hear such echos. And echo times of up to 800 msec have been recorded on the HF-GCS freqs, this would equate to roughly 5 + times around the Earth.
So the delay times sometimes seen on the US Air Force HF-GCS network do not support simultaneous transmission and propagation delay as the only delay source. Propagation delay is definitely going to be present, there is no way around that, but there must be at least one additional major delay.
Network latency could easily account for the extra delay. Remember that when you put data on fiber the propagation delay is larger than transmission through air, so that even with no processing delays the site delays must be increased. Audio signal arrival time over the network to each site cannot be simultaneous (unless the transfer distance is identical to each site all around the World), and that delta must be added to propagation delays. Processing delays and audio codecs can further delay, just look at VOIP as an example. Populating and controlling each node with a non-real time system, such as Windows, can further introduce delays. And since we are not talking about two way messaging when discussing EAMs and Skykings there is no real drive to spend time and effort overcoming the lag and latency. Of course the only way to really do so would be to introduce controlled delays at all sites, so that timing could be aligned and then they would be correct for ONE listening location only. Why bother?
T!
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Token
Member
(edit) Double Post
T!
T!
As stated, it's due to multiple transmitters. As for simulcast timing adjusted, it would be impossible to optimize the system for global coverage without some sort of artifact of running multiple transmitters. Given the choice of ubiquitous global coverage that WILL be heard, or a slightly annoying echo, the echo is a minor nuisance.
I would agree. Considering that the echo does not detract from overall readability of the transmissions, and having multiple transmitters goes a long way toward making the network relatively immune to propagation conditions (compared to a single transmitter), it doesn't make sense that they would expend any effort to suppress it at the transmitter end. Echo suppression devices have existed for decades, and it's always possible that echo suppression is being applied at the receiver end.
I would agree. Considering that the echo does not detract from overall readability of the transmissions, and having multiple transmitters goes a long way toward making the network relatively immune to propagation conditions (compared to a single transmitter), it doesn't make sense that they would expend any effort to suppress it at the transmitter end. Echo suppression devices have existed for decades, and it's always possible that echo suppression is being applied at the receiver end.
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Interesting posts - I get others points of view too but Tokens net latency fits well, to me, as root cause with encounters witnessed in land-line echo issues as the rest applied to radio TX and then RX then flows on delivering the end result - i.e. with their own issues added/combined. Anyhow, we have echo - nice if we didn't.
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I agree, that is most likely. I have seen/heard some serious variations in arrival times, when using multiple intersite pathways.
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prcguy
Member
Yea, sez me and I was trying to weed out some of the reasons mentioned like the Air Force having speakers running near their microphones picking up and rebroadcasting themselves for the last 40+ years.
Token gave a great explanation with varying time delivery of the program material to the various sites being the major contributor and RF propagation delay to the listener being another.
I often simulcast on HF with a remote station in MA via the Internet and my local HF radio in CA and it causes the same sound due to Internet delay to the remote radio. I could delay my local station audio a little to improve the echo but I kinda like the comments it gets on air.
I hope to have a second remote HF radio in TX soon so I can add more layers of echo, and of course be heard a little better on some coast to coast HF nets I participate in.
prcguy
Token gave a great explanation with varying time delivery of the program material to the various sites being the major contributor and RF propagation delay to the listener being another.
I often simulcast on HF with a remote station in MA via the Internet and my local HF radio in CA and it causes the same sound due to Internet delay to the remote radio. I could delay my local station audio a little to improve the echo but I kinda like the comments it gets on air.
I hope to have a second remote HF radio in TX soon so I can add more layers of echo, and of course be heard a little better on some coast to coast HF nets I participate in.
prcguy
PRCGHFS net
You got to email me when you are on sometime so I can hear that.
You got to email me when you are on sometime so I can hear that.
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