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Friday, September 26, 2014

Radio China International Echo Mystery SOLVED!

For the past month or so I have been wondering about a strange echo that I've been hearing on the 31 meter transmissions of Radio China International.  I first noticed it on my "Kings Speech" regen receiver.  Then I heard it again on my "Off the Shelf" regen.  

For a while I thought that what I was hearing was a propagation effect:  Perhaps the very strong RCI relay station in Quivican,Cuba was sending north a signal so strong that it was travelling around the earth along the grey line and coming back to me about .133 seconds after the original reception.  

This sounded plausible (and it does happen sometimes).  But there were reasons for skepticism: Why wasn't anyone else hearing this?  Why wasn't the effect showing up on signals from Radio Havana Cuba?  

Pete Juliano had suggested that perhaps I was getting signals from TWO different RCI transmitters.  I had quickly checked the RCI schedule and didn't see them transmitting on the same frequency at the same time from multiple transmitters, so I kind of put that idea aside.  Hey, the round-the-world idea was just more appealing! 

But then I remembered something strange about the echo:  It seemed to disappear when I tuned close to the center frequency of the RCI signal, but then appeared when I tuned off to one side. Hmm.... That was an important clue. 

I've long been wary of regen receivers and for a while suspected that I was dealing with some weird regen effect.  Regen and Echo seem to go together, right?    Well, as it turns out, no.  But I was right about this being an effect of the nature of my receiver... 

Last night I was listening to RCI English service at around 0030 UTC on 9570 kHz.  Nice clear signal.  No echo. 

At 0100 UTC the program changed, and the echo started.  A very strong echo. 

I went to the RCI schedule.  Here I found the answer: 

At 0030 they were transmitting from their relay station in Cerrik, Albania on 9570 kHz. 
At 0010 they switched programs, frequencies and transmitters.  At 0100 Cerrik shut down, but the Quivican, Cuba relay came on on 9580 kHz.   At the same time the RCI transmitter in Kasi Sabagh in far-off exotic Western China, in Xinjiang, fired up on 9535 kHz.  Both transmitters were carrying the RCI English service. 

You see, my little regens are not very selective, and the RCI transmissions are quite strong.  So if I have my receiver tuned to around 9560 kHz, I'll be hearing BOTH the signal from Cuba AND the signal from Xinjiang.  That would explain the echo. 

To try to confirm this, last night I fired up my old Hammarlund HQ-100 receiver to see if I could discern the two different signals.  I could.  And the echo appeared when I tuned BETWEEN the two.  You can hear this in the video above. 

There is one remaining question here:  Is the echo caused by the RADIO path difference between the two transmitters?   Or are we just seeing the effect of the programming being transmitted at slightly different times, perhaps with this delay caused by INTERNET latency?  Anyone know how RCI gets its signals from its Beijing studio to its distant transmitters?   I calculate that the path difference is about 10,000 km.  With c at 300,000 km/second, that would yield an echo of only about .03 seconds.  The echo we are hearing sounds longer than that, so I suspect we are hearing a difference in studio-transmitter transmission time.  What say the SWL RF gurus?

BTW:  I think the same phenomenon may explain the echo on Brother Stair's "Overcomer" signal.  I see that starting at 2200 UTC he is on BOTH 7570 kHz and 7730 kHz from RMI transmitters in Florida.  Perhaps they are not synched up. 

I think this is all very cool.  Think about it:   Here I am, sitting in Virginia in 2014, listening to the Albanian, Cuban, and Xinjiang relay stations of Radio China International on a receiver first built by some guy in England during the 1930s.  And I'm trying to figure out if the echo I hear is caused by the limits imposed by the speed of light and the size of the earth,  or by the time it takes packets to move through sub-oceanic fiber optic cables.   

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6 comments:

  1. It's articles like this that make me want to dust off the S-38, restore it's innards, and start listening again... thanks for a great read!

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  2. Bill it is super cool, or "way cool" as my nephew in San Diego would say. Been listening to and watching programmes on quatum mech, relativity, and other good stuff to ponder. I like the ideas that network latency, ever-so-slight but significant propagation delays, transmission-reception distances, and the Regen Goblin all combine to produce these effects. We were with XE1H Tony the other day, he has been messing around with low power JT-65 moonbounce,and the issue of speed of light/radio came up, etc. He mentioned that GPS sat sytems have to accomodate very slight time dilation effects (Einstein und freunds). There is no doubt that the radio hobby is a door to many interesting topic areas. Bueno, en cuanto a tu nuevo libro, no lo he leído, pero los otros dos, sí. Saludos, cheers, DSW, namaste, etc.

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  3. Hey Bill --

    perhaps it's the sat downlink equipment differences. And I think I have a personal example: at home, when I turn on two digital TV sets (Sony at bedroom, Samsung at living room), I can clearly notice audio delay (the Sony arrives later). Same TV channel, same antena, similar cable length but different makers.... one decoder is like 0.1s faster then the other.

    -- pu3hag

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  4. Accumulative processing delay from Sat' uplink would also be my suggestion. Packet queuing and FEC overheads.

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  5. Another (redundant) personal example: can watch XEIPN Politecnico Canal Once TV VIA direcTV, cable, and via analogue repeater located on a hill here in GDL: audio echoes as described by others are quite noticeable. For what it's worth, back in late teens had a show on KUOW-FM in Seattle, in Spanish, as part of Enfoque Nacional. I remember the "echo" effect quite vividly when we downloaded content off the big ole dish from NPR back east and then went out over the air....

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  6. First you need to make a correction from RCI (Radio Canada International??) to CRI.

    CRI uses satellite to relay it's audio from the 4th floor control room. Nothing is relayed directly from from the studios.

    Personally I don't think it's that unusual. Even when VOA broadcast was to East and Southeast Asia there was a delay between the Thailand and Sri Lanka sites. Even Radio Australia. Shepperton and lets say the Babcock relay in Singapore or a delay.

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