China Radio International warbling on Dave’s home brew receiver

IMG_7078My buddy and SWLing Post reader, Dave Richards (AA7EE), wrote several weeks ago with an interesting comment:

“I was just now  tuning around the 31M band on a [regenerative receiver] that I am putting the finishing touches on, and noticed that the audio from China Radio International on 9790KHz was not only cutting in and out, but was also warbling, as if the program was being played from a tape machine with a slipping pinch wheel.

I’m finding it a bit hard to believe that in this day and age, a country like China would be using tape machines in their studios still, but am trying to figure out what other explanation there could be for this. The warbling doesn’t sound as if it is being caused by the propagation. Have you heard this before?

I have attached a short recording. Please excuse the bassy audio – I need to modify the receiver circuit to provide some bass roll-off.”

Click here for Dave’s recording or listen below:

I agree with Dave; it sounds like CRI is playing from a tape deck with a slipping pinch wheel. I’m not sure this could be a modulation issue. Perhaps it’s both? Curious what other readers think.


Warble aside, I was also very intrigued by Dave’s home brew 31 meter broadcast band regenerative receiver. I asked him for more details; he replied:

“I built another version of the WBR. The original version, as well as the first version I built, was for the 40M amateur band. I was intrigued to see how it would perform on other frequencies, so I built a version for the 31M band.  The only change I need to make now is a bit of filtering to provide some rolloff of the bass frequencies, as they are hurting the intelligibility in my opinion.”

IMG_7111Wow! What a cool little home brew project! And many thanks to Dave for the brilliant photos of the WBR. I should mention that Dave has an excellent ham radio blog where he documents his radio projects. Indeed, check out this page for more information about the WBR broadcast band receiver.

Dave told me that he is currently working towards his goal of designing and building the “ultimate” regenerative receiver with  plug-in coils for many different bands. I’ll be following him, so I encourage you to do the same by bookmarking his website.

Many thanks, Dave!

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4 thoughts on “China Radio International warbling on Dave’s home brew receiver

  1. Rod KB8DNS

    I have heard this before while converting analog formates to digital. The term is called ‘generation loss’, has to do with format conversion such as .WAV file to MP3 or taken from an analog recording to a digital format. There can be an introduction of ‘noise’ if the cables used are sub-standard, have been poorly routed or perhaps placed near noise generating sources, even near high power RF fields!
    Radio Havana Cuba has been noted for ‘errors’ and ‘artifacts’ with-in their broadcasts on shortwave with simulcast telephone ring sounds on both number stations and shortwave at the same time. CRI could be having a similar effect as their studio may be used for more than one purpose.
    The ‘slipping drive wheel’ effect heard could be this generation loss.

    73 Rod KB8DNS

  2. EA4998URE - David

    One possible explanation I’m just thinking about would be that they just had some kind of a failure in the modern, digital playback systems normally used and had to resort to an old, unused tape machine which had the pinch rollers cracked and worn out.

    Other explanations could be a malfunction of the transmitter, or problems in the audio signal transport and processing stages, in between whatever system is used to relay the signal in between the studios and the transmitter site (I suppose a satellite link is used) and the transmitter itself.

    Just guessing…

    73 from Barcelona!

    David – EA4998URE SWL

  3. Moshe Ze'ev Zaharia

    Well, first of all, amazing radio! amazing construction!
    As for the recording: It really sounds like dried out, cracked worn out pinch roller. as cracks causes excactly this kind of warble, and wear causes the drop outs.
    It looks like CRI can’t afford 50c pinch roller…
    Again, Well done Dave! I watched the videos and photos on your site, and loved the quality construction and preformance.
    All the best,


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