Many thanks to SWLing Post contributor Jeff McMahon, who notes that he’s published a new post on his blog, The Herculodge.
This time, Jeff examines the Tecsun PL-880 vs. PL-660, exposing the quirks and realities of DSP-based radios in his interference-heavy household. Jeff finds that in real-world conditions, the trusty PL-660 often outperforms the PL-880—especially when battling household RFI.
I have a PL-660, and two DSP portable radios. I have very little technical knowledge about radios, so I can only say what I like or don’t like. Having used my radios for awhile, I can say that I’m glad we still have choices (the PL-680 is being made again by Tecsun), because both types of radios work well, but are clearly different to operate. Sometimes a DSP radio tunes a signal better, and other times the PL-660 can get it better. And, now that you mention it, I certainly see what you mean- signals heard on the PL-660 are much less noisy than on the DSP radios.
There are a couple of factors here.
DSP or not, there needs to be a filter made of coils and capacitors to filter out all but the frequency bandwidth of the signal you are trying to receive. This is what makes excellent receivers succeed.
Automatic Gain Control in AM measures the DC which results from the transmitted carrier which reduces as the signal becomes weaker. In DRM and DAB+ there are a few pilot tones of known frequency and amplitude which are used to control the automatic frequency control and the AGC. In broadcast signals there is more than one in each signal so that not only is the overall gain controlled, but there is also an equaliser to ensure reliable decoding of the data. In such receivers, you never hear noise and interference. Once either produces too many errors, the receiver mutes. This occurs when in AM the signal is not listenable to.
If you listen to Single Side Band Suppressed carrier the AGC is controlled by the average demodulated audio level as well as the signal power reduction in the atmosphere. Since the noise and interference is audible it will cause the AGC to reduce the receiver’s gain making not only the unwanted sound softer but also the wanted signal.