Thursday, September 14, 2006
Eton E1, Reception Modes; Audio Quality
In addition to the usual modes AM and SSB (which can be used for ECSS), the E1 is equipped with a selectable-sideband AM synchronous detector (AMSD). The ECSS technique is supposedly well-known among DX-ers so I will not go into that subject here except note that the E1's ECSS reception is very good, once the frequency alignment has been done.
So what does AMSD do? Some say that AMSD is the life and soul of a receiver - hinting perhaps that a non-AMSD receiver is no good. Baloney. For further enlightenment on the possible virtues of AMSD, please read Dallas Lankford's articles on the subject.
AMSD is often associated with reducing distortion caused by selective fading. This is true, but you don't really need an AMSD for that, you can use an Elliptic Low Pass Audio Filter (ELPAF) instead. Selectable-sideband AMSD can also be used for reducing interference from one side much the same way as with ECSS.
AMSD has one downside compared to ordinary AM: It is usually non-transparent; it needs to lock onto a signal. This takes time, and is often accompanied with growling when the radio tries to center on the passband. Some AMSDs loose lock fast, some stay put.
The E1's AMSD has three positions; USB, LSB and DSB (double sideband). I tested its distortion-reducing capabilities on some fading SW frequencies. Reducing distortion will normally be best when using the sidebands, but I found that the AMSD was very effective even in DSB. On MW there wasn't much difference on the signals I tested, but if I had tested with groundwave/skywave blends I believe I might have had the same results as on SW. Comparisons were made with the AM Slow AGC setting. The conclusion is that if I want to listen to a SW broadcast I would probably use AMSD.
And locking to a signal is rather straightforward. It does so within half a second, without muting the receiver's audio (any AR7030 owners out there?), and without growling (any SE-3 owners out there?). Or mostly without growling. If the carrier is far away from the radio's frequency it needs to tune in and does so much more slowly than the SE-3, and it growls a lot.
But what about split-channel DX and interfering stations with very high signal levels? Well there's a mixed bag. I tested this specifically on 1250 (Ontario) with a strong European on 1251. And now we're at the core of the problem. The manual says that the AMSD will lock on the strongest signal in the passband. So even if you choose LSB, adjust the PBT to get away from 1251, and even if you manage to lock on 1250 - if 1251 increases its signal level it will simply take over the lock and you will find yourself listening to 1251 instead of 1250! The problem will of course diminish the further you are away from the offending signal. But there is in fact a problem here. Imagine this happening at 10 seconds before the full hour.
I cannot recommend the use of selectable-sideband AMSD for serious DX unless you are certain that the lock won't be lost and "transferred" as it were, to the interfering station. This is especially true if you operate several receivers and need to let the E1 record audio unattended. On the other hand, if you meet that requirement, the audio quality from the selectable-sideband AMSD is very good.
ECSS audio quality is good, better than expected. There are slight tonal differences between USB and LSB but they are not disturbing in any way. But the difference should have been avoided.