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Old 29-07-2004, 23:41   #32
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Join Date: Jun 2004
Posts: 6
Co-channel and in-channel interference

Here's my two-penn'orth on the filter discussion.

Those getting a strong DRM signal probably need no filtering, as the DRM component is strong enough to win through.

Those getting a weaker signal, however, are prone to losing the lot because of both in-channel interference (another carrier, usually AM, within the DRM signal) and co-channel interference (another carrier at one or both edges of the DRM signal).

I often see a good DRM signal with a whopping great carrier soomewhere within it. The rogue carrier prevents some of the DRM carriers from being resolved and also, I suggest, causes the AGC to turn down the gain. Result - no DRM audio.

Solution - a sliding notch filter which can be placed over the rogue carrier, with variable width so it can be tweaked to kill it without also killing too many DRM carriers.

Conversely, I often see one or two carriers at the edges, 5kHz up or 5 kHz down from the centre frequency of the DRM signal. This is where the bandpass filter in DREAM may help - my initial tests suggest that it does in some cases.

But given the most common co-channel interferer is 5 kHZ away, reducing the bandwidth to 8 kHz would help enormously (except for strong interferers which intrude further into the DRM signal). Someone above has mentioned that using the receiver's 8kHz filer helps in this situation.

Here's another thought - why not reduce the transmitted DRM signal to 8 kHz? This would mean fewer carriers and therefore lower bit-rate or reduced resilience (fewer carriers means you can afford to lose less before decoding is unavailable). But it would enable us to use an 8kHz window at the receiver and get rid of a lot of the energy from the carriers 5 kHz above and below.

Another way to approach it is to leave the transmitted DRM as it stands but establish a DRM DX decoding specification which will allow decoding to take place on fewer carriers? The transmtter could send the full-specification DRM signal but we could adjust the decoding to get a reduced quality signal if interference prevents full decoding, by reducing the bandwidth to 8kHZ, or accepting fewer carriers, or both of these.

For those of us for whom DRM is DX, a DX DRM decoding algorithm would work wonders. I would rather receive slightly reduced quality audio (but still significantly better than analogue AM or SSB) than no audio at all, which is what we get for a lot of the time here. And after all, aren't we talking about long-haul options for shortwave broadcasters?

OK, everyone, what do you think?

Paul VK3DBP Melbourne
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