Re: DRM vs SSBc

Gegroet, Tom,


Tom Holden schreef:
It can image that a AM-decoder can decode this correctly for CHU, but I don't know if this would apply for a broadcasting-signal.

Why not? CHU contains human voice announcements in USBc so why would a SSBc broadcast signal be any different as far as an envelope detector is concerned? 

I was more thining about the effects of being in a more crowded band.

My question is what happens if you tune a normal DSB-AM decoder to a SSBc signal (say in USB), where there happens to be another signal at the 5 Khz below.

Actually, there is a good test for that. 7335 khz is in the middle of a broadcasting-band.
Can you still hear CHU at that frequency (SSBc USB, carrier at 7335 Khz, signal from 7335 to 7340 Khz) with a normal DSB-AM radio if there is a station broadcasting at 7330 Khz in DSB-AM (i.e. signal from 7325 to 7335 Khz)?


If you need a receiver with sync-detection to be able to a SBBc station in these conditions, you can't really say that this is "compatible" with DSB-AM receivers. (as most people do note have a receiver with has sync-detection or SSB)


Now. The question is if this is necessairy bad. (see below).



I wonder what would happen if you would have a situation like this:
- Say that you have a radio-station in 5 Khz USB SSBc at (say) 7200 Khz (hence, taking up 7310 to 7315 Khz; just a random frequency)
(you mean the 5 kHz USBc carrier is at 7310, not 7200)

Yes indeed.
I started this sentence thinking of a frequency of 7200 Khz, but then changed to 7300 khz (7200 is to close to the 7100-7200 range which will become HAM-area in the future) and then to 7310 Khz (to make the frequencies "below the SSBc-station" in the same 100-range).

Funny to see how your own brain works, doesn't it, even when dealing with a simple question of "pick a random frequency".


- and you have a second signal just below (either a SSBc from 7305 to 7310, or a DSB-AM from 7300 to 7310).
(i.e. a USBc or a DSBC at 7305) 

Yes indeed.


How would a AM-decoder react if it was tuned to 7310Khz. Wouldn't he think this is a DSB-AM station from 7305 to 7315 Khz and completely decode this incorrectly?

If the IF shifts the 7310 to the centre of its 5 kHz passband, then both an envelope detector or a sync AM detector centred in the passband are going to see the upper 2.5k of the upper sideband of the lower adjacent freq. As you tune the receiver higher in frequency, less of the undesired sideband and more of the desired will be seen, thus improving the S/I. A correctly designed selectable sideband synch AM receiver would correctly align the passband on the selected sideband and with the synchronous BFO.

OK, but I went from the presumption that you ment a "normal" AM-radio. Relative few radios have sync-AM detection, so I don't think you can really call this "backwards compatible" with DSB-AM radios.

Is there any research on what kind of receivers SW-listeners typically have? Do they have SSB? Sync-detection?

But I do think the situation in different parts of the world:
- here in Europe SW is used to beam news and information from "home" to holidays-destinations
- in the US -as there are no "domestic" SW-broadcasts- (at least, that's what I read somewhere) SW is probably more a "hobby" thing.
- in other parts of the world, SW is also used for "domestic" broadcasting.



But, as I already said, the question is to what degree this really matters. The way I see it, we're asking ourself the wrong question here. The first question would be "who are we aiming this service for and what kind of equipement do they have now"?


I see two possibilities:
- either you direct this at a target-group which already owns a shortwave radio, and -then- backwards compatibility with DSB-AM is important.

- either you are looking at a group of people who currently do not have a SW-radio, and they will have to buy a new one anyway, so -at that moment- backwards compatibility of your service to existing DSB-AM receivers is not important!


So, what is our goal?

- more robust reception?
- better audio quality.
- a better user-interface
- additional services
- reduced broadcasting-power

I think the most usefull approach is to put the two techologies next to eachother and see how well they score for every element.


We can then try to "map" these to what we think is important for the listener, but that will also vary on the kind of listener you're aiming this at.
And this will different for somebody who is interested to learn about foreign countries and who also has access to the internet, then for somebody whos SW-radio is the only way to get news, information and music from the central island of the archipel.




I'm replying in digestible chunks - more later! 

No problem. Is OK for me.




73, Tom 

Cheerio! Kr. Bonne.
.

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