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Does being the client of such a service constitute being a supporter?
Yes it would as it justifies the very existence of these services.
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is it enough just to mention something positive about one of those services?
Please refrain from mentioning anything (good or bad) about payservers as they are only in it for financial gain which contradicts the whole principal of sharing.
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one shares the whole device, which includes the card and card reader
Not exactly accurate, a request (ecm) is sent to the card to clear a specific channel (get the control word key, cw).
This request has to be asked and answered within a specific time period as they change frequently (within seconds)
The faster these requests reach the card / returned to the peer, the better.
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in principle, it is possible to get started with the whole card sharing scene without owning a single card reader. Did I understand this correctly?
You are correct in principal but obtaining a share from someone without offering anything in return is highly unusual.
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what happens when a shared card that I want to use is already being used over the network by someone else? Will I be prevented from using it? Or does this not apply to devices shared over a network?
The card will only accommodate a certain amount of requests (different packages have different limits).
A server trying to fulfill too large a number of requests will be hindered as it takes time for the reader to process each individual request from the card and the returned cw may be obsolete by the time it returns to complete the initial request.
This is when glitching (picture freezing)will occur and is an example of on overloaded server.
Broadband speed will also play a big part here.
I've no experience with Myth tv.