[ale] Wandering OT: Re: Car PC's and internet radio?

[Jeff Hubbs]

Dale Heatherington wrote:
> Relax.
> Satellite radio is like AM or FM or TV broadcasting - IT'S ONE->  
> WAY.  
Do you know this as fact, or are you presuming it?  Can you point me to
a specification or, better yet, otherwise demonstrate that it's
downlink-only?
> The broadcaster cannot know which programs or channels you  
> listen to.  
Ditto?
> A single datastream from the satellite is received by all  
> the radios.  Nothing is transmitted back.  The radios are "receive- 
> only".
>   
Ditto, ditto, ditto?  I'm sure they *appear* to be downlink-only. 

I've never used a satellite radio and I haven't had a chance to examine
one outside or inside (I might be able to *tell* if it could transmit if
I took one apart).  I don't *know* if they can transmit; I am simply
wondering out loud if they do.

I should point out that there have been satellite phones for years; they
manage to do two-way duplex voice, using a handheld unit with a battery
and a tiny antenna.  All the XM radio uplink would have to do is
transmit receiver identification and channel selection data occasionally
- and it could take its time doing so.
> This is much more efficient than any sort of cell phone radio  
> "broadcasting" with a 2 way datastream for each and every subscriber  
> - what a massive waste of bandwidth!  
Again, you're comparing my apple to your orange.  XM radios that act
like I'm suggesting have a transmission rate profile very unlike that of
a cell phone.
> Satellite radios bandwidth  
> requirement is constant regardless of the subscriber count.
I propose that if my hypothesis is correct, what you are saying here is
close to correct; "constant" would be a lower bound.  In practice, if
different content is shunted to different people on a single channel,
then the downlink bandwidth would scale slightly with subscriber base
size.  They would certainly realize that subscriber base size has an
upper bound, albeit one that has been growing near-exponentially (which
is clearly not sustainable).