[opendtv] Re: Specs flap is mobile TVs next test
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 12 Mar 2006 19:55:22 -0500
Tom Barry wrote:
> The battery power is sort of another interesting issue
> I had never really considered in depth.
It's the reason why DVB-H was developed in the first place. The main
goal of DVB-H was to reduce the amount of time the receiver needed to be
on, to save power. CDMA achieves the same goal in a different way.
> There seems to be some math about how much power it
> takes to xmit. But what about receiving? Apart from
> output devices (displays, etc.) is there any theoretical
> minimum power it takes to drive a receiver? Or decoder?
>
> When I was a kid I built a crystal radio with no power
> input except that furnished by the antenna.
Good example.
I don't know of any theoretical limit, but as you point out
convincingly, the theoretical limit would need to take received signal
power as one available source of power for the receiver.
The problem with digital reception is that fast CMOS circuits need lots
of power. That's why they get hot. But there are different design
efforts looking at reducing that.
One is just a byproduct of miniaturization. For equal clock speeds, the
product with shorter gate lengths will have to operate at lower voltage
(to avoid arching), so power draw will necessarily have to go down. The
smaller these measurements, the greater the leakage currents from source
to drain, the harder it is to tell the difference between the FET being
on or off. So as these leakage issues are addressed, power draw is
reduced, and this trend *has* to continue if Moore's Law has any hope of
applying in the future. (I've been reading that 45 nm design rules are
not as problematic as people had originally expected. Looks like 90 nm
was a big hurdle, but once you've done that, 45 nm isn't such a
problem.)
The higher density chips are also forcing a rethink of how clock signals
are used. Looks like a new trend is to use clocks only locally, rather
than a system clock for the whole chip, and this is supposed to provide
for greater speed or less power draw, or a combination of both.
Another trend is "power harvesting," where all sorts of effects, such as
motion or surface stresses or temprature variations or ambient light, or
whatever, would be used to generate power for the device. Sort of the
electronic equivalent of the self-wind watch (they still exist, believe
it or not).
So I think there's hope in the future for fast digital RF links to work
interchangeably with high quality devices such as fixed or vehicular
mobile, and with small receivers like portable radios. Analog circuits
have achieved this, even though originally they couldn't. The first
transistor radios were strictly AM. To me, the bitrate-compromised
separate stream dedicated solely to small portable appliances is not so
interesting.
Bert
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