Canadian TV, Computing and Home Theatre Forums banner

1 - 7 of 7 Posts

·
Registered
Joined
·
54 Posts
Discussion Starter #1
I can't understand why, with the digital technology available these days, that someone hasn't come up with a tuner that uses some kind of 'digitally derived' notch filter that's synchronized with the selected channel.

For instance..if you're currently watching channel 20, why couldn't the tuner automatically assign an appropriate digital notch filter that would block all other channels and amplify channel 20 as required? This would avoid the overload problem altogether and give those weak stations a chance to be seen.

It's so stupid having a wideband amplifier at the head end anymore. Such retro technology...
 

·
Registered
Joined
·
102 Posts
This problem has been brought up in the preamp forum I believe. Basically, the issue with a true "digital" filter is that the filter must be capable of 2x the frequency, and although A/D's exist to do this, they are very cost prohibitive at this time. Furthermore, anything placed before the preamp would likely negatively affect the NF of the amplifier, which would make the problem worse. a SAW filter might work, but these are channel specific devices, and a different SAW filter would need to be switched in/out each time a channel is selected, which is not practical.
 

·
Registered
Joined
·
1,980 Posts
Good idea, but making it practical is a problem. I've worked at cable & sat co headends where they use a dedicated antenna and filter for each channel. That's expensive stuff.
Building that into your consumer TV or tuner, would put them out of the price range for consumers.
The problem stems from RF being an analog signal. even if it's carrying a digital signal, electromagnetic waves are analog. And to select a certain frequency range, you have to tune to that frequency, using analog filters. All this can and is digitally controlled in todays tuners, but you still have to select the analog carrier to find the digital information within.
A better idea might be to shut down analog transmission, yeah, they're working on that. We'll still be left with co-channel or adjacant channel interference (an analog problem) but not as bad as before.
And all this comes from the original ATSC premise, that it had to work alongside NTSC, providing a time for transition.
A better concept might have used a different band altogether.
 

·
Registered
Joined
·
1,950 Posts
It can be done but it would be prohibitively expensive for consumer-grade devices.

My main ham radio transceiver has digital signal processing (DSP) at the IF stage of the receiver to effectively shut down adjacent interference, even as close as 5-10 kHz away.

But, the radio itself costs over $2000 so there you go.
 

·
Super Moderator
Joined
·
5,080 Posts
Concurred with above: Sharp-cut channel bandpass filters are expensive to create. Possible, but expensive.
 

·
Registered
Joined
·
54 Posts
Discussion Starter #6
True...I realize you're dealing with an analog carrier signal, whether it's digitally encoded or not. I also realize you're talking about a very high sampling frequency (at least double the carrier frequency @ channel 51 - can't remember the Mhz offhand).

This could posibly be accomplished with several processor chips running in tandem, double-clocking (counting each clock-up and clock-down transition) etc. Many clever numerical manipulations are possible in the digital domain, you just have to think outside the analog box. Besides, processing power is growing by leaps and bounds as we speak. It could become simple and affordable sooner than you think.

P.S. I had some other ideas about using digital logic to control an antenna rotor, but I'll save that for later. :cool:
 

·
OTA Forum Moderator
Joined
·
24,867 Posts
leglamp said:
P.S. I had some other ideas about using digital logic to control an antenna rotor, but I'll save that for later
It's an interesting topic that others have also raised, leglamp. You'll want to read through these threads in the Antenna Research & Development Forum and post your ideas in the appropriate one(s):

Switching antennas automatically, using modified DisEQc hardware

Smart Antennas, Adaptive Arrays, Automated OTA Gear

Switching antennas automatically, electromechanically

Always looking forward to more ideas in there. :)
 
1 - 7 of 7 Posts
Top