View Single Post
Old 07-28-2009, 04:21 AM   #8  
otaota
Electrical Engineer
 

Join Date: Apr 2007
Posts: 208
Default

Quote:
Originally Posted by geronimo View Post
Is that a product of your tuner's frequency control?
Short answer, probably not.

By frequency control, you're usually talking about being able to lock onto a channel (e.g., a pilot) and track it. In a moving vehicle (e.g. listening to FM in your car), this matters a lot because of constantly shifting Doppler, but for a stationary TV, this is usually a non-issue. Yes, it's possible for moving vehicles and swaying trees to have some effect, but they rarely cause enough of an effect to make the frequency tracking logic wander off.

What is much more common and problematic for 8VSB is multipath. The different paths of signals reaching your tuner create a mix of constructive and destructive interference that the receiver needs to sort out before recovering the data bits.

These kinds of interference vary with frequency. This is mostly because distance traveled by the multipath signals are different multiples of wavelengths depending on the frequency. In some cases you get an additive effect (constructive interference) while in other cases they cancel each other out (destructive interference). Furthermore, materials (e.g., trees & buildings) can have different absorption, reflection, and refraction characteristics at different frequencies.

Even channels coming from the same tower can have a different mix of multipath impairments by the time they reach your tuner. Channels will eventually break-up and get lost if the signal impairments get bad beyond a certain point. This breaking point might show up at a slightly different point on your TV's signal meter depending on exactly how the channel is being degraded and how well the tuner is able to compensate for those signal distortions.

Also keep in mind that signal meters on TVs are not standardized. The numbers that you see do not necessarily have any physical meaning. Most meters estimate signal quality (e.g., number of detectable data errors) as opposed to a real signal strength (i.e., amount of energy) meter.

Cheers,
otaota is offline   Reply With Quote