R
rbrucecarter5
Guest
Bruce,
you are totally incorrect with your numbers. A Local Oscillator in a radio (The LO) does NOT move because of a wider input signal. A LO is a single fixed carrier......In FM radios, a LO is 10.7 MHz above the desired signal (one reason FM radios are banned on aircraft...the LO can interfere with VOR and DME equipment onboard)
With a station running HD, the LO is still on the original frequency for the analog carrier. The HD carriers are on either side of the main carrier and are converted by the LO to sides of the 10.7 analog IF signal. This signal does NOT radiate (if it did, analog radios would have been interfering with each other on ALL frequencies for decades)...sorry your "facts" are totally incorrect. A PLL tuned radio will have a rock stable LO 10.7MHz above the signal it is receiving...Only one. The 10.6 to 10.8MHz issue was a problem with analog/manual tuned radios...but with PLL tuned radios, that does not happen anymore. A radio digitally tuned to 96.5 will only cause issues with a station on 107.2. That issue only happens with cheap home table top and cheap Walkman style radios but with the PLL circuits being as they are, it's cheaper to make a radio with a digital tuner (NOT HD here) than a manual tuner. However, there are still cheap table top FM radios manually tuned...if more than 100kHz off center, they COULD cause a problem but not the distance you claim....the LO does not shift +/- 400kHz while receiving the FM signal...IF someone had a radio at a home causing a problem over a 600khz spread, the radio was defective and should not have been radiating a LO for 1/5 of a mile anyway. Part 15 rules do not allow radios to be sold with that kind of leakage, etc. Home radios must meet the same Part 15 radiation limits as car models do.
Also HD on FM is redundant. A LO at 107.2, IF it interfered with HD carriers on the low side of 107.5, would not cause issues with the HD carriers above the 107.5 analog signal. This feature was built into HD to provide redundancy in the case of interference.
Sorry your argument doesn't hold water.
I never said the LO frequency moved. It is 10.7 MHz. But the stations do frequency modulate, so the IF frequency frequency modulates. Frequency modulation doesn't equal the channel width, in the old pre-HD days it was +/- 75 kHz. I have no idea what HD did to that frequency modulation, but it must have increased - because the jamming is real. Just do the darn test for yourself! I tried it again yesterday, IF does radiate to some degree from every radio, thankfully car radios are shielded better than most, but it varies with car model.
I am glad they built redundancy into the system, but IF images still cause the HD to drop. It is really noticeable on HD-2 because it goes silent.