Axiom Home Page Forums General Discussion Technical Questions getting rid of hum created loss of HD channels

Uh, I went and got two 75 to 300 ohm transformers and put them between my cable line and my cable box. It fixed the hum coming from my sub but also eliminated my high definition stations from my cable. Why is this happening? obviously the digital feed won't come through, but my analog channels are fine. Can someone explain to me why this happens and if there is a cheap way to fix the problem and keep my HD channels.

JH, not knowing the specs on whatever transformers you used, possibly the obvious suggestion(and also possibly wrong, of course)is that there was inadequate bandwidth for HDTV. An isolator specifically designed for cable TV, such as this might work.

I tried using the isolation transformer from parts express and it too had an affect on the picture quality.I ended up grounding the cable feed to the earth ground for the electric and that fixed the hum for me.

Hi jhunt17, Wid and JohnK,

I just spoke to Tom Cumberland Axiom's senior design engineer, and he pointed out that TV-cable systems supplying digital HD signals combined with many other services use a combination of phase manipulation and multiplexing to cram all the services into a minimal bandwidth in order to pass all the stuff through the copper cable.

Using a pair of 75-to-300 ohm balun-transformers worked in the days of analog TV, but according to Tom, transformer-based ground islolators will do "nasty things" to the phase and multiplexing, which explains why you are not getting your HD digital signals.

It's early at this point, but he says a capacitor-based isolation device should, in theory, be OK. Axiom is getting in some prototypes that we'll take apart and test with digital HD signals and eventually offer one that will effectively pass a digital cable TV feed with no degradation while still doing its job of isolating the cable ground and eliminating ground loops caused by the cable feed.

I suspect the cable line isolator sold by Parts Express may be transformer-based, which may explain why it's mucking up the HD signals.

Stay tuned.

Regards,

Great Idea, Alan! With all the complaints about ground loops I run across on the forums, I suspect it would be a welcomed product.