How can a digital signal exhibit an analog artifact?

ThePhysicsProfessor

Hello gurus

Attached is an image from a client using an HDMI to DVB-T modulator to reticulate an HD HDMI signal around a house and a demodulator to extract the HDMI (not using the TV's digital tuner as its processor is too slow). As can be seen from the attached picture, the result is akin to Moire fringing which is impossible in a digital RF signal. Additionally, the amount of fringing is proportional to the speed of action in the video. ie a static video image is near perfect but increasing movement exacerbates the fringing.

This seems to defy all logic in a digital world. Any thoughts would be gratefully received.

Cheers and thanks                                                                                

The physics professor from NZ

andy1249

Ive seen this before.
It depends on your model of modulator and exactly what it is doing.

None of them are lossless , all of them introduce some level of artifact.
The majority capture the HDMI TMDS signals and convert them to mpeg4 , modulate them onto an RF carrier ( analog by default ) and the digital information is retrieved from this analog carrier.

Demodulating back to HDMI will add more artifacts as a large proportion of the original HDMI content is lost in the first conversion process.
These units work best when the endpoint is a DVB T tuner.

Try switching the channels that the modulator and demodulator work on, they are prone to analog interference so switching channels can help.

Note : Live sports are usually 1080i ,interlaced, looking at the picture and taking a guess at whats happening , the HDMI to mpeg conversion is losing a high percentage of each frame and cannot reconstruct a coherent de-interlaced picture at the demodulator end from the lossy data. (Note the lines in the picture)

ThePhysicsProfessor

Hello Andy 1249 (esq)

Thank you for the very lucid reply. Everything you have said makes sense although I must admit to not really seeing how a demodulator (which is a single channel (adjustable) tuner) really differs from a TV's DVB-T inbuilt tuner. I forgot to say in my original post that the modulator works perfectly when demodulated by the TV's tuner (which would, I guess, rather reinforce your point )

In any case, you have given me a really good nudge in the right direction so THANK YOU!

PS: can you direct me to a decent (ideally white paper standard) URL that provides a detailed description of the underlying principles of HDMI over DVB-T?  I really feel I need to do some serious updating of skills in this area

Kind regards and thanks again for your illuminating reply.

Karl

andy1249

Other than basic tech description and user manuals from the likes of Edision you are not going to find anything official, certainly not a white paper.

These things are of questionable legality depending on where you live.
They are not an official hdmi.org licensed product and never will be because they strip away HDCP.
Defeating HDCP or any other digital content protection is illegal in any country where the DMCA or its equivalent is the law.
So of course, no white papers, any white paper would be a straight out admission of guilt and an invite to prosecute.