RF preamps & distribution amps: is it possible to add noise and/or visable aftifacts to a...

Not really. Your friend clearly has no idea what a digital signal is.

Analog yes for C/N measurements, Digital No. Picture is clean or you have macro-blocking and tiling (pre/post corrections)

Let's take 64/256 QAM for this explanation: You have several parts to a digital signal; the RF signal strength,MER, and the measurement of the MER over a period of time = BER. In the BER measurement you will have the total power and Bit rate of that QAM 38.8 Mbps and with a good quality meter look at packets that can be corrected in the stream and process them thru=Pre Errors. Packets that can not be processed thru that are un-corrected are= Post errors, these are what we see and hear with corrupt digital signal = tiling, blocking, freezing, and audio break up. Actual RF signal in digital is one small part, the bigger measurement is MER 35> for 256 QAM should be the lowest. BER test should be done over a period of 5-minutes> to get a good idea of the quality of the signal over a period of time and give the user of the equipment a better idea if there are any Pre/Post errors.

In a CATV plant the amplifiers that are used to amplify the signal more than likely have 2-3 amplification points (Hybrids) Techs will pad and Eq the inputs for flatness and than pad again to get a desired output. Too little input to the amp will give bad MER/BER results and over-driving an amplfier will do the same. There is no magic number just a cliff effect of either good or bad here.

Analog is more affected by visible artifacts due to seeing the C/N increase = good picture. Or decrease <43db C/N and less noisy picture. Beats from over driving an amplifier CSO/CTB where very common in the 80's but with Node + 3,4,and 5 runs is not seen anymore. FCC POP test are still done on those with analog systems and I have yet too see any large CATV not hit their numbers.

Other than the "cliff effect", with QAM carriers/channels, can a amp of any kind (including cheapos) cause any noticeable picture degradation?

No, not at all. Cable and Satellite companies when they Mux the channels into a QAM or DVBS stream the MPEG Clamping will cause extreme loss of Black and White in the frames which gives that nasty macro blocking in bright and dark scenes, especially fast moving or scene changes where P and B frames can not keep up and do the slow motion effect or heavy blocking.

When HD channels came into play for cable companies there was no real compression on the locals and Premium content, bandwidth was there and everything was just converted to QAM. To me it just looks light HD-lite compared to the actual feed before it hits the DCM units and goes ugly. In my opinion the muxed picture should be viewed on a 50"> monitor and not a 20"...

"Higher digital channels tend to generate visible noise with this amp that most cable box filters will display as mosquito noise and slight smearing around objects and on movement. There are better amps in this price range"

I just read thru this and realize that this person is clueless... Amplifiers amplify the RF signal, the actual MPEG content with Video/Audio Pids is not affected by the amp in regard to picture-sound quality. Analog we can all agree is affected by noise in amplifiers. Digital is going to be affected to the point of Bad MER's with video and audio beak up, but if there is no digital break up, the actual Pids are not be tampered with. Maybe this person can tweak the voltage in the amp and make the interleaving going in opposite directions...

For analog I do believe there is a difference in quality between amp manufactures, have seen that first hand where some types do better than others in picture quality.

In short: no. An amp merely amplifies the RF carrier.

If the amp had an insane noise figure, say 20dB, I could see it causing some degradation of a digitally modulated signal. I mean, that much noise very likely will cause grief with the RF, increasing the MER. For an analog signal, that much noise definitely will cause grief. Modern drop amps, though, have noise figures of <4dB. The naked eye could never see a difference between an amplified signal and an unamplified signal (provided that the unamplified signal is of sufficient strength to give a good picture).