Ok, now the picture is clearer for us to talk about using using mixer.
We will need a bi-directional splitter/mixer, available from amazon, that looks like this to combine the two signal sources (actually I believe a normal 2 way splitter will work just as well):
From soundtech you feed the fta digital channels (hd5, dig5, dig 8 and digCNA) on Channels 29 and 38, plus very poor analogue signals, and 15 mysterious scrambled channels - the frequencis of which are unknown - to one input of the mixer.
The other input will come from the scv outlet which comprises the fta analogue channels, some of which have been shifted to different frequencies (CNA, U and octo), plus tv1, one free sports channel on 76.25 Mhz, and many starhub scrambled channels.
The question is whether starhub use Channels 29 and 38. If they don't then you will get clean output for these two channels from the mixer. The other question is whether the 15 unknown channels from sountech will clash with the fta analogue channels from scv output, when the two sources are combined.
I think the poor analogue signals from the soundtech will not be an issue.
Perhaps a kind starhub subscriber can help to answer the question on Channels 29 and 38. But I doubt anyone can clear the air over the 15 scrambled channels your soundtech receives.
So I think you can only know if it works after you have tried it out.
The above may partially explain why your setup didn't work. The other possibility is that, for example, chan 8 signal from starhub and the chan 8 signal from your own antenna may be out of phase - they travel by different paths - and cancel each other out.
If you are willing to give more info on what you did, I can try to figure out why it went wrong.
I tried using a splitter to act as a "combiner" but it didn't work. I guess the reason is because the 2 signals are cancelling each other out. I think if I can filter out 224.25MHz to 503.25MHz on the Soundtech antenna then I should be able to get the result I want. Any device that can do this?