I got the info that the purpose of the 3 muxes from Singapore are actually
T2-MI.
For those who don't know, one of the main purpose of T2-MI is for "easier" deploying
Single Frequency Network (SFN) where it makes sure the digital TV signals along with main and repeater transmitters are
in sync and transmits the exact identical DTT TV signals at the same time. Avoid open this spoiler. You might get headache
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Source:
https://www.etsi.org/deliver/etsi_ts/102700...773v010401p.pdfTL;DR
So this is what the T2-MI mux is transmitting... To viewers like us, it's like duplicating mux.
| T2-MI | >> | Actual DVB-T2 mux |
| CH 37 602MHz | >> | CH 29 538MHz (Channel 5 and Suria) |
| CH 36 594MHz | >> | CH 31 554MHz (Channel 8 and Vasantham) |
| CH 34 578MHz | >> | CH 33 570MHz (CNA and Channel U) |
And yes, digital TV box (STB) must support T2-MI to watch what's in it. Otherwise, it's just able to detect signal that appears "no channels" in it.
Tbh, this T2-MI thinggy is very technical and too engineer for me to understand. All I can understand enough is, SFN.
That picture you attached is outdated... Here's the latest and updated one:

See the ones highlighted in red? That's signal interference with 2 Indonesia's analogue TV channels, RCTI and SCTV.
About the lower CH 21, 23, 25 frequencies can be used for Malaysia DTT, it is unknown to the public as in why MYTV and/or MCMC don't want to allocate it onto those frequencies. As what i know, those frequencies are vacant now.
Btw, the allocation for radio astronomy on Channel 37 is mainly applies in the United States, Canada and Mexico.
https://en.wikipedia.org/wiki/Channel_37 I think Asian countries didn't reserve/allocate that frequency for radio astronomy also...

CH.50 and above still available despite of putting mobile signal on 700MHz range?