Chorus was a cable operator based in Ireland, who used MMDS to transmit their cable service, whilst using the DVB-T transmission protocol. Compared to ntl: Ireland who opted to use DVB-C over their MMDS system.
The main set top box used was a Sagem ITD4000, which featured two card slots, internal Nagravision conditional access and OpenTV EN1 middleware (this is different to the version used by Sky). To use with an MMDS antenna, a converter (similar to an LNB on a satellite dish) would convert the MMDS frequencies in the 2.5Ghz band to the lower band used by the STB tuner.
Essentially it is similar to a standard freeview (or ONdigital, which existed at the time) receiver with the ITD4000 being able to decode DVB-T broadcasts. Because of the differences in middleware and encryption, ONdigital receivers were not compatible, and it is unknown if the ITD4000 receivers can be returned to Freeview DTT signals.
Chrous and ntl:Ireland would merge, and would later rebrand to UPC Ireland following purchase by Liberty Media.
Screenshots of the Chorus TV guide software
Card slots on the Sagem ITD4000