The only information which I have with regards to the CCITT Group 4 specification is as follows (information provided by Jim Fleming): CCITT Group 4 specifications may be found in: CCITT Blue Book Volume VII - Fascicle VII.3 Terminal Equipment and Protocols for Telematic Services Recommendations T.0 - T.63 ISBN 92-61-03611-2 The specific information on Group 4 is Recommendation T.6: Facsimile Coding Schemes and Coding Control Functions For Group 4 Facsimile Apparatus This is found on pp 48-57 of the above volume. Note: the CCITT no longer exists due to a reorganisation. It's work is now part of the International Telecommunication Union (ITU)