Klaus describes the changes since VDR 1.7.20 as follows:

• Fixed detecting frames for channels that split frames into several payloads (reported by Derek Kelly).
• Now initializing Setup.InitialChannel to an empty string to avoid problems in case there is no setup.conf.
• The start time of an edited recording is now set to the time of the first editing mark (thanks to Udo Richter). This obsoletes the CUTTIME patch.
• Direct access to the members start, priority, lifetime, and deleted of cRecording as well as to position and comment of cMark is now deprecated. Plugin authors should switch to the new access functions for these members. For now the macro __RECORDING_H_DEPRECATED_DIRECT_MEMBER_ACCESS is defined in recording.h, which exposes these members, so that existing plugins will still compile. Comment out this #define to check whether a particular plugin needs to be modified. This #define may be removed in a future version.
• The new functions cRecording::NumFrames() and cRecording::LengthInSeconds() return the number of frames and length (in seconds) of a recording (suggested by Steffen Barszus).
• The subtitle PIDs are now stored in the channels.conf file as an extension to the TPID field (thanks to Rolf Ahrenberg).
• The new function cDevice::ProvidesEIT() is used to determine whether a device can provide EIT data and will thus be used in cEITScanner::Process() to receive EIT data from the channels it can receive (suggested by Rolf Ahrenberg). Note that by default it is assumed that a device can't provide EIT data, and only the builtin cDvbDevice returns true from this function.
• The Audio and Subtitles options are now available through the Green and Yellow keys in the Setup/DVB menu (thanks to Rolf Ahrenberg). This is mainly for remote controls that don't have dedicated keys for these functions.
• The SVDRP command HITK now accepts multiple keys (up to 31).
• The Recordings menu now displays the length (in hours:minutes) of each recording (thanks to Rolf Ahrenberg). Note that the "new" indicator has been moved from the recording time to the length column. This new format is also used by the SVDRP command LSTR, so in case you have an application that parses the LSTR output, you will need to adjust it to the new format.
• The dvbsddevice plugin now supports the new option --outputonly, which disables receiving on SD FF devices and uses the device only for output (thanks to Udo Richter).
• Fixed detecting frames on radio channels (reported by Chris Mayo).
• Revoked the changes to cFrameDetector that have been introduced in version 1.7.19. Detecting frames in case the Picture Start Code or Access Unit Delimiter extends over TS packet boundaries is now done by locally skipping TS packets in cFrameDetector.

An overview of earlier VDR developer versions of the 1.7 branch can be found here.