ZIP
From zedwiki
Discussion
The ZIP file format would be a good candidate to use as a container for Daisy books as it concatenates multiple files into a single file. It also allows to have compressed and non-compressed files in the same archive. So of all the files that make up a DAISY book, the text files would be compressed and the audio files, already heavily compressed, would be left uncompressed which would allow devices with limited memory and processing power to access those audio files with the same ease as if they were not compressed and directly from inside the zip archive.
The main reason for not allowing compression of audio files is to enable devices with limited resources to easily be able to directly seek to specific positions in an audio file within a zip file without having to decompress the whole file. Also since audio files are already compressed, adding a layer of compression would yield very little space reduction for all the processing overhead it would incur.
Unfortunately, using a zip archive would prevent progressive download as the Central Directory (list and position of all files in the zip) is located at the end of the zip file.
The zip password protection method could be applied on compressed files as they have to be processed to be uncompressed anyway. Applying a zip password to audio files would not be possible as it would eliminate the possibility of accessing the files randomly. Using PDTB2 would work seamlessly though.
