time machine backups

i have a question about time machine backups...

i dont need to backup my files often and the drive i am using for a time machine backup is also my backup drive for other computers. currently when i backup it creates a folder on the drive as a backup. i only need to do a backup about once a month or when i feel i made a major change in my computer.


my question is how is each backup on the hdd different from one another? do they depend on each other? since i only need to do a backup once a month, i would like to backup, then delete the old backup, only keeping one backup at a time on the drive to save space.

is this possible? i did a 2nd backup for the hell of it and the 2nd backup seem to go a lot faster than the first. why is that? so is all time machine doing is juss backing up the chances in the 2nd backup and the 1st backup is still needed? or was it only faster becuz time machine scanned the drive and saw most was unchanged and both backups are independant of each other?
thanks