I want to compress backup files to tape using compress on our AIX 4.2

- Our TAR does not have compression.
- I do not want to use local storage to compress as most of the filesystems are pretty full.
- the only compressing tool we have is 'compress'
- tapes are 5Gb 8mm


I am trying this :

change tape block_size=512 (default dd obs value)
tar -cvpdlBf - a_folder | compress | dd of=/dev/rmt0

but it is giving me :
dd: 0511--53 The write failed.
: A system call received a parameter that is not valid.


but using something like this works:
tar -cvpdlBf - a_folder | compress | dd of=/tmp/zzz.Z