Hi, I have a real-world problem encoding and decoding 7- & 8-bit values. The context is a MIDI SysEx file. In MIDI, the high bit of a byte is used to signal end-of-file, so all the data bytes must be 7 bits. When sending 8-bit data, my synth (a Korg Karma, if you care) encodes eight-bit data as follows: 
+---------------------------------------------------------
|  Internal 7byte data <--convert--> MIDI 8 byte data                 
+                   |
|  example: Internal data(bit image) MIDI data(bit image)             
+                   |
|                Aaaaaaaa            0GFEDCBA                         
+                   |
|                Bbbbbbbb            0aaaaaaa                         
+                   |
|                Cccccccc            0bbbbbbb                         
+                   |
|                Dddddddd            0ccccccc                         
+                   |
|                Eeeeeeee            0ddddddd                         
+                   |
|                Ffffffff            0eeeeeee                         
+                   |
|                Gggggggg            0fffffff                         
+                   |
|                Hhhhhhhh            0ggggggg                         
+                   |
|                Iiiiiiii            0NMLKJIH                         
+                   |
|                   :                0hhhhhhh                         
+                   |
|                   :                   :                             
+                   |
|                Vvvvvvvv            000000WV                         
+                   |
|                Wwwwwwww            0vvvvvvv                         
+                   |
|                                    0wwwwwww                         
+                   |
|                                    11110111 (EOX=F7H)               
+                   |
+---------------------------------------------------------

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Now, I wish to decode the 7-bit data to 8-bit data, and back again. The question is, what's the most efficient approach? I'm sure something using unpack & vec should be in order, but I'm unsure which is the most (time-)efficient approach. I'm gonna be doing this a fair bit, so I'm willing to trade readability for speed. Ideas gratefully received, Thanks.

In reply to Converting 7 & 8-bit values by fluffy

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