The problem is, it has to be an always on situation. If you allow for officer control, you will have cases where they will "forget" to turn it on.
Case in DC where 11 responding cars all had "broken" cameras comes to mind.
You can argue the officers right to take a crap without being listened in on, something I can sympathize with, but unless it's an always-on, there will be room for abuse, and there will be those who will abuse it.
Another point in support of 'always on' is how many times have we posted videos where someone has said 'you don't know what was said/happened before the clip shown'. Always on negates most of that.
ANY such system should also be monitored by a unbiased 3rd party. A real-time system could also be used where officers doing stops can be monitored in real time. Again, how many clips have we seen where a lone officer has been attacked and often killed? Real time monitoring would allow officers performing stops to more rapidly receive assistance and back up when needed.