California requires both parties know they are being voice recorded, it cannot be hidden.
This was a law before video cameras were available and affordable to the average person. As a result pure video recording without audio typically does not require one to inform the other.
The funny result is a hidden video camera with audio can be violating the audio privacy law, while the footage itself could be legal without the audio.
However the audio law is not mutual consent, but rather the people being recorded need to know they are being recorded. They must merely be informed in some manner, not consent to it.
This stems from telephone taps. Many states only require that one side of the telephone conversion know that a telephone call is being recorded (while no sides knowing and the call being recorded by a third party without a warrant is a crime.)
Some states such as California on the other hand require that both sides of the telephone call know that they are being recorded. They do not have to specifically consent. If they are informed the call is being recorded everything after that point would be legal.
For example most banks and many other businesses will have a voice recording that informs the customer the call 'may be' recorded or monitored 'for quality assurance' or some other such nonsense. This is to allow them to legally record the call in states that require both parties to be informed.
The result is in California if a cop was to pull you over and you hit record on say your cell phone and recorded the audio of the entire incident without them knowing you could be committing a crime.
However if the phone is out and being used as a video camera clearing pointed and used in a manner that shows it is being used to record the incident, both the video and audio are valid because the act of openly recording is informing the person being recorded what is being done. (Of course a cop is not going to let someone use their cell phone, video camera, or other such device in many situations, and will have them in cuffs. One should also exercise caution in pointing objects at officers, in low light conditions a cell phone pointed can be mistaken for a gun.)
Likewise if you have a video camera you are clearly and openly using to record something, everyone is informed they are being recorded, making the audio valid. However if it is a hidden camera then the audio is a crime because those being recorded were not informed. A way around the hidden camera scenario is a sign that informs people they are being monitored or recorded by surveillance. This then makes the audio valid too, but only for a certain distance, a distance where it informs the regular person. You cannot simply have a sign at your house that someone needs to be within 100 feet to read and a surveillance system that can hear low audio down the street. (And yes there is both directional microphones and laser devices that can hear audio like a normal conversion from miles away. The lasers essentially become microphones where they are pointed, picking up vibrations from the hard surface, they can be pointed in a window from a mile away and hear what is being said within the room as if they were in the room. Or at certain hard objects outdoors near an outdoor target.
I have seen law enforcement openly using directional microphones configured like a rifle and aimed to listen in on drug deals miles away. They liked a hill that had a view of a bad part of town, and could often be observed pointing it towards people and listening to what was being said by individuals further than you could see with the naked eye. It was setup like a rifle, had a scope, and headphones (and recorded.)
In some areas of California it was not uncommon for incriminating recordings of the police that the police were aware about to go missing after being seized or taken as evidence in some areas. Or for the footage to be lost, damaged, or some other such 'accident'.
So the added risk when they know they are being recorded is that if the footage is seized it may become worthless, while if they don't know it is being taken it may be inadmissible in court and a criminal offense.
