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mode. Introduction The system to be described was developed to enable the use of multiple cameras for recording events at a school. The school's cameras are Sony domestic camcorders which do not have a genlock input, hence the need to use frame synchronisation prior to camera switching. Camera switching is performed by a Leitch AVS481 8-input vertical interval synchronous switcher. The 5 camera version of the system is shown in the diagram, but up to 8 cameras may be used. This is limited (fairly obviously) by the number of inputs available on the synchronous switcher, and is also limited by the number of mixers available for frame synchronisation. Use of Panasonic WJ-AVE5 mixers as frame synchronisers: In the absence of very costly professional PAL composite frame synchronisers, it was decided to use instead Panasonic WJ-AVE5 mixers, of which the school owns one. There are also other such mixers available on loan to the school, from parents or governors. The Panasonic WJ-AVE5 vision mixer was sold to the consumer and semi-professional markets, and was intended for use in simple 2-camera setups. It is a dual-bus mixer incorporating timing circuits which synchronise the the signals passing through each bus to the internal master clock. However, these timing circuits may be over-ridden by introducing a master sync. signal into the "ext. camera in" socket, thus enabling this relatively simple and inexpensive domestic/semi-pro mixer to be locked to studio or station master syncs. The "ext. camera" input also incorporates a luminance overlay facility which may be used in conjunction with an external text/title generator for adding titles to the mixer's output signal; however, this additional facility is not used in this scheme. Inputs and switch settings:
The mixer now behaves as a dual-channel timebase corrector (TBC), with the camera signal which is connected to input 1 being available at the main output, and the camera signal which is connected to input 2 available at the preview output. Note that the main output(s) are available as either composite or S-video signals, but the preview output is available only as a composite signal. As the switcher used in the school's system will handle only composite signals, it is these outputs which are taken to the switcher. After passing through the mixer, both the camera signals are synchronised to the master sync. pulses present at the "ext. camera in" socket. If the master sync. signal is derived from another camera (as in the following diagram) rather than a sync. pulse generator (SPG), the maximum permitted number of cameras in the system is twice the number of mixers available, plus one. If a professional SPG is used to provide the master syncs, the total number of cameras which may be used in the system is equal to twice the number of mixers available. Distribution of the master sync. signal: If a camera is to be used as the master sync. source, a video distribution amplifier (VDA) will be necessary to ensure that the camera is capable of producing sufficient outputs to provide both an image signal to it's own switcher input and a sync. signals to the mixers. Such a set-up is shown in the diagram below. There is an exception to this: for a 3 camera (1 mixer) set-up where the sync. camera is close to the equipment, it should be possible to use the composite output from the camera as the image source for it's switcher input, and the S-video output from the camera to feed the S-video "ext. camera" input on the single mixer. Similarly, if an SPG is to be used to provide the system master syncs, a VDA may be necessary to ensure that sufficient signals are made available to fulfil the system's requirements. However, many SPGs have multiple outputs, and this may alleviate the necessity for a VDA for sync. pulse distribution. Use of other models and makes of mixer for frame synchronisation: Most 2-bus Panasonic mixers may be used in the manner described above, but note that older Panasonic models such as the MX10 and MX12 are only single bus mixers and cannot be used to synchronise 2 cameras to a separate master sync. signal. However, if the system uses ony 2 cameras in total, these single bus mixers will successfully synchronise the camera connected to input 1 to the sync. pulses from the camera connected to input 2. This is because the signal connected to input 2 is used by these single bus mixers as the system master sync. source. Thus they can be used to synchronise a single camera to a system by connecting that camera to input 1 and the system master syncs to input 2. This method of single camera synchronisation also applies to the Videotech VMX400 and VMX410 (both also single bus mixers). The writer has no personal experience of any other makes of mixer. Post sync. processing: Selection of the synchronised camera signals for recording must be made using vertical interval synchronous switching. An example of this is shown in the following diagram. The switcher shown in the diagram is the school's 8-input Leitch AVS481 (only 5 of the 8 inputs used), but any broadcast quality synchronous switcher with sufficient inputs for the system's needs may be used. After the selection of camera signals,
the subsequent processing, fading, distribution and/or
recording may be done. The diagram shows the simple
recording set-up used at the school, but the possiblities
are limited only by the needs of the user and the
complexity and variety of the equipment available. |
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