VALID stands for Video and Audio Line-up and IDentification. In its standard definition form it’s been in use for several years all over the world. It produces a virtually indestructible signal – you can compress it, record it, replay it, transmit it – provided some video and audio survives, then the VALID signal can be measured for video/audio delay.
Now there’s an HD version of VALID.
So what is VALID8? The signal can be broken in 4 distinct sections. Firstly there’s the test pattern background – normally that’s colour bars with some additional markers for 16:9 and 4:3 aspect ratios. The Pro-bel V6811 VALID8 generator can actually produce a variety of test patterns, but the basic 75% bars is the default standard. So the signal can be used in any situation where colour bars are used for line-up.
Secondly, there are the dynamic display elements. If you see standard colour bars, there’s no way of knowing if the image you’re looking at is normal video, or something frozen. So VALID8 has a rotating ring at the center of screen. This ring normally takes 4 seconds to complete a 360 degree rotation. The other dynamic element is a black cross which flashes on screen every 4 seconds as well. The dynamic elements are the video equivalent of the film clapperboard – they provide a visual identity to a specific moment in time – once every 4 seconds. In reality, this duration can be adjusted to suit individual circumstances, but for truly international compatibility, the factory recommendation is for 4 seconds.
Thirdly, there is the audio. This consists of 8 channels, each with a different audio frequency. They are logically organised as 4 “Left – Right” pairs. In each pair the audio has defined breaks, which match the 4 second repeat rate of the video signal – so that you have the audio side of the clapperboard, not with a “clap” but with a moment of silence.
The final part of the VALID signal is the ID – the Generator allows up to 4 lines of user defined captions, plus a central indication of the original signal standard and audio level – vital where the signal may pass through a variety of conversion processes in a typical broadcast chain.
In use, it’s entirely practical to detect A/V delay on a purely visual/aural basis. It’s easy to see when the rotating circle reached “top dead center”, and at that time there should be a single break in the LH audio, followed by a double break in the RH. But for accurate measurement, Pro-Bel produce a VALID8 reader, the V6821. This can detect the black cross in the video, and the breaks in the audio, and can display the error to milliseconds. The reader can also indicate if there has been any channel cross-over or even if there is a phase error between left & right channels. It also displays the actual level of signals with options for a variety of metering formats – VU, PPM, etc.
The best way to operate VALID8 is in an “end-to-end” situation. Take a typical sports operation. The remote truck would be equipped with a VALID8 generator, which also provides standard line-up and remote ident facilities. The incoming feed from the remote can be fed to a VALID8 reader, and any delay errors can be measured and suitable audio delay applied. Now you can be certain that the source signals from that remote are in lip-sync.