Broadcast-safe video (broadcast legal or legal signal) is a term used in the broadcast industry to define video and audio compliant with the technical or regulatory broadcast requirements of the target area or region the feed might be broadcasting to.[1] In the United States, the Federal Communications Commission (FCC) is the regulatory authority; in most of Europe, standards are set by the European Broadcasting Union (EBU).
Broadcast safe standards for 625 lines of standard-definition (Inaccurately referred to as PAL, a colour encoding that is usually used with such systems) video are:[2] [3]
| Resolution | Aspect ratio | Pixel aspect ratio | Form of scanning | Framerate (Hz) | |
|---|---|---|---|---|---|
| Vertical | Horizontal | ||||
| 576 | 352 | 4:3 or 16:9 | non-square | interlaced | 25 (50 fields/s) |
| progressive | 25 | ||||
| 480 | 4:3 or 16:9 | non-square | interlaced | 25 (50 fields/s) | |
| progressive | 25 | ||||
| 544 | 4:3 or 16:9 | non-square | interlaced | 25 (50 fields/s) | |
| progressive | 25 | ||||
| 720 | 4:3 or 16:9 | non-square | interlaced | 25 (50 fields/s) | |
| progressive | 25 50 | ||||
Broadcast safe standards for 525 lines of standard-definition (System M, NTSC, NTSC-J, PAL-M) video are:[5] [6] [7] [8] [9]
| Resolution | Aspect ratio | Pixel aspect ratio | Form of scanning | Framerate (Hz) | |
|---|---|---|---|---|---|
| Vertical | Horizontal | ||||
| 480 | 640 | 4:3 | square | interlaced | 29.97 (59.94 fields/s) 30 (60 fields/s) |
| progressive | 23.976 24 29.97 30 59.94 60 | ||||
| 720 | 4:3 or 16:9 | non-square | interlaced | 29.97 (59.94 fields/s) 30 (60 fields/s) | |
| progressive | 23.976 24 29.97 30 59.94 60 | ||||
Digital broadcasting is very different from analog. The NTSC and PAL standards describe both transmission of the signal and how the electrical signal is converted into an image. In digital, there is a separation between the subject of how data is to be transmitted from tower to TV, and the subject of what content that data might contain. While data transmission is likely to be a fixed and consistent affair, the content could vary from High Definition video, to SD multicasting the next, and even to non-video datacasting. For ATSC 1.0, 8VSB transmits the data, while MPEG-2 encodes pictures and sound.
| Resolution | Aspect ratio | Pixel aspect ratio | Form of scanning | Frame rate (Hz) | |
|---|---|---|---|---|---|
| Vertical | Horizontal | ||||
| 720 | 1280 | 16:9 | square | progressive | 23.976 24 25 29.97 30 50 59.94 60 |
| 1080 | 1920 | 16:9 | square | interlaced | 25 (50 fields/s) 29.97 (59.94 fields/s) 30 (60 fields/s) |
| progressive | 23.976 24 25 29.97 30 50 59.94 60 | ||||
Broadcast engineers in North America usually line up their audio gear to nominal reference level of 0 dB on a VU meter aligned to +4 dBu or -20 dBFS, in Europe equating to roughly +4 dBm or -18 dBFS. Peak signal levels must not exceed the nominal level by more than +10 dB.[10]
Broadcast audio as a rule must be as free as possible of Gaussian noise, that is to say, it must be as far from the noise floor, as is reasonably possible considering the storage or transmission medium.
Broadcast audio must have a good signal-to-noise ratio, where speech or music is a bare minimum of 16db above the noise of the recording or transmission system. For audio that has a much poorer signal-to-noise ratio (like cockpit voice recorders), sonic enhancement is recommended.
Although almost any video gear can create problems when broadcast, equipment aimed at consumers sometimes produces video signals which are not broadcast safe. Usually this is to reduce the cost of the gear, since a non-standard video signal in the home might not create the problems that one might find in a broadcast facility.Potential flaws exist with:
In nations that have fully converted to digital television, broadcast safe analogue television takes on a slightly different meaning. All broadcasting systems will have been mostly converted to digital only outputs, leaving fewer entry points for analogue television signals.
What this means is that all devices that feed to the television transmitter must take in and feed standard analogue television signals into the transmission chain. Mostly it is up to the switcher to notify if there is non-broadcast safe video to the programmer. However, due to the limitations of many switchers for DTV and HDTV it ultimately is up to the automation systems to alert the programmer of non-broadcast safe video inputs.
As a matter of broadcast engineering practice, 4:3 analogue television signals will always pose the most problems with broadcast safe compliance. The use of portable and cheap timebase-genlock systems for analogue television inputs in the digital television studio will be clearly mandatory for the next 50 years.