30-09-2021, 07:06 AM
A quick look at possible routes from camera to screen.
The image is originated as an array of pixels and lines by a camera or graphics generator. The data rate at this point is very high, far too high for transmission.
The images are stored either at full data rate, or very lightly compressed.
The data rate must then be reduced to something suitable for transmission. Within each frame this is usually done by block based systems, similar to those used for JPEG. There is a tradeoff between quality and data rate. For moving pictures you need only transmit changes between frames. That's what MPEG does. There are clever and complex methods that work very well indeed.
You then need to package up this data for transmission. The coding method is chosen to suit the nature of the channel. For terrestrial TV (Freeview) this is done by a coding system called COFDM. The details are not important here. Suffice to say that it's resistant to ghosting and other propagation problems. One weakness is that it doesn't work well with fast moving receivers, but that isn't usually a problem. Satellite doesn't have the same propagation problems and so uses a simpler coding system. Broadband internet uses COFDM to send data reliably over the relatively poor quality channel of a copper pair. That data can be anything from emails to full HD video.
The receiver reverses all of these steps.
One inherent problem is that the processing time rises as compression gets better. This isn't just a matter of faster chips. If you're looking over a group of frames to establish what's moving and what's not, the delay is inherently that long. For normal broadcast this means that the viewer sees things a little later, usually of no importance. Where it matters is on contribution circuits. We've all seen interviews done over a connection with excessive delay. There are odd sounding pauses.
The image is originated as an array of pixels and lines by a camera or graphics generator. The data rate at this point is very high, far too high for transmission.
The images are stored either at full data rate, or very lightly compressed.
The data rate must then be reduced to something suitable for transmission. Within each frame this is usually done by block based systems, similar to those used for JPEG. There is a tradeoff between quality and data rate. For moving pictures you need only transmit changes between frames. That's what MPEG does. There are clever and complex methods that work very well indeed.
You then need to package up this data for transmission. The coding method is chosen to suit the nature of the channel. For terrestrial TV (Freeview) this is done by a coding system called COFDM. The details are not important here. Suffice to say that it's resistant to ghosting and other propagation problems. One weakness is that it doesn't work well with fast moving receivers, but that isn't usually a problem. Satellite doesn't have the same propagation problems and so uses a simpler coding system. Broadband internet uses COFDM to send data reliably over the relatively poor quality channel of a copper pair. That data can be anything from emails to full HD video.
The receiver reverses all of these steps.
One inherent problem is that the processing time rises as compression gets better. This isn't just a matter of faster chips. If you're looking over a group of frames to establish what's moving and what's not, the delay is inherently that long. For normal broadcast this means that the viewer sees things a little later, usually of no importance. Where it matters is on contribution circuits. We've all seen interviews done over a connection with excessive delay. There are odd sounding pauses.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
