[Orwella]

My wife was complaining about how much space our AVCHD footage takes up on our hard disk (currently around 180GB), and why I have so many backup DVD's.

That got me thinking why HD Camcorders actually need such high bit-rates. The two camcorders I use regularly (Sony SR12 and Canon HF100) record at 16Mb/s and 17Mb/s, so each hour of "taping" requires approx 8GB of disk space.

Now 17Mb/s seems a lot but when broken down into 25FPS (PAL progressive) or 50FPS (PAL Interlaced) it is actually very low.

17,825,792 / 25 = 713,032 bits (87KB) per frame

So, each 1920x1080 frame requires 713,082 bits (0's and 1's) to reproduce an image containing 24bit colour, detail and sound. That is smart compression considering a frame consists of 2,073,600 pixels.

Each bit accounts for 3 pixels displayed on screen. Now that is pretty damned amazing when I can make out individual pixel detail.

No wonder you need a beefy PC to re encode the information contained within the complex algorithms AVCHD employs.

[A n d r e w]

Don't forget audio. That said, AVC/H.264 uses a 4:2:0 colour space so each pixel is not sampled independently for colour (whereas it is for luminance) - that lowers the headroom considerably.

Andrew.