http://foro.speccy.org/viewtopic.php?f=6&t=3071
http://k1.dyndns.org/Vintage/Sinclair/80/Sinclair%20ZX80/Tech%20specs/Wilf%20Rigter%27s%20ZX81%20Video%20Display%20Info.htm#14
http://www.pictureviewerpro.com/hosting/zx81/fredn.htm
