Digital Ballasts use the latest technology to be far more efficient and quieter than older analogue ballasts, whilst being more efficient and softer on the bulb. Instead of the old transformers and capacitors, these new digital ballasts use a micro processor like you would find in a home computer.
Digital ballasts use a soft start process to which helps to lengthen the bulb life, by supplying low power to the bulb before bringing it to full power.
The Maxibright Digilight from Teignbridge Horticulture is the latest technology for digital ballasts allowing the powering of 600w 400v glasshouse lamps from a 240v supply. This outputs gives you 10% more PAR per watt than a regular HPS bulb.