In almost every respect, digital broadcasting is lightyears ahead of the older analog system.
The analog system relied upon a carrier wave frequency onto which the television pictures were modulated. In Australia we use the PAL system. Other parts of the world use different standards - most notably NTSC in the US and SECAM in France - although digital broadcasting will do away with these eventually.
At any given time there are only about 300 lines of picture being displayed on screen. The other approximately 300 alternate in a method known as interlacing. Here you brain leaves the image lingering in your mind while the other half of the image is displayed. Unfortunately the bigger you make the picture, the more the picture quality shows it's inherent low resolution. Analog signals are also at the mercy of the weather and as the signal quality deteriorates, effects of noise (such as snow) appear on screen, degrading the total image (but still often watchable) The picture quality depends upon the signal quality and strength.
With digital broadcasting there are different philosophies at work. A digital broadcast is actually a data stream of 0's and 1's that is recorded very much like your digital camera.
The transmission works on a carrier frequency that is modulated on and off (high and low) to simulate 0's and 1's. Because quality and perfection of wave shape is not critical - only the existence r not of a signal, digital is far less susceptible to interference than analog.
The entire screen is arranged into small groups of pixels or blocks, and the information about the colour and detail in that block is only transmitted if and when the contents of that block change. This saves transmitting a lot of information that is essentially the same as an instant ago. In addition to this there are checksums sent with the bundles of data. These are the totals if you added all the data in the bundle together. The digital Set Top Box (STD) compares the broadcast checksum with the checksum it comes up with. If they are the same, then that block of pixels is perfect. This is a nice form of error trapping. If the checksums aren't the same, the STB knows there has been a reception error and will not change the block of pixels - leaving it the same as it was last instant.
So being less susceptible to interference, and having built in checks on image quality means that digital broadcasts are 99% of the time 100% perfect, the other 1% means the signal has be so interfered with, or there is so much strength loss that the signal is unusable. In which case it appears to freeze until the signal returns to within normal parameters.
Analog gradually fades away into snow, digital stays perfect until there is nothing - this is known as the digital cliff. There won't be any option after about 2008 when they turn off analog in Australia.
In Australia we have Standard Definition of 576 lines of interlaced signal.
High definition is defined to be anything higher than this. It includes 576 lines of Progressive signal, 720 lines progressive and 1080 interlaced. These are known by their shorthand of 576p,720p,1080i.
Not all broadcasters offer a continuously High definition service, additionally when they do broadcast HD signals they are not broadcast in all levels of HD. So while HD is definitely a better picture quality, it's not a complete solution. Thankfully most of the best HD STB's offer upscaling of SD signals to HD output.
These emergent technologies leverage the enormous capacities of modern storage devices, as well as featuring their ability to read and write simultaneously at many points on the drive.
They will in the medium term change the way we use our home entertainment systems. Once we stop cooking our meals to be ready in time for a TV show will be gone. The HD recorder will be able to record everything you would want to watch on TV allowing you to watch it at your leisure. This is the precursor of the video on demand systems that will undoubtedly come over the next few years.
Hard drive recorders are ideal for programs that are regularly scheduled such as sitcoms or documentaries. These can later be compiled onto the DVD and backed up, although most recordings will tend to be viewed just once. The size of the hard drive determines the amount of recording time. There is no consumable when writing to a HDD, you can do so as often as you like.