ls. methods of meeting all these targets are considered technically Other impairments in digital video may be due to aliasing, clock feasible, although it is recognised that 'compatibility' must be a jitter, error rates and the impossibility of transmitting perfect pulses. relative term since reception of PAL plus in either 'letter box' or Digital video signals may also call for added complexity for moni- 'side panel' formats would involve some loss to viewers with toring and measuring the impairments, and some still uncertain cost standard PAL receivers. Cross effects could be removed or mini- factors in equipment and maintenance. mised by degrees of band segregation or phase segregation; in- The advantages of digital transmission include freedom from the creased resolution by the use of spectral folding. System ill effects of differential phase and differential gain and the ability specification (based on letter-box format) was agreed in late 1991. to regenerate an exact replica of the input data stream at any point in the chain, thus avoiding cumulative signal to noise degradation. 57.2.5 Digital and component systems It was Shannon's communication theory that first underlined math- ematically the outstanding efficiency of digitally encoded trans- Since the early 1970s, digital techniques in video/audio processing mission systems. and transmission have been under intensive investigation and im- However it is important to realise that digits do not eliminate all plementation. This work has led to the first worldwide production problems. From the earliest days of manual and machine cable standard Recommendation 601 of 1982), the implementation telegraphy it has been recognised that the transmission of high speed of a few experimental and operational all digital studios, the wide- pulses within a channel of restricted bandwidth can present severe spread use of 'stand alone' digital studio equipment for special practical problems, including inter symbol interference and suscep- effects, noise reduction, standards 