poynton@vector.Sun.COM (Charles A. Poynton) (01/11/90)
TN30-Calculator Shortcuts for Television Engineers Charles A. Poynton Sun Microsystems, Inc. 90/01/10 Many different approximations to the values of various television parameters are in common use. However, the fact that television parameter values are based on ratios of small integers can be used to quickly compute exact parameter values, as an alternative to entering multi-digit approximations. Each line of the table on the following page gives the name of a parameter, a ratio of integers giving its precise value, the unit of the value, and its numerical value rounded to ten digits (unless the exact value is already expressed numerically). This table contains expressions for time periods (e.g. frame, field, line, subcarrier, and sample): invert the numerator and denominator of any ratio which defines a time period (duration) to obtain an exact expression for the corresponding frequency. The values derived are in scaled units (e.g. nanoseconds). The power-of- ten scaling required to obtain a value expressed in its basic unit (e.g. seconds) is implied by the metric multiplier of its unit. For example, to convert the ratio for NTSC subcarrier period in nanoseconds (88/.315) to seconds, multiply by 10^-9. Even though all but two of the PAL values are simple integers, all values have been included to make the table useful even to neophytes. The number of subcarrier cycles per line in PAL (283.7516) is exact; this value is the sum of 1135/4 (exactly 283.75) which accomplishes the 90 degree line-to-line subcarrier phase shift necessary for PAL, and 1/625 (exactly 0.0016) which accomplishes the much-discussed (but not much celebrated) 25 Hz offset. Now that you have read this far, I can reveal that the true purpose of this document is not to reduce computation time but to increase accuracy. Television engineering has suffered on many occasions from inaccurate calculation (case studies are in Technical Note 19), and it is actually that problem which I hope this Technical Note addresses, through the back door. CCIR 601 Parameters CCIR Y sampling period 1/.0135 74.074 074 07 ns CCIR 8-bit Y quanta 3x73 219 levels CCIR 8-bit U, V quanta 7x32 224 levels 525/59.94 Parameters Sync amplitude 2/7 0.285 714 285 7 V Black-to-white excursion 37/56 0.660 714 285 7 V Pedestal amplitude 3/56 0.053 571 428 57 V Blanking-to-white excursion 5/7 0.714 285 714 3 V 1 IEEE unit 1/.14 7.142 857 143 mV Frame period 1001/30 33.366 666 67 ms Field period 1001/60 16.683 333 33 ms Line period 572/9 63.555 555 56 us 625/50 Parameters Sync amplitude 300 mV Black-to-white, blanking-to-white excursion 700 mV 1 IEEE unit 7 mV Frame period 40 ms Field period 20 ms Line period 64 us 525/59.94 NTSC Parameters Subcarrier cycles per line 455/2 227.5 cycles Subcarrier period 88/.315 279.365 079 4 ns Sample period (at 4xFsc) 22/.315 69.841 269 84 ns 625/50 PAL Parameters Subcarrier cycles per line 1135/4+1/625 283.7516 cycles Subcarrier period 64/.2837516 225.549 389 0 ns