The bottom line
I just read your April 2005 article about active lines. In the NTSC standard, there are 485 lines. But other articles have talked about 480 lines. Which is right?
Michael Robin responds:
The SMPTE 170M standard, Composite Analog Video Signal — NTSC for Studio Applications, describes the composite analog color video signal as NTSC, 525 lines/picture, 59.94 fields/sec, 2:1 interlace with an aspect ratio of 4:3.
The standard assumes that the original picture is scanned horizontally and vertically in the camera and the synchronized reproduction device. It specifies the duration of the field blanking and the line blanking required to normalize the display characteristics in terms of vertical and horizontal resolution.
The specified vertical blanking duration for the odd fields is 20 lines, plus 1.5ms (or a total of 1272.62ms). For the even fields, it is 20 lines (or 1271.12ms).
The vertical blanking duration for the complete picture is 40 lines, leaving a nominal number of active scanning lines. So much for the heritage analog composite video world.
MPEG-2 4:2:2 starts with component digital video signals as per ITU-R BT601. ITU-R BT601 features a luminance sampling grid of 720 horizontal active samples by 485 lines. The digital field 1 has 262 lines, and the digital field 2 has 263 lines to avoid half-lines. The number of active lines is 485, just as in analog NTSC.
One of the basic functions of the MPEG-2 compressor is to divide the picture into blocks and macro-blocks. The luminance information is transformed into 16 pixels by 16-line macroblocks. It is essential to have an integer number of macroblocks per active picture width, as well as per active picture height. Given 720 active horizontal samples, the picture can accommodate 720:16 = 45 macroblocks per active picture width. To obtain an integer number of macroblocks per active picture height, the number of active video lines is reduced from 485 to 480, which can accommodate 480:16 = 30 macroblocks per active picture height.
It is, therefore, in the MPEG-2 compressor that the number of active lines is reduced to 480 from the original 485. Evidently, an NTSC composite analog signal derived from an MPEG-2 compressed video signal will only have 480 active lines and will thus be slightly different from the SMPTE 170 standard.
PAL color bars
Why are 75-percent color bars used in the PAL system?
Michael Robin responds:
Seventy-five percent color bars are used in cases where some element in the television distribution chain, for instance NTSC or PAL transmitters, cannot handle 100 percent color bars.
The difficulty in handling 100 percent color bars is due to the frequency division multiplexing of NTSC and PAL chrominance information with the luminance information, resulting in excessive video signal amplitude and transmitter overload.
Because normal camera-generated video signals are unlikely to reach chrominance signal levels equal to those of 100 percent color bar signals, under normal operating conditions, the transmitter will not be overloaded.
Early VTRs also had difficulties in handling 100 percent color bars. Current digital equipment and systems don't have this problem, so A/D and D/A converters are aligned using 100 percent color bar signals.
Problems may arise when synthetically generated video signals (e.g., from character generators) with excessive analog video signal amplitudes reach an analog NTSC or PAL transmitter. These problems will disappear with the imminent demise of analog television.
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