mike@adm.BRL.MIL (Michael John Muuss ) (11/21/89)
There seems to be quite a lot of speculation on television here lately. NTSC is quite old, and well documented. From pages 4.60-4.61 of ``Television Engineering Handbook'', K. Blair Benson, ed.: "The color picture signal shall correspond to a luminance component and a simultaneous pair of chrominance components transmitted as AM sidebands of a pair of supressed subcarriers in quadrature having the common frequency of +3.579545 MHz." The line rate is 15,735.264 video lines/sec (1 line per 64 uS), the field rate is 59.94 video fields/sec. Of the 525 lines transmitted, 486 carry active picture; the remainder are used for vertical sync (8.5 lines/field) and blanking (16 lines/field). Vertical resolution is 340 lines, and horizontal resolution is 330 lines (660 pixels). Note that V and H resolution are measured differently. The composite video signal is treated as a luminance signal (Y), and two color difference signals Red-Y (R-Y, or U) and Green-Y (G-Y, or V). "To produce a composite color television signal meeting the standards of the FCC, the video frequency chrominance signals should be limited in bandwidth according to the following specifications: The bandwidth of the channel handling the E'[I] video chrominance signal shall have an attenuation of LESS than 2 dB at 1.3 MHz and an attenuation of MORE than 20 dB at 3.6 MHz. The bandwidth of the channel handling the E'[Q] video frequency-chrominance signal shall have a bandwidth with less than 2 dB attenuation at 400 kHz, ..., and at least 6 dB attenuation at 600 kHz." "The standards specified by the FCC are that the color subcarrier frequency shall be 3.579545 MHz +/- 10 Hz, and the maximum rate of change of chrominance-subcarrier frequency shall not exceed 0.1 Hz/sec." The effective bandwidth for Q is about 550 kHz; for I, about 1.5 Mhz. For transmitting NTSC video in digital form, CCIR standard 601 is used; this is also known as "4:2:2" digitial video encoding. It may be transmitted between devices in D-1 or D-2 form. For CCIR 601 digital video, scanlines are considered to have 720 pixels; a full frame (2 fields) has 486 visible scanlines. According to the ``A60 Digitial Video Interface Manual'', Abekas Video Systems, the luminance (Y) channel is sampled at 13.5 MHz and the two color difference channels (U, V) are sampled at 6.75 MHz. Each sample is 8 bits. For each two samples in Y there is only one each in U and V, hence the name 4:2:2. The color difference signals U and V are normalized to the range -0.5 to +0.5 by these equations: Cb = 0.564 * (B-Y) Cr = 0.713 * (R-Y) The samples are then sent as a byte stream: Cb Y Cr Y ... two luminance (Y) samples for each color sample (Cb, Cr). This results in a video signal with luminance bandwidth of 5.75 MHz, and chrominance bandwidth of 2.5 MHz, which is significantly better than the bandwidth the FCC allocates for transmission (see above). However, this (720*486) is a useful resolution to prepare images in. It is quite a pitty that CCIR 601 digital VTR devices are still $100k and higher. Best -Mike -- Mike Muuss