gclark@utcsri.UUCP (Graeme Clark) (02/10/87)
I'm curious about the claims of a particular LAN design (FDDI) which
talk about sending video data and digital data over the same baseband
channel. Can someone tell what bit rate would be required to encode
a standard colour TV brodcast channel? How about monochrome?
I can make order-of-magnitude estimates, but I'm looking for slightly
more precise values.
Thanks.
Graeme Clark -- Dept. of Computer Science, Univ. of Toronto, Canada M5S 1A4
{allegra,cornell,decvax,ihnp4,linus,utzoo}!utcsri!gclarkcmcmanis@sun.UUCP (02/10/87)
In article <4095@utcsri.UUCP>, gclark@utcsri.UUCP (Graeme Clark) writes: > I'm curious about the claims of a particular LAN design (FDDI) which > talk about sending video data and digital data over the same baseband > channel. Can someone tell what bit rate would be required to encode > a standard colour TV brodcast channel? How about monochrome? > Thanks. > Graeme Clark -- Dept. of Computer Science, Univ. of Toronto, Canada M5S 1A4 According to a recent talk I attended FDDI is designed for a baud rate of 125Mhz and a bit rate of 100Mhz (For those who don't know the differences between bauds and bits, you should expect 100 million bits to come out of the wire every second) The 'dot' clock on a video interface that produces NTSC video (60Hz, 262.5 line frames interlaced, 640 dot horizontal resolution) are on the order of about 10Mhz. Multiply that by some factor to color (pixels being the multibit critters that they are) and you can get a raw bit rate. Simplistically you would also be transmitting the black around the edges too. However there are a number of schemes that reduce this considerably. So for a ball park I would guess it would take an aggregate throughput of about 40Mbits/sec to get a "standard" colour picture out the other end. That's definitly moving right along. 'Course it would be great fun to mount HBO from a server on ones desktop :-) -- --Chuck McManis uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com These opinions are my own and no one elses, but you knew that didn't you.
dsi@unccvax.UUCP (02/10/87)
In article <4095@utcsri.UUCP>, gclark@utcsri.UUCP (Graeme Clark) writes: > > I'm curious about the claims of a particular LAN design (FDDI) which > talk about sending video data and digital data over the same baseband > channel. Can someone tell what bit rate would be required to encode > a standard colour TV brodcast channel? How about monochrome? For NTSC colour video, a sampling rate (minimum) of 3 * Fsc (the colour subcarrier frequency) is required minimum to prevent chrominance cross-contamination. The minimum accepted number of bits for broadcast quality composite video is 8 bits. Hence, the data rate is 85.9091 megabits per second. Monochrome video still requires a sampling clock which is a harmonic of the horizontal scanning frequency (to prevent disturbances in straight vertical lines, etc) and typically, a rate of 8.4 mHz can be used with good results. Of course, if one can tolerate reduced bandwidth video, even 2.5 mHz is acceptable, with a sampling rate of 5 mHz. There are encoding schemes which reduce the number of bits required for real time NTSC digital video. See this month's SMPTE Journal (JAN 87) for one such scheme. David Anthony DataSpan, Inc. P.S. I might add that if one is willing to insert synchronizing words into the data stream rather than encode the entire synchronizing pulse with data at the same rate as the video, then a reduction in data rate of a little more than 1/6 can be obtained....