James, EXCELLENT REPLY!
Do you have a URL to download the most recent HDMI spec? Thank you for correcting me on interpreting less than, instead of more than. Even after you told me, it still took 5 more reads before I saw that indeed that should read MORE THAN 75ps. Yes, I believe lousy cables will cause problems. That was kind of my question: GIVEN a lousy cable, which transmit/receive technique is quietest? Regards, Robert > Hi Robert, > > The HDMI spec (section 4.2.4) details the electrical characteristics of > the TMDS lines, including rise and fall times. I think your > interpretation of the spec isn't strictly correct in that "rise time / > fall time" actually means "rise time OR fall time". If it were the case > that rise and fall times were meant to be different then they would be on > different lines of the table. Also, the risetime must be greater than > 75ps, not less than. > > However the spec is different between HDMI 1.3 and HDMI 1.4 > > HDMI 1.3 75ps < Tr/Tf < 0.4 Tbit > HDMI 1.4 75ps < Tr/Tf > > Presumably they've removed the 0.4 Tbit because if your rise time starts > to infringe on this parameter then you'll be in danger of closing the eye > and infringing the central mask of the eye diagram (instant fail, no > conditional pass here). Also, it might not have been compatible with the > higher clock frequencies and data rates called up in HDMI 1.4 > (speculation). > > You are right in that it doesn't specify matching of rise and fall between > the pairs. However all the measurements I've made on HDMI sources show > good matching between the rise and fall times for each pair. The worst > deviation I've measured was about ±5ps on our products. > > RM >> "Wouldn't steering current through either conductor have more > potential for EMI generation from physical reality limitations, than if > the same cabling were to be driven using LVDS with matched impedances?" > > You aren't strictly "steering the current" as each line in a TMDS pair has > it's own termination pullup to the 3V3 termination supply. From what I've > seen from my measurements on some sources, HDMI drivers can take a small > amount of current (1 to 2mA) in the "high" state, presumably to control > the rise time and any ringing. > > In my experience with HDMI and EMC, you will have significantly more > issues with cheap cables than from any other issue. The area from > transition of cable to connector is often poorly shielded with long > "pigtail" ground connections. Measurements with near field probes clearly > show this to be a major problem area for emissions. Also, because the data > frequency is 5 x the clock frequency you get the harmonics ganging up on > you! For a 74MHz TMDS clock you'll likely see 370MHz and 740MHz harmonics. > > FYI the convention for HDMI is to refer to the "source" (transmitter) and > the "sink" (the load e.g. a TV) > > I hope this helps, > Best regards, > James > > James Pawson > Leading Hardware Engineer > EchoStar Europe > T: +44 (0)1535 659000 > e: [email protected] > > > > -----Original Message----- > From: Robert Macy [mailto:[email protected]] > Sent: 18 November 2009 23:30 > To: [email protected] > Subject: [PSES] HDMI Transmitter: EMI Tutorial > > EMC Group Members: > > The following questions came to mind after looking at the HDMI electrical > interface specifications. Especially, questions regarding the potential of > EMI generation for the technique that is specified in HDMI 1.2, assuming > 1.3 is similar. > > The spec looked like it could cause problems, because instead of matching > rise and fall times, they have tr and tf specified VERY unbalanced, at > least potentially, ie, risetime < 75ps and fall time < 0.4Tbit [related to > bit time] Doesn't that yield a huge spike of common mode at each > transition? > > It was my understanding from reading the spec that the transmitter > consists of a pair of current switches to shield ground, where either is > on, much like driving the cable with ECL Logic, except in the HDMI > transmitter the impedance was kept as high as possible, approaching > infinite. I realize using current switches may give you 6 dB larger > signal over LVDS for the same power supply, but...here's the big question: > Wouldn't steering current through either conductor have more potential > for EMI generation from physical reality limitations, than if the same > cabling were to be driven using LVDS with matched impedances? And one can > only drive over shorter distances using current switches? Plus, there has > to be ringing, after all, you are only load terminating the line and never > source terminating. Shouldn't long lines should be be terminated at both > ends? > > Also, consider a standard 100 ohm differential shielded cable: It is my > understanding that most 100 ohm 'differential' cables barely make it to 75 > ohm and 75 ohm to shield and 300 ohms between the two conductors. I > believe some of the Belden models for balanced twisted pair show 61 ohm, > 61 ohm and only 560 ohm between. Is it possible to even get better > 'forgiveness' to the balance and have 100-100 and 200 between them? > > Now consider an unbalanced cable caused from manufacturing tolerances, any > 'unbalance' in the cable will exacerbate EMI generation, because the > shield current is subtracting large numbers to get zero. In other words, > driving a slightly off balance pair of conductors, CAUSES EMI when driven > with LVDS. But, that effect must be much less when driving with switched > currents, only the voltage levels become unbalanced? > > Any papers covering this in detail out there? > > What happens when you go through a connector where the structure usually > becomes 50 ohm to shield, 50 ohm to shield, and zero between during this > transition? > > Has anyone compared the two driving techniques, current switching to LVDS? > > Has anyone compared the maximum distance obtainable for the two > techniques? > > Regards, > Robert > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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