Thursday, July 22, 2010

Back from a job - a few thoughts on DVI, HDMI & audio

Having just got back from a slightly unusual build (all HDMI capture off games machines for a 24-7 HD channel on Sky) I had a few observations that might save you some head-scratching;
  • DVI extenders; I normally recommend extending DVI and HDMI over fibre; twisted pair copper isn't really up to it. It works most of the time so long as patch cords and wallboxes are avoided and you can't realistically do that in edit suites. Anyhow - on this job we discovered we needed to extend and extra DVI into a couple of the edit rooms after cables were run and we'd used up all our fibres on the edit machine's GUI displays. So - with a spare cat6 we gave it a try. The model we used had an eq tweak on the receiver with an LED that lit when the signal was tuned. Only it didn't! Turned out the lit LED meant 'sub-optimal'! Hmm - OK when I discovered!
  • HDMI vs DVI and interlaced 1080 video - the Samsung domestic panels that we were using for the time-line display wouldn't work with 1080i video over DVI, only 1080P (at whatever fancy framerate you wanted - 23.976P, 24P etc). Turns out that model only does DVI progressive. Changes the cable from the extender to feed the monitor's HDMI port and it's all good.
  • AJA k-box's unbalanced audio outputs aren't buffered. We had a set of PPMs that were loading the signal earths (but not enough to effect the accuracy of the meters) but it was enough to load the unbalanced outputs which fed the speakers - cut the screen on the XLRs behind the PPMs and it's all good.
On the subject of HDMI and DVI it should be stressed that from a signal point of view the TDMS data lanes are the same in each - you can use DVI extenders to carry HDCP and audio data which aren't part of the spec, it's only the equipment at each end that generates/won't recognise those things.

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