Thursday, February 24, 2011

Ever need to slow down ethernet?

I've had a few occasions when I've had to force gigabit down to 100BaseT or even 100 down to 10BaseT. My preferred method is to force the NIC down to the appropriate speed but if you aren't using Windows (OS-X, Linux or an embedded device) then a hardware solution is needed.

  • Distance - 100BaseT only goes 100m over cat5e but 10BaseT goes 300m; If you find yourself in that situation then an old 10BaseT hub at the far end does the job.
  • Equipment reports 100BaseT but is only reliable at 10BaseT; my Squeezebox network MP3 player is running a hacked OS and works a lot more reliably at 10BaseT. I achieved this by swapping the green/white and orange cores in the network cable. This degrades the common-mode rejection performance of the cable and means the ethernet switch ramps the circuit down to 10BaseT.
  • Gigabit too fast? Just make off a cable with the blue and brown pairs excluded. Gigabit needs all four pairs and if the switch only sees the Green and Orange pairs it will assume 100BaseT.

Thursday, February 17, 2011

My Presentations at BVE 2011

I've been representin' at the Broadcast Video Expo show in Earls Court. I'll blog about some of the things I've seen at the show later this week but in the meantime here are the slides from my three seminars;

These are all tasters for the training days I run at root6.

Friday, February 04, 2011

Rise times in HD TriSyncs

To my shame I haven't blogged for a month! We are starting to pick up at work with projects on the go and the BVE trade show at Earls Court in just over a week (I'm presenting each day - will post my slides next week).

These are two traces from two separate TriSync generators - The blue trace represents a correct waveform and as every superhero will realise you're looking at the line timing pulse. Here is the diagram from rec-709 (the spec for HD video);

It clearly shows that rise time is to be equal between the start, middle and end of the line-sync pulse. The rep from the manufacturer of the green pulse insisted that his waveform was a lot sharper - but given the ringing on it I think they just aren't filtering it properly to comply with the rise time spec - seen here (again from rec-709) as being 4 clock cycles +/- 1.5

Thanks to an unnamed ex-colleague(!) for engaging me in this conversation! The link in the title is to the summary document PDF of rec-709.