Wednesday, June 22, 2016

USB current draw by HID devices and Amulet Zero Clients

One of the complaints I often get from Amulet users is that they get strange results when using power-hungry USB devices. The prime culprits are Wacom tablets (particularly the Intuos and Bambo series) but today I went to a facility where they were getting intermitent behaviour from the left hand control and shift keys which in Media Composer is a real pain. 
The most common third-party Avid coloured-keycap keyboards are from Logic
They come with a USB hub built in and so you'd expect them to draw a bit more than an HID device, but it's worth looking up what the spec is for USB power draw. This is stolen from Wikipedia;

There are limits on the power a device may draw, stated in terms of a unit load, which is 100 mA, or 150 mA for SuperSpeed devices. There are low-power and high-power devices. Low-power devices may draw at most 1 unit load, and all devices must act as low-power devices when, starting out as, unconfigured. High-power devices draw at least 1 unit load and at most 5 unit loads (500 mA), or 6 unit loads (900 mA) for SuperSpeed devices. A high-powered device must be configured, and may only draw as much power as specified in its configuration.
Or, in summary;

However, flipping the keyboard over gives a different story;

Wow, 1.1A - more than twice what the spec says. However - these keyboards are used the world over and it seems that motherboard USB ports are more than able to power them. I thought I'd get my own reading to see what the current draw was so I stuck one of those USB volts/current monitors in line. The keyboard continued to work but the USB-meter gave me a 0mA reading; it was working the day before, but today, no dice. When I got it back to the workshop I opened it up and discovered the current-shunt resistors was blown away!

 see bottom left on the +Vcc rail - where's the resistor?!

So I conclude that the keyboard is actually pulling a lot more than 1.1A (at least at in-rush).

So, Amulet can supply 2.0A across the USB hub on it's Zero Client with a maximum of an amp on any one port. All much better than the spec but somewhat less than the Logic keyboard is pulling. In the case of power-hungry, non-compliant keyboards and tablets I stick in a powered hub and that normally fixes the problem.

Thursday, June 09, 2016

Stuck pixels on LCD & OLED monitors

Because they're semiconductors (and pretty tiny ones at that!) the pixels on modern HD monitors can become "stuck" such that you get a dot that is either black or white (in the case of all three sub-pixels; R, G, and B being stuck on or off) OR a primary (or secondary) colour where less than three of the sub-pixels have stuck. It usuall looks something like this;

see how the blemish aligns perfectly with the pixel raster

You have to get your face about an inch away from the screen to see a single stuck pixel on a 1920x1080 25" display, not exactly edit/grading viewing distance!
A piece of software that has helped me in the past is JScreenFix which is a little Java app that allows you to hook up your laptop to the monitor - thankfully all current model broadcast displays have HDMI but you might have to make arrangements (DVI to SDi converter, for example). 

Here's the interface as I used it today to mark three dead pixels on a broadcast OLED;

This is the random pattern (video noise) that it fires at the hundred or so pixels around the area of interest. The idea is that you leave it running for a while and hopefully it will provoke the thin-film-transistor (in the case of an LCD) and the diode (in the case of an OLED) to recover it's ability to start switching again.

In the case of the monitor I had a go at today I was able to clear down two of the three stuck pixels, but the final one that remained resistant to repair was interesting; a photo shows it to be probably some contaminant in the panel rather than dead pixels or sub-pixels;

the blemish seems to be in the inter-pixel space?

Tuesday, June 07, 2016

Colourimetry, Rec.709 and HDR video; a presentation.

A couple of weeks ago I was up in Glasgow doing a presentation at Root6 Scotland's PlatformOne event. Gerry shot some video so here it is (in case it's useful). 

The PDF of the notes is here, the URL of the YouTube clips is

Wednesday, June 01, 2016

The damage 100m of good-quality coax does to a 1080i signal.

I've posted measurements of the degredation that cables does to video signals before; see here, but I was grabbing a couple of screen caps for an industry colleague and here they are.