Saturday, 28 June 2014

DSLR cameras for live TV: not so simple!



DSLR cameras for live TV: not so simple!

In the course of setting up an amateur radio DVB-T transmitter, I have sought high quality video and audio.

The modern DLSR (digital, single lens reflex) cameras seem to be a good candidate as they are capable of recording very high quality video, have a wide range of relatively inexpensive lenses and have some auto-focus capability for use by a one-person camera operator. They are generally better than consumer-grade camcorders.

DSLR cameras are very popular with cinematographers, with an ability to do Full-HD and even up to 4K resolution (4K is used by cinema projectors). See sources such as http://nofilmschool.com/ Digital Filmmaker magazine

For DVB-T the maximum resolution is Full HD 1080i at 24, 25, 30, 50 or 60 fps (frames per second), and more commonly 720p at the same frame rates. The frame rates are related to the frequency of the electricity supply in the country, 50 Hz in Britain and Australia, 60 Hz in Northern America.

However, there are three problems using DSLR cameras for live TV video:

First, the video feed from the DSLR's HDMI port is fixed at 1080i 60 fps when switched to video mode and not recording. This is a problem when a different resolution is needed. There is no firmware fix for it at the moment. The only way to change it that I am aware of is to use a HDMI to HDMI Converter or Scaler. These start at less than $100 for a Chinese one on eBay and seem to be a suitable quick-fix. There is less of a problem scaling down resolution than changing the frame rate. The frame rate is usually changed by adding or deleting frames.

Second, the video feed is not clean from the cheaper DSLRs (<$3000), having all the camera information seen in the DSLR's LCD view-finder. These artefacts can be reduced to the auto-focus frames, but not eliminated. For some DSLRs, Canon in particular, third party firmware is available to give clean video, albeit making the camera a bit more difficult to use. Magic Lantern firmware is available for all but the most recent Canon DSLR cameras that can do video. http://www.magiclantern.fm/

With Magic Lantern firmware, it is possible to use Canon DSLR bodies that cost about $300 and up. I am using a Canon M series (which is technically not a DSLR, but uses the DSLR firmware), with Magic Lantern beta firmware to get a clean live feed. I have a Canon 70D that has a very effective auto focus system, but the Magic Lantern firmware is still being developed for it.

Third, a lesser problem is that the length of HDMI cables is limited to about 5m or less. There are HDMI repeaters available, but I have not tried using them for a live video feed and have had limited joy even running long (>10m) HDMI leads to projectors.

Most DSLR camera can record/feed audio as well as video. It is best to use an external microphone, either a shotgun type mounted on the camera or a microphone on a cable near the sound source.

An annoyance of using the HDMI output live, is that the operator cannot see the image. This is a problem with cinema cameras as well, but small focusing monitors are available that have a HDMI pass-through.

DSLR cameras have somewhat morphed into low-cost, high quality cine cameras, such as the BlackMagic Design's Pocket Cinema Camera. A small cinema camera that can use DSLR/SLR lenses, uses SDI output and costs about $1200.

In summary, modern DSLR cameras can be a quality video and audio feed for live TV via HDMI, but have some problems which can be overcome to some extent.


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