Digital Video - July 2008 - (Page 14) DV UPDATE LETTERS HONORING GEORGE DIBIE I very much enjoyed the too short piece on George Spiro Dibie [ASC] in the March issue of DV. I knew I had something written by him in my clip file. Sure enough I have notes based on a lighting technique he used on Barney Miller: the “X Theory” or “Dibie Square” method. Still works for me. Also, I’m very much enjoying Stefan Sargent's column. I think that DV consistently has the best material — lots of hands-on/user pieces. Too many other magazines just have too many recycled press releases. Kevin Sio Via E-mail COMPRESSION ISSUE I just read through the June issue of DV magazine and I noticed a questionable statement. It was contained in a sidebar “3 Things” on page 10, written by Justin Stanley, which quoted some information from Jim Martin at Birns & Sawyer. The statement was in the second point, called “Tape or Memory Cards?”. In the paragraph, the statement was, “Memory Cards do not have as much compression as tape…”. I take issue with this statement. Whatever the method used to record digital data, such as compressed video and audio, the media does not determine the compression ratio or the bitrate of the stream. This is determined in the camera’s processing circuits and then the resulting stream is dumped onto tape, or written into flash memory. If 25Mbit DVCPro is the codec being used, I believe the resulting data stream is exactly the same if recorded to tape or written to flash memory. The bitstream is the same either way, as far as I know. If there is something I am missing here, please fill me in. Thanks for the great magazine. I read it every month. Ed Fraticelli VP Technology Production Masters, Inc. (PMI) Via E-mail DV Technical Editor Jay Holben responds: In one respect, you are absolutely correct, it is the recording format that determines the compression. If the camera is recording in DVCPro then it will be the same compression regardless of the recording medium. What Justin is referring to is a lack of data rate ceiling in digital media as opposed to tape. Tape has always been a limiting factor in record formats and has dictated the compression formats most commonly used. For instance, MiniDV tape is not capable of more than 35-40Mb/s. To achieve a higher bitrate, you have to move to a larger or faster moving tape. DV is 25Mb/s and runs at 18.81mm/s tape speed. On the same tape, to achieve DVCPro 50 (50Mb/s) is to more than 14 dv july 2008 there is with tape. Digital media doesn't care what bitrate is being fed to it — as long as the pipeline is good enough to accept it. This is why cameras like the HVX200 record 25Mb/s DV to tape, but 100Mb/s DVCPro HD to P2 card from the same camera. It is not possible to record DVCPro HD to a miniDV tape. So, although you are correct that the record format, not the media, dictates the compression, it is the media that dictates the maximum allowable record format and digital media has no discernable ceiling as to record formats. HARD-DRIVE ACHILLES? I just bought the Canon HG10 and found it to be a fabulous camera, but with one major drawback. As it is a tapeless camera with a hard drive, it cannot be used in altitudes above 9,800ft. Use above that height will cause hard drive damage. I called Canon about this, and they informed me that this was correct. I’ve mostly used higher-end tape cameras when working on productions, but thought the HG10 might be convenient when doing some of my documentary work, especially in sensitive places, but the elevation restriction is a problem when shooting in [places like] the Rockies or overseas in remote locations, as I don't generally carry an altimeter with me. I thought this should be something that is put up on a blog or you guys put out as a warning, as [DV is] the front line of great reporting for prosumer and professional video equipment. Howard Donnell Via E-Mail The “Dibie Square” method is an excellent basic lighting technique and easily Googled. double the speed of the tape at 67.7mm/s — it's the only way the tape can handle the increased data rate. This means you're getting nearly 1⁄4 the record time of a normal tape or you need a tape twice as large. DVCPro HD (100Mb/s) is running at 135.4mm/s — double, again, the speed of DVCPro 50. Tape has a limitation as to the data rate it can record. This was also the issue with HDCAM, which is a 144Mb/s format, but due to the limitations of the physical tape, was only able to record a 3:1:1 color space even though the first major HDCAM camera — the Sony F900 — was capable of 4:2:2, tape was, and always has been, the limiting factor. This is also why cameras like the Dalsa Origin do not (nor cannot) have a tape deck to record 4:4:4 4K images. That is only possible to a hard drive. Where Justin is referring to digital media having no compression is the fact that there is no ceiling (except for file size capacity) of digital media like This issue regarding hard-drive-based cameras has been known for some time, but it’s a good tip to pass on. Note that this is an air-pressure-related problem, and does not apply to pressurized aircraft cabins. www.dv.com http://www.dv.com
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