As DCT winds down its useful life at the start of the HDTV era, here's an actual take from an actual user from the site Hal and Flick referenced.
pacificnet.net
Picture Degradation in 601 Video Tape Recorders (my opinion...or is it fact?...)
Error Correction is a process where corrupt digital data is exactly "repaired" using redundant and error correction data, and sophisticated algorithms. This means that the missing data (pixel) is perfectly replaced with it's original value. Concealment is a process whereby pixels from spatially adjacent pixels are used to approximate what the true value of the corrupt pixel would have been. The original data cannot be recovered, and the original value of the pixel is guestimated.
Ampex DCT has Zero allowance for concealment; the machine attempts to always error correct any missing pixel data (it's spec allows one concealment per hour or less). -If it can't perfectly error correct, a warning is always given to the user. With D1, warning is only given when concealments occur excessively, because a even a mostly error corrected tape cannot be obtained. The operator is given the option of being warned when the error correction rate is excessive, but even the bottom of this adjustable threshold is well above what the error correction engine needs to accomplish to prevent concealments.
Because of this, D1 and the other poorer error correction machines actually yield more video distortions than the compression algorithm used in DCT machines.
DCT uses the mildest compression of all Standard Definition VTRs that utilize compression. In fact, it is a ratio that is frequently
referred to as visually "loss less" because it is so low. But considering the concealment rate tolerance of all other tape formats,
DCT actually has the lowest mathematical distortion of the video signal as well. (This can be proven or disproved with an add-invert of an original 8 bit source signal (such as D1) with the signal recorded to the DCT. Note that 10 bit signals would show the difference of the last 2 bits in the form of "noise" that these data bits represent).
Average Error Correction Performance
DCT = 10^11 (1 concealment per hour at worst)
Digital Beta & D5 = 10^8 (average 1 concealment per minute)
D1 = 10^4 (1 concealment per second at least)
If the same number of multi-generation recordings of any full length feature film are made to DCT, D1, D5, and Digital Betacam, the actual video distortions will become apparent. Ampex did this in ~1995, and the result was that the DCT had the lowest picture distortion of all competing component-digital tape recorders (D5 and Digital Betacam), indicating that >Ampex has found the optimal balance of compression, head-to-tape data-rate, and error correction algorithm. Because of its precision mechanical design and optimal head-to-tape contact and data-rate, DCT also statistically has fewer catastrophic head-clog and tape interchange problems which can ruin long dubs or simply cause downtime during editing.
Shelf life and muti-pass over a tape is also an important consideration to prevent eventual concealments over the tape's life.
DCT's design is the only one to use air cushioned tape guides, and a slip and "pinch" free ("pinch roller") capstan design. Because of it's unmatched precision and reliability, the U.S. military and government use the DST data recorder, which is based on the same mechanical design).
Engineers enjoy the most user friendly, efficient, and thorough diagnostics of all video tape machines. Its modular design and
precise mechanical and electrical tolerances allow most parts to be replaced in under five minutes, with little or no alignment needed. (capstan, individual play heads, power supply) (-individual record heads take 5 to 10 minutes) Machine Owners enjoy that it has the lowest cost of ownership of all mastering machines. Tape owners appreciate that it is the fastest yet most gentle linear editing transport for 601 video. Tape operators might even enjoy the fact that they can clean the rotary scanner themselves! (you have to be an engineer to clean the more delicate & encapsulated D1, Digital Beta, and D5 scanners, or use a dry cleaning tape, which causes premaure head wear)
This American designed and built machine is the top rated of all component digital mastering machines at:
-Editior, Operator, and Engineer preference (yes, it's true - ask experienced users).
-picture distortion
-record time
-lowest maintenance cost and downtime
-speed of editing
Q: Why is it used less than other formats?
A: The same reason that the 6 mm digital tape formats are now selling better than half inch tape competitors- size, transportability and cost. The competing tape formats that were introduced at the same time cost less, and only one was available with portable camera capability.
DCT does enjoy a good portion of tape format usage. It is the primary tape format at various post production facilities in Hollywood (including the authors, we have 25), and most hi-end facilities have at least one or two to ensure their compatibility with client preferences.
Ken Martinek. |
