The teardown train keeps rolling along, with this post focusing on MiniDV, a format of metal evaporate cassette used for recording DV (standard definition), DVCAM and HDV (high definition) formats. This compact tape is 1/4″ wide, has a recording bitrate of about 25Mbit/s, and stores close to 13Gb of data on a 60 minute cassette. It still sees a few applications today, although it is dwindling due to several factors. These include the cost of media as compared to flash-based alternatives, the slow workflow from “real-time” playout into a computer, the lack of robustness of the recording mechanism which can have dropouts due to mechanical shock and the unreliability of the mechanism which is prone to needing service and alignment when under heavy use.
Despite the possibility of re-use, these were often not re-recorded due to the slow accumulation of drop-outs as the media wears out. These can cause visual glitches (blocking or banding) in SD format DV. When used in HDV, results are more disastrous due to the non-keyframe MPEG2 compression, resulting in half-second video freezes which made the format very difficult to adopt in critical situations (e.g. wedding videos) and resulted in the need for higher quality HDV-branded Mini DV cassettes which have a claimed lower drop-out rate.
That being said, I did use MiniDV as a logical progression over Video8, entering the digital realm of no-fuzz nonsense. It was amazing, with the ability to stream video over Firewire (a good reason to keep the port alive) both ways to and from the camcorder. MiniDV could also be used to back-up post-edit video as well, with the minimal compression for standard definition being visually lossless.
Seeing as I no longer had a working deck, I found a Fuji DVM60 cassette at the bottom of my drawer, so lets take that apart!
This particular cassette from Fuji claims to have 70.4m of tape in it. Not a bad feat for such a small shell, as the metal evaporate based tapes all tended to have thin bases as well to “cram” as much tape as possible into the shell. It features an “easy tear” band at the top, and is Made in Japan (as are all the high quality cassettes).
Many different brands of Mini DV cassettes existed (more on this in another posting), but mixing and matching brands can sometimes lead to trouble. The story (possibly an “urban legend”) goes that the lubricants on different brands of tape would mix together and cause head clogs or accelerated head wear. The key guys were Sony with a wet type lubricant, and Panasonic (practically every other brand) with a dry-type lubricant. Recommendations included cleaning the deck between every change of brand. Other people had found that there was no issue – however, I did find some head clogs myself.
This tape comes in a hard plastic case, similar to the sort you would find around compact cassettes.
The cassette has a pre-applied label in the labelling area, branding the cassette. This became less common in later batches of tape, probably to reduce costs, as it was entirely unnecessary “marketing decoration”. A small vertical window on the supply reel shows the status of the tape, although in many cases, DV decks could sense the remaining tape by monitoring the rotation rate of the reel.
The top features a multi-part lid which covers the tape from the top and rear. Small sprockets are used to drive the tape, with the reel lock disengaged through a slot. Write protection status is detected by a hole on the underside.
A space to apply a label is provided on the spine, along with a slide switch for write protection which engages a small flap which covers a sensing hole at the bottom. Four channels can be seen underneath, which lead to nothing in this cassette, as this area is for the IC memory contacts for cassettes with some EEPROM to store information about the recording and tape. This feature was relatively uncommon on consumer equipment and consumer-oriented cassettes.
The sides show grooves which allow the top section of the lid to slide back. A small lever is used to unlock the top flap.
Inside, we can see the clear leader tape and the dark golden metal evaporate tape. The tape is on a base so thin that it really likes to curl when exposed between the two rollers. Blowing air across the tape makes it vibrate and produce a very tinny sound, implying the thinness of the media.
Standard inclusions include the inlay card in the case which can be used to mark important information.
You also get care instructions and a warranty statement separately.
Finally, you also get a set of labels, three spine labels and three top labels.
The cassette comes after undoing four screws from the rear.
The top side shows the spring loaded lid, and the two reel support springs which are individually mounted by melting the plastic shell over them, and canted at 45 degrees. The bottom lid flap cover can be seen “dangling” as it was pulled from its support rails in the other side of the cassette.
The cassette itself is very simple, with the tape travelling across from one reel to another identical reel. Both front and back plates on the reel have no perforations or patterning. A clear window is used to sense the clear leader to detect end of tape condition, and a single central spring loaded reel lock mechanism is used to release both reels in a way similar to VHS. The reel lock utilizes the spines at the edge of the rear plate.
The cassette itself has no rollers or wipers, which means that dust protection relies on the multi-part hinged lid. It also means the precision of the moulding in the support edges is vital, as any unevenness will cause scoring of the back of the tape as it is run across the supports. While it is simpler, it probably is not ideal for long-term heavy duty service, and these design decisions seem to imply a more consumer/low-end-prosumer-oriented cassette.
I found an old Mini DV cartridge I didn’t have much of a use for, and I tore it apart. It was both interesting and disappointing to see the simplicity of its internals. Maybe we will take a look at the variety of Mini DV cassette outers instead … stay tuned for the next post.