IMovie HD6: HDV to AIC to HDV... quality loss?

Question

Hi All,
I'm curious, when I use my normal workflow (HDV to AIC (imovie 6) to HDV), does it lose quality?
If so:
* Is there a way to avoid this?
* How much quality is lost? Is there a visual comparison available?
Thanks for any input!

Answer 1

Dear catspaw,
Here are my thoughts, based on my experiences, and what I think I understand of all this..
1. Standard-definition DV (those little tapes, or the larger 'broadcast' tapes) is pretty much compression-free ..we-ell, strictly speaking there's some, but relatively little, compression used in DV. It looks perfect, although it is slightly compressed. The material recorded onto tape - and imported into iMovie - contains every frame which the camcorder optics see. So editing it is simple: all the frames get copied into iMovie, and you can chop out, or insert, anything you want. Using iMovie HD 6, or earlier, you can then copy the edited material back to a DV camcorder ..all the frames get shuffled out of the computer and back onto tape again. (You can't do that with iMovie '08, as it has no option to Export to Camcorder.) What you see in iMovie - after importing from a DV camcorder - isn't exactly the same as what you've imported, because iMovie runs on a computer, and uses a computer display, and that generally shows complete "progressive" frames of video, whereas a TV ..or TVs with cathode ray tubes; precursors to the latest LCD or DLP or plasma TVs.. will generally show interlaced 'half-frames' one after the other, each comprising half the TV picture, but shown in such rapid succession that they blur into each other, and our brains see a succession of complete frames.
(..Here's a good visual representation from one of Adam Wilt's pages:
..There are two 'fields' of video, each made of half the entire number of lines down the screen, superimposed on each other, and blending into a full frame of video comprised of all the lines. That's what happens on a TV screen when the interlaced 'fields' of video blend together..)
So standard-def DV is really plain and simple, and there should be no quality loss after shooting, importing, editing, exporting.
2. Hi-def. A can of worms. There are several different varieties of "hi-def". What we're working with in our 'amateur' movie program, iMovie, is generally the HDV version of hi-def, or the AVCHD version. (And a few people may be working with JVC's version of 'progressive' frames, but with a lower total number of lines down the screen: 720p, instead of 1080i. 720p has 720 pixels down the screen, and records and presents an entire 'progressive' ..one-line-after-the-other.. frame of video at a time, whereas 1080i shows 1080 pixels down the screen, consisting of half that number, 540; all the 'odd-numbered' lines.. at a time, immediately followed by the other half ..the even-numbered lines.. slotting in-between the previous lot. That repeating pair of 540 'interleaved' lines gives a total of 1080 interlaced lines in every frame. Movement appears smoother using 1080i (..after all, the picture is refreshed twice as often as with single-complete-frame 'progessive' video..) but may not look as super-sharp as progressive video, because at any moment there's only half the total information of a frame onscreen. 'Interlaced' video is smoother, and any action flows more "creamily", whereas 'progressive' may be considered 'sharper' (..it is if you freeze a frame..) but more jerky.)
So our 'amateur' hi-def movies may be recorded as HDV, AVCHD or some other similar format. 'Professional', or broadcast-intended, hi-def may consist of several other non-amateur formats, some of which are completely uncompressed and require extremely fast links between the cameras and recording equipment, and massive-capacity hard discs to capture and edit the huge quantity of data which such cameras..
..deliver ..for $150,000. Or here's a remote-control broadcast hi-def camera for (only) $7,995..
(..Tell me if I'm boring you..)
The hi-def cameras which we're more likely to be using..
..record compressed video in MPEG-2 format, or H.264, or some similar codec. The idea behind HDV was that the companies which make 'consumer-grade' (amateur) camcorders wanted a method to record hi-def - with about 4x the data of standard-def - onto the little miniDV tapes which we were all familiar with. So a method was found to squeeze 4x the data onto a tape which normally records standard-def DV data at 25 megabits per second. The method decided upon was MPEG-2 ..the same codec which is used to squeeze a two-hour Hollywood film onto a little 4.7GB capacity DVD. (Bollywood movies, as distinct from Hollywood movies, tend to be three hours long!)
If MPEG-2 was good enough for the latest cinema releases, in nice, sharp, sharper-than Super-VHS form, then it was thought to be good enough for 'domestic' hi-def recordings. The only awkward thing about that - from an editing point of view.. (..but which of the camcorder manufacturers are seriously interested in editing..? ..they primarily want to sell 'product' which - according to their advertising - is terrific at simply recording and playing-back video. Like car advertising shows you how wonderful cars are to sit in and for travelling to places, but the adverts don't tell you about how tricky it may be to get into the rear sidelights and replace a blown bulb..) ..is that in HDV there's only one 'real' frame for every 15 frames recorded on the tape. The other 14 are just indications of what's different between the various frames. Therefore, for editing, the 'missing' frames must be rebuilt during import into iMovie.
Steve Jobs heralded 2005 - at MacWorld, you may remember - as the "Year of HD!" ..It became possible to import and edit hi-def in iMovie ..that is, the HDV version of hi-def, not the uncompressed 'professional' broadcast version of hi-def, of course.. but ONLY with a fast enough computer ..and many weren't fast enough to import and convert HDV to editable-format in real-time (..no mention of it being the year you would import at half, or a quarter, or an eighth, real-time ..ugh-ugh).
So HDV gets converted to AIC to make it editable ..and then what d'you do with it? ..Few (none of them?) HDV camcorders let you import HDV back to tape from iMovie. No Macs had/have Blu-Ray burners ..although you can burn about 20 mins of hi-def onto normal DVDs with a Mac's normal inbuilt SuperDrive DVD burner with the appropriate software ..DVD Studio Pro, or Toast, etc.
(..Once again, there was some omission from the hoopla ..yes; you can import HDV! ..yes; you can edit HDV! ..er, no, sorry; no mention that you can't burn a 1 hour hi-def home video onto a hi-def DVD with a Mac ..iDVD would/will only burn in standard-def, and there are no Blu-Ray burners built into Macs..)
Then came AVCHD (Advanced Video Codec; High Definition). This compresses video even more than HDV (whose compression is pretty much invisible, and is in regular use for broadcast material) by using a different method. And along came progressive hi-def recording, trying to supersede HDV's generally 'interlaced' 1080i hi-def.
But the problem with progressive, non-interlaced AVCHD is that if there's rapid movement in a scene - if you move the camera, or something rapidly crosses the picture - instead of the "creamy flow" of interlaced video, there's a jerky lurch from one frame to the next. And with the added extra compression of AVCHD this jerkiness can be (..to my mind..) even more horribly evident.
Anyway, unscrambling ..and then re-assembling.. hi-def interlaced MPEG-2 HDV is pretty much invisible - to me, anyway. The video looks sharp, moves smoothly, looks 'true-to-life' and doesn't have terrible artifacts and jerks.
Unscrambling ..and then re-assembling.. hi-def interlaced or progressive AVCHD (..which is sometimes described as MPEG-4 or H.264..) - I know that you know this, but I'm also writing for others here - isn't quite as simple as doing the same for tape-based MPEG-2 hi-def HDV. Here's all the gobbledegook about what AVCHD can consist of.. http://en.wikipedia.org/wiki/MPEG-4_AVC
..Oh, and here's a bit about the "usability" of AVCHD: http://en.wikipedia.org/wiki/AVCHD
There are many more 'varieties' of encoding in AVCHD than in 'simpler' hi-def, such as HDV. There's less data sent in an AVCHD data stream than HDV (..AVCHD has jumped from 17MBits/sec to 24MBits/sec ..just below HDV's 25MBits/sec..) so the video is more compressed than HDV. And there are all sorts of video formats (interlaced, progressive, HD, 'Full' HD) which are recorded by different cameras under the all-embracing 'AVCHD' label. iMovie - or a Mac - has to work much harder to unscramble and convert the more-compressed AVCHD format(s) than uncompressing HDV. And has to work harder to compress the output of iMovie to H.264 (an AVCHD codec) than when re-compressing to MPEG-2 (the codec for standard-def DVDs and hi-def HDV).
To - finally! - come back to your question "..is there therefore no advantage in using DV tape-based vidcams for editing purposes.." I'd say that there ARE advantages in using tape-based vidcams for editing purposes ..using your two categories:
1. Non-hi-def tape-based DV is ..to all intents and purposes.. lossless. And the material can be imported in real-time, and be output - with no loss - in real-time, too, using any Mac from an old G3 onwards. Importing non-tape material into iMovie ..e.g; miniDVDs, or chip-based, more compressed video.. is more long-winded, and generally has to go through various external bits of software (..e.g; MPEG Streamclip or somesuch..) to put it into a format that's editable in iMovie. AVCHD can, theoretically - as 'AVC', without the 'HD' - be used for recording in standard-def, but there are currently few AVCHD camcorders which are built to record standard-def video as well ..there is the Sony HDR-SR12. But only iMovie running on an Intel-powered Mac will decode AVCHD, apart from separate standalone Mac software such as 'Voltaic'.
2. Hi-def tape-based recording IS an advantage on anything that's less than the fastest, or highest-powered, of Macs, because it needs less "horsepower" to "unpack" the compressed data and to get it into an editable format through recovering, or rebuilding, the necessary individual frames. I think it's an advantage in every case, as not only can tape-based hi-def be edited on older, slower Macs (including pre-Intel Macs) but also:
(a) HDV data's less compressed, and so motion is generally expressed - currently - more "fluidly" than with the more compressed hard-disc or chip-stored AVCHD,
(b) HDV original material is "self-archived" onto its tapes ..you don't have to "empty" a camcorder's hard disc or memory chips onto something else - such as a separate hard drive - in order to re-use, or continue using, the camcorder: you just drop in another cheap 1-hour tape,
(c) Tape-containing camcorders tend to be heavier, less lightweight, than fewer-moving-parts chip-based AVCHD camcorders. They're therefore inherently less "wobbly" and don't tremble so much in your hand ..that gives smoother, less "jiggled-about" recordings ..even taking into account the stabilisation built into most camcorders,
(d) Tape-based camcorders are less likely to lose an entire 'shoot' by being dropped or mis-treated. Material already recorded onto a tape will not be damaged if you drop the camera and its tape-heads thereby become misaligned. The data can be recovered by simply ejecting the tape and popping it into another camcorder. If a hard-disc camcorder is dropped, subsequent head misalignment may mean that all data already on the hard disc is irrecoverable. If a memory chip becomes corrupted, all data may similarly become irrecoverable. If a tape becomes damaged, it's usually only a few seconds' worth which be lost. (..I dropped a tape-based camcorder in the sea when I was trying to get shots of waves coming in onto the beach from an offshore viewpoint, and a wave washed right over me and knocked me down. The camcorder was a write-off, but I managed to prise the tape out, and recover the 30 minutes of movie I'd already recorded. I don't really want to test it, but I have doubts about whether I'd have been able to recover my video from a similarly-drowned hard-disc based camcorder ..maybe, in the interests of factual objectivity I'll try it some day with an old, no-longer-used 2.5" hard disc..)
(e) AVCHD camcorders - unless you're looking at 'semi-pro' or professional 'cost-a-plenty' record-to-chip camcorders, or that Sony HD12..
..are generally built for "point-and-shoot" amateurs. This means that AVCHD camcorders generally do not have the assortment of manual controls which you find on most tape-based HDV camcorders (..because the camcorder makers also aim, or aimed, HDV at low-cost broadcast users, too). There's usually far greater flexibility and more shooting options (shutter speeds, exposure, audio handling) on tape-based HDV camcorders than can be found on AVCHD camcorders. If you're just pointing and shooting, that doesn't matter ..but if you want to shoot good-looking video, there are generally - and it is a generalisation - more adjustment options to be found on a tape-based camcorder than on a chip-based or hard-disc AVCHD camcorder. In my experience - yours may be different - people tempted by AVCHD camcorders tend to buy (..and manufacturers tend to publicise..) high pixel counts (like "Full HD 1920x1080") and that magic word "progressive" (perhaps because it has the flavour, in English, of "futuristic" or "more advanced") rather than their being concerned with choices of apertures or shutter speeds and the clearest representation of what the camcorder's pointing at.
In summary ..at last!.. "..is there therefore no advantage in using DV tape-based vidcams for editing purposes.." Yes; the advantages, I believe, are that HDV converts fast into AIC for editing; my perception is that HDV delivers smoother action (onscreen movement) than AVCHD; and with a suitable deck..
..HDV can be returned back to tape, whereas it's more long-winded and needs more subterfuge to export AVCHD back to a chip, or a camcorder's hard disc, for in-camera replay ..and thence out to an HDTV.
As always, these are simply my opinions ..others may disagree.