Re: DVD vs Tape - time and quality
- From: "PTravel" <ptravel@xxxxxxxxxxxxxxxxxx>
- Date: Thu, 4 Aug 2005 16:36:28 -0700
Ummm, no. Not even close.
See below.
"Rick Merrill" <rick0.merrill@xxxxxxxxxxxxxxx> wrote in message
news:qYCdnbnqjbDuC2_fRVn-2w@xxxxxxxxxxxxxx
> DVD has two different formats, 'video' is compatible with dvd players,
> when 'finalized.' Both are " readable " by conversion software, but the
> 'video' mode is compressed and resists editing inside of
> a single file [problems include screwing up the audio, loss of
> resolution]. The 'raw video' (VR mode) is uncompressed and therefore
> easy to edit.
Recordable DVD has two formats _at_the_moment, DVD-R and DVD+R (I'm
ignoring, for the moment, DVD-RAM). Both are compatible with most set-top
DVD players, though DVD-R is compatible with more brands and models than
DVD+R.
Video DVDs (which is what you are talking about) record video as
specially-formatted MPEG2 files. DVDs can also be used for storing data.
Digital video is data, just like anything else, and can be stored as a data
file on a DVD. A data DVD can be read only by computers, not set-top DVD
players. A data DVD will store approximately 22 minutes of DV-25-format
digital video. DV-25 is the format employed by miniDV camcorders. It _is_
a lossy, compressed format. It compresses frame-by-frame at approximately a
5-to-1 ratio. Video DVDs compress video in mpeg format. Mpeg compresses by
using base or reference frames and then calculating only the delta, i.e.
that which changes, in succeeding or preceding frames. DVD-format
mpeg-encoded video results in a compression ratio of 10-to-1 or higher.
Digital video, when stored on a computer, is frequently stored as an AVI
file. AVI is a "wrapper" that contains the video information which may or
may not be compressed, depending on the particular codec employed.
> To get tape (VHS or MIni-Dv) into an editor it must be played at
> real-time speed. So it takes as long to get the data into the editor as
> it took to record the data. 2 cameras for 2 hours would take 4 hours
> just to read the tapes into the editing station! [Am I missing something
> here?]
No, you're not, though you're also, I think, a little confused about the
process. Video from a miniDV source is stored in digital format and is
transferred as a data file, usually via an OHCI-compliant 1394/Firewire
port. I'm not aware of any miniDV source that transfers faster than real
time, but there is no reason why it couldn't be done -- it's just digital
data and could, in theory, move at the maximum rate of the 1394 bus. VHS is
an analog video format. In order to transfer analogue video to a computer,
it must be digitized. Consumer units require that the video be played at
the NTSC standard, i.e. real time. There are professional duplication units
that can copy analog NTSC to analog NTSC at higher speeds. I see no reason
why a digitizer couldn't also accept video at higher speeds.
>
> To get a DVD recording into an editor it may be played, also at
> real-time speed, OR it can be transferred as data-files at disk transfer
> speed (measured in seconds instead of real-time minutes).
I don't know what you mean by an "editor." To transfer the video on a DVD
to a computer, the data on the DVD is read by a DVD drive, just like any
other data. It must, however, be converted into a format that can be
manipulated by the software on the computer. Transferring video from a DVD
by playing it in a set-top player and digitizing the resulting analogue
video output is an extremely poor idea, as it introduces and additional
digitization step that will adversely effect video quality.
The speed at which data can be read from a DVD is limited only by the DVD
drive, the computer CPU and the bus speed.
>
> The editor station will, for example, have software that converts .VOB
> (Visual OBject with both audio and video)into MPEG files.
Again, I have no idea what you mean by "editor station." DVDs store video
data in VOB files, which are a specific species of mpeg. There are other
files on the DVD that contain menuing and chapter data. If you are using
unecrypted, i.e. non-commercial, DVDs, you can usually simply rename the
..VOB extenstion to .MPG and then treat the file as any other mpeg-encoded
video.
"Conversion" is not, therefore, strictly necessary. There are programs
called "rippers" which read the VOBs on a DVD and write either AVIs or
MPEGs.
>
> When transferred as MPEG files from the editor the data-files (.MPG)
> are playable on all Video Servers.
I don't know what you mean by "Video Server." Software that can read MPEG
data can read MPEG data. Software that can't read MPEG data can't read MPEG
data. There are a number of ways mpeg data can be formatted. Some software
is limited to DVD-compliant mpeg, some isn't. Some software can read any
"legal" variation of mpeg, some are more limited.
>
> As for "Video Quality" when used at SP (standard play mode) and RAW
> VIDEO (VR) all digital recordings have the same quality regardless of
> whether they are on tape (MINI-DV) or on disk (MINI_DVD).
Absolutely wrong.
SP and LP on miniDV cameras and decks result in exactly the same video
quality, i.e. both store video in the identical DV-25 format. LP packs data
more closely together than SP and, as such, is more prone to drop out.
Because the tracks are closer together, alignment of heads is more critical
and an LP tape recorded on one machine may not play on another machine.
However, the data, whether in LP or SP is exactly the same, hence the video
quality is exactly the same.
MiniDVD, i.e. the crappy consumer DVD camcorder format, uses more
compression in LP modes than SP modes. More data is discarded in LP modes,
so the video is of correspondingly poorer quality.
> [ You can see
> the ease with which a verbal conversation about the differences could be
> confusing! ]
I can't, but that's okay.
>
> DVD [disk] camcorders will have the same or better quality as DV tape
> camcorders (in the same price range) as both are digital recordings.
Absolutely not.
DVD camcorders use much higher compression ratios and much lower bit rates
than DV tape. DVD camcorders produce much WORSE quality video than miniDV
camcorders. Of course, there are other factors that determine video
quality, e.g. lenses, sensor size and density, etc. However, given
identical camcorders, except that one stores video on miniDVD and the other
stores video on miniDV, the miniDV will have far, far better video than the
miniDVD camcorder. This is why miniDVD camcorders are constrained to
bottom-of-the-line cheapie consumer machines, whereas miniDV is used in
better-than-broadcast-quality prosumer camcorders.
>
> Rick
> Merrill
>
.
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