are the DV artifacts I keep hearing about?
DV artifacts []
come in three flavors: mosquito noise, quilting, and motion blocking.
picture defects []
encountered are dropouts and banding (a sign of tape damage or head
clogging).
The most noticeable spatial artifact is mosquito
noise around any sharp, contrasty edges. These are
compression-induced
errors usually seen around sharp-edged fine text, dense clusters of
they show up as pixel noise within 8 pixels of the fine
or edge causing them. The best place to look for them is in fine text
superimposed
on a non-black background. White on blue seems to show it off best.
magnitude of these errors and their location tends to be such that if
monitor the tape using a composite video connection, the artifacts
often be masked by dot-crawl and other composite artifacts.
A spatial quilting artifact can
sometimes appear at the boundaries
between 8x8 pixel blocks, most noticeable on shallow diagonals or on
slightly-defocused
backgrounds, typically when there is some motion in the scene to make
fixed "grid pattern" a bit more obvious. Some DV codecs seem to be
prone to this than others, and with a few the quilting really starts
only after a few generations of rendering.
Motion blocking occurs when the two
fields in a frame (or portions
of the two fields) are too different for the DVC codec to compress
together. "Bit budget" must be expended on compressing them
separately,
and as a result some fine detail is lost, showing up as a slight
blockiness
or coarseness of the image when compared to the same scene with no
Motion blocking is best observed in a lockdown shot of a static scene
which objects are moving: in the immediate vicinity of the moving
(say, a car driving through the scene), some loss of detail may be
seen. This
loss of detail travels with the object, always bounded by DCT block
boundaries.
However, motion blur in the scene usually masks most of this artifact,
making this sort of blocking almost impossible to see in most
circumstances.
Finally, banding or striping of
the image occurs when
one head of the two on the scanner is clogged or otherwise unable to
data. The image will show 10 horizontal bands (12 in PAL countries),
every other band showing a "live" picture and the alternate bands
a freeze frame of a previous image or of no image at all (or, at least
in the case of the JVC GR-DV1u, a black-and-white checkerboard, which
frame buffers appear to be initialized with).& Most often this
due to a head clog, and cleaning the heads using a standard
manufacturer's
head cleaning tape is all that's required. It can also be caused by
damage, or by a defective tape. If head cleaning and changing the tape
used don't solve it, you may have a dead service
be required.
This sort of banding dropout about once
tape in my experience. Usually it isn't even noticeable -- a single
of banding due to a momentarily clogged head won't be visible unless
motion in the scene to show off the frozen stripes. Have a look
your old tapes frame by frame (on a slow day, of course!) and you
be surprised how often you'll be able to find a single, subtly banded
For what it's worth, I've only rarely found such a banded frame on any
footage I've shot, which indicates to me that DV is right on the edge
reliability. DVCAM, with its 15 micron track width, or DVCPRO with
its 18 micron track, are sufficiently on the safe side of the bleeding
edge so that this sort of droput is much less likely to occur.
Bear in mind that analog BetaSP typically has several
dropouts per
the last time I measured visible dropout rates on Hi8 and S-VHS I got
in the range of a dropout every 3-5 seconds (Hi8)
and every 7-20
seconds (S-VHS). One visible dropout per hour-long tape, on average,
not something to get flustered about. But if it does bother you, shoot
DVCAM or DVCPRO instead.
Digital-S, DVCPRO50,
DVCPROHD/100, and HD Digital-S
What are Digital-S and DVCPRO50?
JVC's D-9 (formerly known as Digital-S) and Panasonic's
use two DV codecs in parallel. The tape data rate is doubled to 50
(video) and the compression work is split between the two codecs. The
is a 4:2:2 image compressed about 3.3:1. It's visually lossless and
gorgeous. Think of Digital Betacam, albeit at 8 bits instead of 10, at
a bargain price.
JVC's D-9 uses the 1/2" SVHS form factor for tapes and VTRs,
the tape cassette itself is more robust and the transport is equipped
sapphire guide roller flanges and tape cleaner blades and a new
design. One of the D-9 players will also play back analog SVHS tapes,
its use for editing existing libraries of SVHS tapes as well as newer
footage. Head life (so far, in on-air broadcast usage) is well in
of 4000 equipment cost is very low (comparable to 25 Mbps DVCAM
or DVCPRO); and maintenance expenses are well below those of the
Betacam decks
that D-9 is typically displacing. Only JVC is supporting this format,
which has resulted in a less-than-headlong rush by the video community
to embrace it. Watch it, it's hot. If you're doing
EFP on a budget, this is the format to use.
Panasonic's DVCPRO50 uses the same DVCPRO tapes and
transports as its
25 Mbps DVCPRO products (there is also a 93-minute DVCPRO50 tape,
which Panasonic says should only be used in DVCPRO50
mode. When using standard DVCPRO tapes, the maximum recording time is
61 minutes since the P123L cassette is being run twice as fast).
DVCPRO50 VTRs will also play back DVCPRO tapes, and most will play DV
and DVCAM tapes, too,
though some of the decks cannot accommodate the miniDV cassette size,
even using the cassette adaptor.
DVCPRO50 kit is real jack-of-all-trades stuff. Most of the
camcorders record either
DVCPRO (25Mbit/sec) or DVCPRO50 (50Mbit/sec) in either 4x3 or 16x9.
One, the AJ-SDX900,
can record in 24p (using either of the
used by the DV-format AG-DVX100A), 30P, or 60i at the flip of a
Unlike D-9, second-sourcing is available from Philips,
Hitachi, and
DVCPRO50 kit is also a lot more portable and lightweight
so it's the format of choice if you're doing high-end EFP with a
bigger budget and you want to keep your camera operators from wearing
as quickly!
Panasonic also had DVCPRO-form-factor progressive-scan
cameras and VTRs
that use the 50 Mb/sec data rate to encode a 480-line proscan 16:9
image at 60 frames per second.
DVCPRO-Progressive It looked very good -- but outlets for 480/60p
media, ouside of the Fox network,
are far and few between. People using 480/60p for digital filmmaking,
example, upconverted from 480p to 1080i HDTV, and sent the HD out to
film. With the advent of the 480/24p-capable SDX900, as well as the
720p variable-frame-rate Varicam,
Panasonic's DVCPRO-Progressive format has passed largely into history.
Four codecs
Both JVC and Panasonic showed working prototypes of 100 Mbps
DV-derived
products at NAB '99 for handling HDTV; Panasonic was shipping by NAB
Both firms gang four DV codecs together to get the 100 Mbps
datastream,
while preserving the same equipment form factor and operational
methodologies
used in the current 50 Mbps products. Panasonic calls their stuff
while JVC used the D-9HD moniker, reflecting the SMPTE standard number
for their DV50 format. DVCPROHD is readily available, while D-9HD
much died on the vine and is not (as of 2004) a current product line.
In 1080/60i formats, DVCPROHD records 1280 Y samples and 640
Cr & Cb samples
per line, compared to HDCAM's 1440 Y and 480 Cr & Cb samples.
the 1080-line DV100 formats have slightly lower luma resolution than
HDCAM but slightly
better chroma resolution (see the
for a discussion of sampling).
1080/50i DVCPRO is said by certain uncorroborated
documentation to be sampled at , and if still 4:2:2, that
means there would be 720 Cr & Cb samples per line! I have not
found a second source for this
information, so I can't confirm it.
720p DVCPROHD records the
image subsampled to 960 Y
samples per line,
maintaining 4:2:2 sampling to give it 480 Cr & Cb samples per
720p DVCPROHD always records 60 frames/second, but can, with the
Varicam, record lower frame rates from the camera section by
performing
in-camera "pulldown", repeating frames as needed to record 60 per
second. 24p, for example, is recorded with
a standard 2:3 pulldown like the one described ,
but with full frames instead of fields.
It should be noted that both Panasonic and JVC are
well-placed to serve
the growing DTV market whatever image format a broadcaster selects.
is selling switchable 720p/1080i D5-HD (not based on DV technology)
DVCPROHD VTRs.
D5-HD has already become the studio standard VTR format for the
dawn of US DTV. Some Panasonic camcorders, starting with the
AG-HVX200,
can record 480i, 480p, 720p, 1080i, and even 1080p (at 24 and 30fps),
all using various DV/DVCPRO formats recorded on solid-state P2 cards.
JVC's NAB '98 and '99 displays featured D-9 variants of
most popular ATSC DTV formats -- 480i, 480p/30, 480p/60, 720p, and
1080i, although the company seems to have pulled back from the
market, instead focusing on an excellent series of 720p HDV camcorders
Sony's HDCAM format uses compression technology "derived
from DV and
with certain similarities", but it is not on the main branch of the DV
family tree. Its data rate of 135 Mbps yie it's
you'll see a noticeable artifact in an HDCAM picture.
4:2:2, 4:1:1,
What are 4:2:2, 4:1:1, and 4:2:0 anyway?
These are all shorthand notations for different sampling structures for
digital video. They are also used for CIF and QSIF and suchlike MPEG
sizes, but in the discussion that follows, I focus on the numbers for
(standard-definition TV) digitized to the ITU-R BT.601 standards: 13.5
MHz sample frequency and 720 pixels per line.
The first number refers to the 13.5 MHz sampling rate of the
because (a) it's nominally almost approximately sort of four times the
NTSC and/or PAL color subcarrier frequencies, and (b) because if it's
the other numbers can be integers whereas if it were "1" the formats
be "1:0.5:0.5", "1:0.25:0.25", and "1:0.5:0" respectively, and which
you rather try to read off in a hurry? The 13.5 MHz sampling yields
pixels per scanline in both 525/59.94 and 625/50 systems (NTSC and
PAL/SECAM).
This number applies to D-1, D-5, Digital Betacam, BetaSX, Digital-S,
all the DV formats just the same.
The other two numbers refer to the sampling rates of the
color difference
signals R-Y and B-Y (or, more properly in the digital domain, Cr and
In 4:2:2 systems (D-1, D-5, DigiBeta,
BetaSX, Digital-S, DVCPRO50)
the color is sampled at half the rate of the luma, with both
color-difference
samples co-sited (located at the same place) as the alternate luma
Thus you have 360 color samples (in each of Cr and Cb) per scanline.
In 4:1:1 systems (NTSC DV &
DVCAM, DVCPRO) the color data
are sampled half as frequently as in 4:2:2, resulting in 180 color
per scanline. The Cr and Cb samples are considered to be co-sited with
every fourth luma sample. Yes, this sounds horrible -- but it's still
for a color bandwidth extending to around 1.5 MHz, about the same
bandwidth as Betacam SP (which, were it a digital format, would be
characterized
as 3:1:1).
So where does 4:2:0 (PAL DV, DVD,
main-profile MPEG-2) fit in?
4 x Y, 2 x Cr, and 0 x Cb? Fortunately not! 4:2:0 is the non-intuitive
notation for half-luma-rate sampling of color in both the horizontal
vertical dimensions. Chroma is sampled 360 times per line, but only on
every other line of each field. The theory here is that by evenly
subsampling
chroma in both H and V dimensions, you get a better image than the
unbalanced 4:1:1, where the vertical color resolution appears to be
times the horizontal color resolution. Alas, it ain't so: while 4:2:0
well with PAL and SECAM color encoding and broadcasting, interlace
diminishes vertical resolution, and the heavy filtering needed to
process 4:2:0 images caus as a result,
multigeneration
work in 4:2:0 is much more subject to visible degradation than
multigeneration
work in 4:1:1.
"Now how much would you pay? But wait, there's more!" In US
implementations
of 4:2:0, the color samples are supposed to be vertically interleaved
luma, whereas in European 4:2:0 they're supposed to
be co-sited.
Practically speaking, this is a headache for developers of codecs,
and DVEs, but for DV purposes it's not especially exciting, since only
European DV is 4:2:0.
Why does PAL DV use 4:2:0?
The best explanation I can come up with why PAL DV went with 4:2:0 is
both PAL and SECAM show reduced vertical color resolution and better
horizontal
color resolution compared to NTSC, so 4:2:0 seemed a closer match to
native display systems in PAL/SECAM countries. As PAL DV was intended
a consumer format for off-air recording or camcorder acquisition,
multigeneration
losses in 4:2:0 were considered a less important factor than the
optimization
of first-generation performance. PAL DVCAM also used 4:2:0.
When Panasonic developed DVCPRO, they opted for 4:1:1 even
in PAL versions,
specifically for the multigeneration advantage. Thus PAL DVCPRO decks
the pleasure and responsibility of handling both 4:1:1 DVCPRO playback
and 4:2:0 DV they have extra hardware to digitally resample
4:2:0 signal and come up with a decently synthesized 4:1:1. Sometimes
a reason for the higher prices that the poor Europeans are saddled
when it comes time to purchase gear...
chroma-key with 4:1:1 / 4:2:0?
Yes indeed. Many early DVEs were 4:1:1 plenty of digital
out there still are (such as the Panasonic WJ-MX50 and Sony FXE-series
vision mixers, both of which chroma-key). As previously mentioned,
could be considered a 3:1:1 format in terms of component bandwidth, and
BetaSP is used for chroma-key applications all the time. With some
care, DV25 keys at least as well as BetaSP; read on...
Part of the standard JVC sales pitch for D-9 is the
superiority of 4:2:2
(which is true), and the utter doom and degradation that awaits you
you try to do anything -- including chroma-key -- with a 4:1:1 format
is, shall we say, a wee bit exaggerated). But that doesn't mean that
can't do very satisfactory work in 4:1:1.
JVC had an excellent D-9 demo tape showing multigeneration
performance
comparisons of DV, D-9, and Digital B watch it if you can. Just
be sure you take the hype with a grain of salt.
True, the chroma performance of 4:2:2 formats is superior to
4:1:1 formats,
especially in multigeneration analog dubbing. But by the same token,
is as superior to 4:2:2 as 4:2:2 is to 4:1:1 or 4:2:0.&
Where DV can get into trouble is that the coarse resolution
chroma signal (only 180 samples per scanline in 4:1:1) leads to a very
regular, "steppy" key signal, most noticeable on near-vertical edges
vertical edges
where motion is present, especially if the codec's decompression
replicates
the chroma sample across the intervening pixels instead of low-pass
or interpolating between samples. The 4:2:0 sampling in 625 DV/DVCAM
better in this regard, but it has its own problems vertically, so
always a tradeoff.
The single most important factor in good DV
chroma-keying is low-pass filtering or interpolating the chroma
to applying
the keyer,
so that the 搒teppy edges