DV compression question

[John Jay]

My question concerns the signal at the 1394 port on any DV camcorder

Is each field of a frame compressed with the DV compression codec or is the compression done when a whole frame (2 fields) is ready before output at the port?

[Rob Lohman]

You will get the whole frame at the port. Actually the frame is
devided into several firewire packages, I can get you specifics
if yoy want.

Whether the compression is field or frame based I'm uncertain.
But that doesn't matter since you WILL get the whole frame
(+ audio samples and extra information) at one time.

Hope this helps!

[Bill Ravens]

hmmm...interesting reply, Rob. Considering that the codec is NOT used in a firewire transfer...the data is merely copied from one medium to another. The only time the codec is employed is during the "write to tape" internal to the camera and during a rendering process inside the computer. I'm not so sure the data is useable, either in field or frame format, at the firewire port. It's, quite simply, a data stream...in packets, yes, but, I don't know if those packets bear any relation to the discrete frame info cantained within.

[Andre De Clercq]

Like Rob writes, the compressed DV video data stream consists of so called isosynchronious packets which contain compressed whole frame data. There is basically no "field after field" involved. Therefore field order setting in NLE and convertors is important. On the compression level itself DCT uses, depending on the motion in the picture a field based (2:4:8) or frame based (8:8) pixel structure for the DCT transformation, which is only the first step in the DV compression concept.

[John Jay]

thanks all

the reason I asked is concerning those cams which support frame mode by two line averaging.

If the compresion is at the frame level then the signal should be better (less DV artifacts) since compression is applied to effectively half the signal whereas field compression would have no such advantage from a signal standpoint since it is just copied to the other field

[Robert Mann Z.]

importing into a computer will compress the signal even more, check out the comparison featuring todays nles and card system, click on my signature link

[Jeff Donald]

skyonic newyork,

Only the very last image displays. I can't see what you referring to.

Jeff

[Robert Mann Z.]

in order to preserve the integrety of the images they are bmp @ 1 meg each, however the first image is a collection of all codecs

[John Jay]

what were the settings used on the avid and main concept codex?

i ask because there are a choice of quality settings

[Robert Mann Z.]

vegas was set to best...i know of no best settings on avid xpress

[Rob Lohman]

Skyonic,

Taking footage in a computer will NOT alter its quality at all, since
it will only be a digital copy. It will alter when it is being displayed
(since it is going through a decompressor and color space conversions
and what not). But the main fact were people are loosing quality
is when changing the footage and/or outputting it again (back
to DV or MPEG2 etc.).

[Robert Mann Z.]

Rob,

>>Taking footage in a computer will NOT alter its quality at all, since it will only be a digital copy. <<

hardly true, you never edit raw dv, i know of NO such app, everything is either an avi or qt wrapper with appropiate codec...the reason it's called dv codec is it's re-compressed into a wrapper from the camera...

even further if you use the matrox codec to bring in your footage your dv signal will be converted into rgb from yuv..


>>But the main fact were people are loosing quality
is when changing the footage and/or outputting it again (back
to DV or MPEG2 etc.)<<

yup... you lose even more at this stage, this actually a heated debate in the industry currently, you might see ads promoting a products lack of gen loss (generation loss). No Gen loss is based on a codecs ability to maintain quality even after 20 renders...


so the myth that dv footage is just moving data back and forth is just that...

[John Jay]

skyonic

do you know if there are differences in the firmware compression between cams like Sony, Canon, Pana

ie do they use the same algorithms or are there differences in the way they are implemented?

(might explain why video recorded on a Pana plays back better on a Sony)

[Robert Mann Z.]

jay this is very good question, i only know of the audio sampling differences, i was told a while back that most cams use sonys chip, but i could be wrong...

[Jeff Donald]

I've had some Panasonic, Sharp and Canon DV cameras apart and I don't recall seeing any Sony chips in the Panasonic or the Sharp. I could be wrong, too. I wasn't looking that close, but I think I would have remembered a Sony chip in a Panasonic.

Jeff