Best Downconversion workflow for 720P in FCP

[Douglas Toltzman]

This article actual contradicts my previous understanding on this topic, so I hope everyone can forgive me if I've said anything misleading in the past. The following quote is from this URL:

http://www.hometheaterhifi.com/volum...e-10-2000.html

How Progressive Players Work

How The Information is Stored on Disc

It’s important to understand at the outset that DVDs are designed for interlaced displays. There’s a persistent myth that DVDs are inherently progressive, and all a DVD player needs to do to display a progressive signal is to grab the progressive frames off the disc and show them. This is not exactly true. First of all, a significant amount of DVD content was never progressive to begin with. Anything shot with a typical video camera, which includes many concerts, most supplementary documentaries, and many TV shows, is inherently interlaced. (Some consumer digital video cameras can shoot in progressive mode, and a handful of TV programs are shot in progressive, particularly sports events.) By comparison, content that was originally shot on film, or with a progressive TV camera, or created in a computer, is progressive from the get-go. But even for such content, there is no requirement that it be stored on the DVD progressively.

DVDs are based on MPEG-2 encoding, which allows for either progressive or interlaced sequences. However, very few discs use progressive sequences, because the players are specifically designed for interlaced output. Interestingly, while the sequences (i.e., the films and videos) are seldom stored progressive, there's nothing wrong with using individual progressive frames in an interlaced sequence. This may sound like a semantic distinction, but it’s not. If the sequence is progressive, then all sorts of rules kick into place which ensure that the material stays progressive from start to finish. Whereas if the sequence is interlaced, then there are fewer rules and no requirement to use progressive frames. The encoder can mix and match interlaced fields and progressive frames as long as each second of MPEG-2 data contains 60 fields, no more, no less (or 50 fields per second for PAL discs). The progressive frames, when they are used, are purely for compression efficiency, but the video is still interlaced as far as the MPEG decoder is concerned.

The input to a DVD encoder (the instrumentation that is used to author a DVD) is almost always an interlaced digital master tape, even if the original material was shot on film. The video transfer is typically done at a different facility, and the output of the transfer is interlaced. Since the DVD encoding software doesn't even have access to a progressive master, it must rely on the same kinds of algorithms that a deinterlacer uses to put the proper fields together. Since there is essentially no requirement that it actually always put the proper fields together, other than compression efficiency, many encoders are conservative about using progressive frames. If the encoder cannot be sure that a frame is progressive, it will typically mark it interlaced, because the only real loss is a few bits of disc space.

When the mastering engineers view the disc for quality control, they view it on an interlaced monitor. They don't necessarily care how well it deinterlaces, because that's not part of the DVD spec. Some mastering houses do pay attention to the flags produced by their encoder, and some do view the disc on progressive players just for quality control, but that's not at all required.

In short, the content on a DVD is interlaced conceptually, and is stored in interlaced sequences. Frames can be marked "progressive" to help compression, but are not always marked that way, even when it would be correct to do so. In interlaced sequences, the encoder can either keep the fields separate, or combine them together into one frame, whichever is best for compression purposes. There is a flag on each image stored in the MPEG-2 stream called “picture_structure” that can be either “frame” for a full 720x480 pixel frame, or “top field” or “bottom field” for a single 720x240 field. (We’ll learn about top and bottom fields later.) And it is allowed, but again not required, to set a flag called “progressive_frame” as a hint to the decoder that the fields in that frame were taken from the same frame of film. This allows for better pause and slow motion modes, and better down-conversion of 16x9 images for 4x3 displays. But this is again, purely optional. The content will play fine whether the data is structured as fields or frames, and whether the flag is present or not.

In fact, the encoder is allowed to combine fields that are not from the same film frame together, as often that produces better compression, even for inherently interlaced video. In such cases, the encoder is not supposed to set the progressive_frame flag, but again, if it does happen to get set, it will make no difference for normal playback on an interlaced display. And since interlaced displays are the only thing DVD was designed for, sloppiness with flags is more common than you’d think.

The flags on the disc, and the structure of the frames, are purely hints. Interlaced video can be stored on the disc as frames, and progressive frames can be broken into fields. The progressive_frame flag can be there or not. It doesn’t make any difference for interlaced playback. As we will see, though, it can make a big difference for progressive playback.

[Sean Adair]

Originally Posted by Stephan Ahonen

I can't find my source but I'm very sure I've read that the HDV used by the HD100 operates at 60fps and uses repeat flags on 24 and 30p material. Yeah, useful, I know.

720p60 downconversion for standard def is a no-brainer. 60i, not 30p. By going wtih 30p you're losing half your temporal resolution and making it look juddery. And I've mentioned before that going 30p will not result in any more vertical sharpness because 30p in standard def is treated exactly the same as 60i.

When you watch standard def TV (especially sports), often times you're looking at downconverted 720p60. I can't name a single broadcaster that chooses to maintain a progressive look in downconversion at the expense of temporal resolution. They all downconvert to 60i.

Well if the HD100 recorded 60p, there's sure a lot of you guys out there who'd like to be using it! <gr> Also I was informed by JVC that although 24p takes the same time on a tape by tape design, on the firestore it uses less space, since less frames are recorded to the drive...

From 60p, 60i may very well be better for many things - certainly sports... but for material with more stately rhythms, viewed on progressive oriented equipment (LCD/Plasma "smart" dvd player, 30p, ensuring flagged progressive playback could have advantages over the "unflagged" 60i. Fields with slight differences (interlace artifact) would be created - although I realize that we are used to seeing these, they are a factor. 30p originated footage, WOULD be treated the same at 60i, since fields would originate from the same frame. But interlace is also known to compress less effeciently.

Progressive is inherently better for both acquisition and broadcasting, and part of why many of us chose this camera. The temporal resolution limitation is purposely chosen by many (ok often abused) witness the popularity of 24p settings with the panasonic DVX.

But bring on the darn blu-dvd's or hd-rays asap. I wish apple would put it's big posterior on the line and give us machines with the burners and software with the authoring. Like, this year!

Meanwhile progressive broadcast is also clearly the path for HD. In Europe they have this path clearly envisioned.
http://www.jvcpro.co.uk/getResource2...bc.pdf?id=6594
article "EBU backs emerging progressive standard"
60p HD broadcast, 720 now, 1080 to come. Leave interlace behind!

[Stephan Ahonen]

Originally Posted by Sean Adair

Well if the HD100 recorded 60p, there's sure a lot of you guys out there who'd like to be using it! <gr>

While the container stream is running at 60fps, the encoder in the HD100 isn't sophisticated enough to fit 60p into the bandwidth allotted to it, which is why it records 30p with repeat flags to fit in the 60p container.

[Sean Adair]

I realize the encoder is the weak link here. But why waste bandwidth with 60 frames instead of 30?
I was just teasing about the frame envy issue...

BUT, the firestore taking up less drive space with 24fps than 30fps is the fact that makes this 60 frames stream with flags seem strange. I don't have a firestore, but I remember this clearly as an interesting and useful feature in the JVC presentation. Going to tape, they have the same size file.

This isn't a challenge, Stephen. I'm just curious to have a better understanding of this stuff!

[Stephan Ahonen]

The repeat flags don't take up any (well, hardly any) bandwidth at all. They just tell the decoder "play this frame twice."

Without more detailed knowledge of the HDV spec I couldn't say exactly why this is done. I know DVCPro HD can only do 60fps in 720p, and Varicam uses the same technique to record slower frame rates on tape. Maybe HDV is the same way.