A friend of mine was nice enough to burn me a DVD-R of a 50-minute documentary I created a while ago. He’s a professional video editor, so he should know what he was doing.
Three times I’ve tried to make VHS dubs off of the disk, but all attempts were thwarted by a glitch somewhere in the works. The movie will play fine for a while, then freeze.
Here are the symptoms:
So far, each freeze-point has been different; the earliest one was about 20 minutes after the start of the doc. The audio goes silent when the video stops. The screen does not go blank/black/blue; it freezes on the existing video image. Hitting the PLAY (or any other) button does not make the playback resume; I have thad to power down the player to start again. The bottom side of the disk appears clean as a whistle, but I wiped it between trys anyway and it still froze.
The player is, admittedly, a cheapo Ampex AD-2100, but it has always performed like a champ for me in the past. I checked the troubleshooting chart in the back of the manual and it makes no mention of this problem or how to solve it.
Assuming it’s not commercially encrypted simply burn the flaky DVD onto another DVD-R or DVD+R burnable DVD disk and see if the problem repeats with the new disk. If it does then it’s an original media transfer issue, and short of dumping it into an DVD editing program on your PC and fixing the munged sections, then recompiling and burning a new copy there’s not much you can do.
If you do have a DVD burner and a media editing program like Roxio 7 or 8 you may be able to fix it yourself.
Even DVD burner drives can have an issue reading a brand of media it was not designed to read. The different dyes used by manufactures are only garanteed to be compatable with their equipment. I can put a DVD media into 3 players and only one plays back the content. Commercial DVDs are made in a different way and the DVD players are made to read the commercial DVDs. More data is stored in the same area on a DVD than a CD, so the laser must track and focus better.It’s easier to mess up the lasers focus due too laser light and dye incompatability. Have him try to burn it on a different brand medium. Spend a little more and try a RW media. You can use it hundreds of times and an error in burning means you don’t keep using new DVD blanks. Have him burn it at a slower speed, this can be the difference between a readable DVD and garbage.
The video on DVDs is encoded (perhaps obviously) as a series of frames, but there are different kinds of frames, called I P and B frames -these work approximately like this:
I frames are complete frames
B frames are nothing more than information about what is changing between frames - they are encoded relative to both past and future I and P frames
P frames are a sort of hybrid between I and B - they contain some static image data and some motion/change data, but relative to past reference frames
I and P frames are ‘reference’ frames, the information in the B frames can only be interpreted in their context, so in order to display the video footage, the player needs to see an I frame, then some B frames, perhaps a P frame, then some more B frames, then another I frame etc.
If the player fails to read a reference frame, it will be unable to properly construct the video footage expressed in the B frames - some players will attempt to display some kind of moving image (but it will break up in a blocky way, because the B frame information is being interpreted against the wrong reference frames. Other players will simply not try to refresh the display until they get another reference frame.
The pattern of I P and B frames is not fixed, it can be laid out pretty much any way you like when encoding the stream; fewer reference frames means greater overall compression, but bigger problems if something goes amiss. Furthermore, the bit rate - that is, the amount of data that will be used to describe the individual frames, is variable too - the temptation is to use a really high bit rate for quality, but this can sometimes mean that the read buffer in the player fills up and it skips a frame - if the skipped frame is a reference frame, then the picture will freeze or break up.
Commercially-produced DVDs are (or so one would hope) encoded with great care to ensure that the pattern of frames and the bit rate are tweaked to ensure optimal quality - leave it too long between reference frames and the image reconstructed from the B frames will look ‘ragged’ - put in too many reference frames and you use up the available storage space on the DVD too quickly; too low a bit rate means you will start to see blocky compression artifacts, too high a bit rate means you risk saturating the buffers and skipping frames and also that, again, you use up the available space too quickly.
The resources and expertise available to the film industry make is just more likely overall that they will produce a DVD that plays properly on the widest range of players; the same is unfortunately not true for the amateur.