The Bitrate Trap

I’ve been experimenting a bit with downloading recordings from the Strong SRT-5490 and the Topfield TF6000PVRt digital TV receivers to my computer, editing and then burning DVDs. But there has been a problem: time.

I’ve been using Ulead Movie Factory 3 as my DVD authoring application, and this was taking hours to prepare the video for burning. The reason was that it was re-rendering everything, but this had never been an issue with material I had ripped from DVDs. With DVDs, the video preparation phase of creating the DVD would move rapidly into video/audio multiplexing, and creating a fairly full DVD would take maybe ten or twenty minutes, plus the disc burning time.

This had been puzzling me for a while, but I’ve finally stumbled upon the answer. It is all to do with the bitrate value recorded in the video header. In general, devices sending video need to communicate to receiving devices what they are in for, so there are a number of header fields defining the format. One of these is ‘bit rate’. This field doesn’t have anything to do with the average bit rate of the video actually in the recording. It simply defines the maximum allowable within that video stream.

DVDs support a maximum video bit rate of a touch under 10Mbps (leaving some room for audio). Now the recordings I was taking were mostly from the standard definition transmissions of Southern Cross 10 here in Canberra. I’ve just examined the header of these recordings, and have discovered that they are set to 15.00Mbps. That is well above what DVD is capable of. So when you dump these recordings in Movie Factory 3, it decides that it had better re-render the whole video stream to bring it within the capabilities of DVD, thus taking forever.

This is easily solved. If you use VideoReDo to open the recording (you’ll probably want to do this anyway to edit the video), you can select ‘Options’ during the ‘Save as’ process. One of these allows you to change the header bit rate value. Remember, this value is only used to indicate the highest actual bit rate that may follow in the video stream. Obviously you’re taking bit of a chance in changing this because the station, in a fit of quality improvement, could actually use a very high bit rate in transmission. In practice, though, they don’t. The actual bit rates for SD material are well under 10Mbps. In VideoReDo, if you change this value to 10Mbps it will set an actual value with sufficient room for the audio as well.

Do this and the resulting MPEG files import into Movie Factory 3 and burn to DVD without the need for re-rendering. A pleasant side effect is that the aspect ratio is preserved, rather than reset to 4:3.

These are the header bitrate fields from the five SD channels in Canberra:

So you don’t need to change the header for SBS or WIN.