Bitrates and CBR/VBR on Toshiba recorder - explanation?

[moxiecat]

I've run some tests on my Toshiba RD-XS35, and I'm wondering if a knowledgeable person can explain what I've found.

I've recorded short clips using a variety of the bitrate settings, ranging from picture quality rate 6.4 to 9.0. Using the clip information in Womble Video Wizard, I've found that at 8.0, the recorder changes from VBR to CBR. For PQ rate 8.0 and above, clips are CBR. 7.8 and below are VBR.

I am wondering if someone can explain which is better out of these two:
Picture quality rate 7.8: 9200 kbps, VBR.
Picture quality rate 8.0: 7800 kbps, CBR.

I may be wrong, but doesn't this show that the "lower" rate of 7.8 has a better bitrate than the "higher" settings (which use CBR and actually a lower bitrate)? I'm a little confused by this and I'm hoping that someone who is familiar with how DVD recorders handle CBR/VBR and bitrates may have an answer. Thanks!

[p_l]

Short answer: VBR means the bitrate varies, therefore goes above and below the average, giving you higher quality when needed at the expense of a lower bitrate when not needed, for the same file size. Since at 8.0 you are very close the the maximum bitrate for the DVD spec, very little quality improvement would be had by doing VBR, so CBR at least keeps the minimum bitrate high, or rather doesn't allow the bitrate to dip because the bitrate is constant, in fact.

[RabidDog]

How can a recorder do VBR? VBR is generally a product of a two pass system.. a recorder just gets a stream without knowing what bitrate it requires.. I suspect this may be pseudo Vbr e.g. only slight variations above and below the set bitrate?

[samijubal]

I've never seen a recorder that didn't do VBR.

[lordsmurf]

The video is not encoded "live" but rather after a delay of a second or two, from the buffer. The buffer time allows for VBR decisions. Not large ones, but VBR nonetheless.

[RabidDog]

Ah so Pseudo-Vbr, like Pseudo-Democracy?

[wabjxo]

The video is not encoded "live" but rather after a delay of a second or two, from the buffer. The buffer time allows for VBR decisions. Not large ones, but VBR nonetheless.

Exactly. The Philips 3575 and 3576 have a 3-sec buffer.

[samijubal]

They all use a buffer, at least any decent recorder. The buffer is what allows VR recording that can be partially erased and then recorded in the erased spaces seamlessly. Without a buffer there would be pauses as the laser moved around the disc.