CBR/VBR LQ bitrate Vs HQ bitrate to Divx

[flambot]

Greetings,

I've been trying to learn a little about re-encoding (transcoding??) with various pieces of S/ware using test captures via a video card. I've read a lot of posts and searched through the forum looking for answers to help me understand the principles. Most I don't understand, so I started testing with video to see first hand the results(especially as quality is such a subjective experience).

I've been using the "Standard DVD" capture (a card setting) of 6400-8000Mbps VBR as the test subject and re-encoding it to Divx/avi for a visual quality check on a W/S TV via XBMC. (I've had the best success with TmpgEnc XP 4 for quality and my basic undertanding)

Then it occurred to me I could adjust the bitrate of the capture card to a lower mpg setting. I tried 2Mbps (CBR) and (VBR) and also 3Mbps (CBR) and (VBR). When I viewed them on the TV I expected the CBR to be better, but in fact I thought the (VBR) looked better from both bitrates. Should this be the case? I thought CBR would be better.

I was surprised at how reasonable the picture seemed for these low bitrates (for mpg at least) and realised the video didn't necessarily need to be re-encoded (to say divx) but could be recorded using mpg at a lower rate. What things should I look out for at these mpg lower bitrates? Supposedly, fast moving video has problems with low bitrates?

Just wondering from an experienced point of view (you guys ) for quality(subjective I know) whether I should just use lower bit rate mpg or use the 8mbps video and re-encode to divx/avi for better quality. I thought a 2 hour video at a final size of 1.5Gb (approx -2gb if needed) was a good point to aim for (while okay for divx/avi, not sure if the same PQ is achievable with LQ mpg).

Thx for the help.

[Irakli]

Then it occurred to me I could adjust the bitrate of the capture card to a lower mpg setting. I tried 2Mbps (CBR) and (VBR) and also 3Mbps (CBR) and (VBR). When I viewed them on the TV I expected the CBR to be better, but in fact I thought the (VBR) looked better from both bitrates. Should this be the case? I thought CBR would be better.

VBR is better than CBR at any bitrate. The reason for this is the fact that CBR will just encode video at a constant bitrate. This means that hard to encode scenes will not get enough bitrate (hence lower quality in such scenes), whereas very simple scenes will get too high bitrate (which is unnecessary).

VBR, on the other hand, can allocate more bits to hard to encode scenes and less bits to easy to encode scenes, hence the overall quality will be better than in CBR.

I was surprised at how reasonable the picture seemed for these low bitrates (for mpg at least) and realised the video didn't necessarily need to be re-encoded (to say divx) but could be recorded using mpg at a lower rate. What things should I look out for at these mpg lower bitrates? Supposedly, fast moving video has problems with low bitrates?


Yeah, videos with fast movement, grain/noise or compex texture (such as jungles) need high bitrates to look good enough.

I would suggest you encode few samples at different bitrates and see which one gives you acceptable quality.

Just wondering from an experienced point of view (you guys ) for quality(subjective I know) whether I should just use lower bit rate mpg or use the 8mbps video and re-encode to divx/avi for better quality. I thought a 2 hour video at a final size of 1.5Gb (approx -2gb if needed) was a good point to aim for (while okay for divx/avi, not sure if the same PQ is achievable with LQ mpg).

I'm not really that experienced in video, but I think that it's better to stick with original MPG that you've captured, because reencoding will lead to loss of quality, even if your source is at 8mbps.

Regards,
Irakli

[Alex-Kid]

Some good points:
Quality captures are made with lossless codecs, or lossy codecs with only I-frames (like MJPEG and DV) as "last resort".
VBR is always better than CBR, even in audio, because bit allocation is more efficient than CBR.
Quality video (like DV footage) needs high bitrates for MPEG-2 encoding, specially on high motion scenes (water flows, camera shaking). Analog captures doesn't have high quality, the picture looks blurrier than digital ones.

Saludos

By ALEX-KID

[flambot]

Really appreciate the replies. It's helped a lot. So the reality I think is to capture using high bitrate mpg then convert (Still using a reasonably high bitrate) to another format that takes less space. That helps me move forward. Excellent! :)