TMPGEnc - Setting ideal bitrate

[neo]

I have posted on this subject before. I am capturing Dishnetwork with a Canopus ADVC-100. I am then compressing with TMPGEnc to a 250 GB hard drive, not to DVD. I want the smallest file size with no quality loss.

I am using a resolution of 704 x 480. I am using the Constant Quality (CQ) mode with Quality set at 65.

I was using a max bitrate of 3400 but this seems to result in quality loss in terms of resolution/sharpness. I increased this to 5000 and it seems better.

How does one tell the Max bitrate to use without losing quality and keeping the file size as small as possible? It is hard to do subjectively.

Thanks much!

[Savant]

Constant quality at such a low bitrate is your problem here... It just won't look any good at that rate.

Switch to 2-pass VBR with an average of 2500, a max of 3500, a min of 0 and you should see a marked improvement in quality with a similarly smaller file. You could even try dropping the average down to 2000 and see if it is acceptable.

Take a small capture that you have (or make one to test with) and then try it out, I think you will find the difference worth the extra encoding time.

Regards,

Savant

[bugster]

How does one tell the Max bitrate to use without losing quality and keeping the file size as small as possible? It is hard to do subjectively.
Thanks much!

Thats the only way to do it unfortunatley.

There simply is no definate answer to this question for several reasons

Quality is simply too subjective.
Different types of material requires different bitrate settings for the same quality (Studio discussions and talking heads can get away with low bitrates where fast action movies or music videos with lots of dancing and flashing lights need higher bitrates).

Try switching to encoding at 352*480 and see if that quality is acceptable to you at the lower bitrates you desire.

[neo]

Constant quality at such a low bitrate is your problem here... It just won't look any good at that rate.

Switch to 2-pass VBR with an average of 2500, a max of 3500, a min of 0 and you should see a marked improvement in quality with a similarly smaller file. You could even try dropping the average down to 2000 and see if it is acceptable.


Regards,

Savant

I have worked with just about every function of the program and generated log files to study.

When I was testing 2-pass it seemed to give worse quality at the same file size compared to CQ. This was some time ago but as I recall from studying the log files the first pass did not appear to take samples often enough to represent the rapidily changing bitrate (BR) needs. It also appeared that in the second pass there was a much lower difference in the variation of bitrate. In other words, when the material did not need much bitrate the CQ dropped the BR much lower and then increased it much higher. It seemed much more responsive for each GOP.

With CQ at 65 and max BR at 5000 this looks much better. I believe the 65 sets the average BR at 65% of max but I am not sure on this. If so, this should give me an avg BR of 3250.

At these settings I get about 85 minutes/GB.

neo

[energy80s]

I think your bitrate is too small for your frame size. At this low bitrate, the pictrue is going to look a bit the worse for wear. For bitrates under 4Mbps I would only use Half D1 frame sizes (ie. 352x576 (480)). If you want full (or Cropped) D1 then you need to start looking at bitrates of 6Mbps plus. With TMPGenc, CQ coding is much better quality than multipass VBR, but the file size will suffer. To get decent results, I would recommend using Half D1 res.

[neo]

I think your bitrate is too small for your frame size. At this low bitrate, the pictrue is going to look a bit the worse for wear. For bitrates under 4Mbps I would only use Half D1 frame sizes (ie. 352x576 (480)). If you want full (or Cropped) D1 then you need to start looking at bitrates of 6Mbps plus. With TMPGenc, CQ coding is much better quality than multipass VBR, but the file size will suffer. To get decent results, I would recommend using Half D1 res.

Thanks for the input. When you say a BR of 6+ is that max or average? I jumped the max to 5 which looks pretty good. Dish Network does not provide DVD resolution, I do not believe. I researched this for months and received an entire range of answers. The only thing I am fairly certain of is that they use different resolutions and bitrates depending on the program. I usually am interested in paid movie channels like HBO or the most popular TV series which have the best quality. I belief they are tight on bandwidth and do all they can to conserve it especially with HD rolling out.

With my testing, I found CQ to give better quality at the same file size due to the greater dynamic range in BR per GOP with CQ as can be seen in the log files.

Are you suggesting I move the max BR to 6 with a Quality of 65?

Thanks much

[energy80s]

I normally do full D1 encodes with the CQ set to 80 and max bitrate at 8000 and min set at 4000. This will make a large file however. I imagine that Dish Network is something similar to Sky Digital here (shite quality) but to re-encode this material needs a higher bitrate than was used originally by the broadcaster. So even though DISH may only be using bitrates of 4Mbps, you will need to increase this "just to stand still" and not drop off the picture quality. I suppose 6 and 3 could be used as the max and min settings, with a CQ of say 70 or 75. With TMPG it is important to have the different bitrate settings as "half" or strange things can happen. This is not true of other MPEG encoders.

[bugster]

With TMPG it is important to have the different bitrate settings as "half" or strange things can happen. This is not true of other MPEG encoders.

Do you mean min as half of max? If so why? I have been using TmpGenc for a couple of years and never seen this mentioned by anyone. Is this specific to using CQ mode?

[neo]

With TMPG it is important to have the different bitrate settings as "half" or strange things can happen. This is not true of other MPEG encoders.

I am curious about this as well. I have been using 65 for quality in CQ mode with no problems. It is the default in the program for DVD. For VCD the default is 50. I also found a big difference in file size with perhaps better quality if I set min. to 0. This way it can use the BR where it needs it and not waste it where it is not.

neo

[neo]

I decided to look at the help file in the program and it explains more. It explains the modes. In CQ mode the Quality controls the B frame spoilage, not the average BR compared to Max. I am thinking that it uses whatever BR it needs at the time, not to exceed the Max. This makes more sense because when I increased the Max BR from 3400 to 5000 there was little increase in file size.

I use a GOP structure of 1, 5, 2: IBBPBBPBBPBBPBBPBB (18). Since 2/3 of my frames are B frames I tested changing the Quality. When I changed the Quality from 65 to 100 the file size for a short 4+ minute test increased from 70 MB to 226 MB. I did not see that much improvement in picture quality.

Now that I understand this better, I will do some more testing both with BR and Quality.

neo