View Full Version : HDTV encode - what is a reasonable bitrate?

12th November 2005, 08:32
I am going to be converting 1080i HDTV material.

I am trying to determine what is a reasonable bitrate to use?

I will be storing data on my NAS and playing it back with a soon-to-be purchased Avel Linkplayer 2.

Obviously the higher the bitrate, the better the quality, but it will also take longer to encode, higher storage requirements, and ata a certain point could cause playback problems for the linkplayer.

Additionally, I understand that if you ask 100 experts what the "best" setting is, you will get 100 different answers.

So, that being understood what is generally accepted as a decent starting point for 1080i Xvid encoding?

1 GB/hr - approx 2200 Kbps?
2 GB/hr - approx 4600 kbps?

Thanks for the inputs.

12th November 2005, 13:36
What resolution will you be encoding it at? This is the critical question. Is your 1080i input truly interlaced (i.e. interlaced video camera source), or is it progressive material (i.e. created on film) which has been converted?

The problems with interlacing at HD are the same as at SD. If the original is progressive then you should be able to recover the full progressive frame through IVTC and it would be appropriate to encode at the full 1920x1080 res if you don't want to sacrifice quality. If it's an interlaced source you have more problems, and need to consider whether you want to try some sort of deinterlacing adaptive blend while retaining the resolution, or simply cut one of the fields and resize it down. These problems are why many appear to consider 720p to be superior :/.

To give you a rough guide, I've seen some very good xvid encodes done of material at a res of 960x544 with a bitrate of 1444kbps. This is done by experts who've tweaked the settings, and probably used custom matrices as well. You could try scaling this rate up to the resolution that you'll be using then tack on a bit more for safety.

13th November 2005, 01:02
Thanks for the info,

I should have been more clear on my encoding plans, I was planning on not resizing and encoding it at 1080i as well.

The first problem is that I don't think my TV will display 720p, but I know that if the Linkplayer is set for it, it will always output 1080i, so I figured it would be best to properly process the source material instead of letting the player convert it.

The source is coming from a Comcast HDTV DVR, so I would suspect if anything it is hybrid material.

I have been treating it as interlaced and running
as part of my AVS script.

14th November 2005, 03:26
Well if you're encoding frames of 1920x1080x29.97fps and aiming for a bits/(pixel*frame) Qf value of 0.14 (which is reasonably safe - some get away with 0.11, but they're wringing the most out of the encoder) you'd need 0.14*1920*1080*29.97 bps = 8.3Mbps. Even if you get the Qf down to 0.11 you're still looking at around 6Mbps.

[Edit]It is possible that you'll be able to get away with a lower Qf value for HD encodes than you would for SD encodes on the same size display, as macroblocks will be rendered much smaller. If you really want to retain quality on HD encodes without massive bitrates though, I'd suggest using h.264 (MPEG-4 AVC) instead of xvid.

Sorry, just remembered you want to play it on a standalone player, so that rules out h.264.

14th November 2005, 04:43
SilentBob have you done any encodes yet? I'm curious as to how long yours will take.

I have converted 1920x1080 files to SD resolution, and the last one I did took six hours on my 3.2GHz P4. I'm assuming yours will take an order-of-magnitude longer perhaps (so maybe you're still running one).

I converted only one to a >SD file, 1056x592 @ 2141Kbps and bits/pixel of .143, some 4.6Gb to 700Mb, and it looks darn good, holds-up well to the original HD. But this took over 9 hours to convert.

14th November 2005, 05:17
charleski, thanks for the inputs.

That is somewhat disappointing I guess, perhaps I'm going to have to wait until my player arrives before I start encoding. I was hoping to get a head-start.

Looking at the player FAQ, it looks like I may have to limit things to 1.5Mbps, but that doesn't sound like it is going to give me the greatest results....


laserfan, I've done some experimental encodes to figure out the tools, but I haven't done any "for real" encoding.

I don't care too much about time...I have a second computer that I use for my video work, it is only a 2.4 P4. What are you using to play back the recorded content? 1056 x 592 seems like a strange resolution

14th November 2005, 07:14
Check out the link SilentBob posted. DVD Player Manufacturers should use that FAQ as a standard for their own sites.

14th November 2005, 19:15
While the FAQ they provide is certainly a lot beter than most, I think the 1.5Mbps value they give for XviD and Divx is a bit misleading. Their player is certified for Divx HD, and the Divx HD FAQ says What is the bitrate for DivX HD videos?

DivX compression delivers 720p HD resolution video at 4Mbps. This means a full-length (approximately 90 minute) DivX HD movie can fit on one DVD.So they must support 4Mbps at least!

I think they were just being needlessly conservative for those values. They have the bandwidth inside the device to accomodate up to 25Mbps, so you should be safe with a 6-8Mbps XviD stream.

14th November 2005, 19:29
The Player is DivX High Definition certified, and that seems to be 720p @4Mbps, and you should be able to get more or less the same bitstream with Xvid, but that wouldn't fit with the 1,5 Mbps restriktion... I'm afraid unless you find a good datasheet somewhere, or someone who knows what bitrates, Mpeg4 ASP/Xvid features and other stuff the player supports you will have to just try it out till you find something that looks good and plays well.

Edit: Too slow...

I wouln'd try HD encodes at much less than 4 Mbit. I haven't done any, but if you look at the bitrates for lower res and estimate it for 1080i, depending on source, settings, your personal taste and other stuff I'd guess 4-8 Mbps should be reasonable in most cases. But that's just my guess.
The ~8.3Mbps mentioned earlier would be for good quality 1080p res if I'm not mistaken, and as far as I know Interlaced has ~half the bitrate, so it would also lead to ~4Mbit, so I would take that as a starting point if the player supports it.

If you really are restricted to 1,5 Mbps I would forget about high quality HD content. (Maybe you could try 720i with optimized encoder settings, but I'm not too sure about that)

Edit: The supportet bitrate for Xvid probably depends strongly on the used features. With the Divx HD standard features ~4Mbps, with max Xvid settings about 1,5 Mbps? And with easy MPEG2 about 10Mbps. I think that could fit...

Cu MM.

14th November 2005, 20:04
Yeah, that's true. Instead of using TomsMoComp (which converts it to a 1080p frame) try leaving the stream as-is and switching to interlaced mode (in the Profile@level dialog of the XviD encoder). All my encodes have been progressive, so I can't help much further on that, the best thing is to test and see.

I'm not sure how much this will save you on bitrate, a 1080i stream is actually made up of 1920x540 fields at a rate of 59.94Hz, so you still have much the same amount of information. It might allow the encoder to detect repeated fields (from any telecine 2:3 pulldown) and save the bits it would waste on encoding the same field twice though.

14th November 2005, 20:30
Thanks for all of the inputs. I'm doing a few encodes at various bitrates right now to get ready for the player arrival. I'll post more details once it arrives and I do some testing.

14th November 2005, 20:40
...laserfan ...What are you using to play back the recorded content? 1056 x 592 seems like a strange resolutionI have a Pinnacle ShowCenter SC200, which has the Sigma Designs EM8620L as I think does the LP2--that's why I'm into this discussion. It'll play whatever resolution you want!

I dunno why Avel would post 1.5Mbps as a max bitrate. The 2000+Kbps file I did plays fine, and the 'King Kong' trailer in Xvid (1280x720) is at 3Mbps and looks pretty sensational on the SC200.

I do agree though that you should get your LP and play with it a bit before deciding these things. While you may start-out thinking that you need to approach 1920x1080i@15Mbps (the MPEG2 of which also plays fine on the SC200 BTW) you will find that once you start building your library that you just never have enough storage space. In any case finding your "sweet spot" (that point where you trade-off playback quality with resolution/bitrate) will not be easy and may take you a while before you decide what you want to live with.

16th November 2005, 18:50
I would recommend using the eqm_v3ulr custom matrix and keeping the AC3 sound, after all you'll be using 2k+ bitrates for video and AC3 is a lossy compression format. With that in mind, with a resolution of 1920x1080 you could use up to 105 minutes on a standard DVD-R or 195 minutes on a DL DVD. With a resolution of 1280x720 you could fit 210 minutes on a DVD-R or 390 minutes on a DL DVD.
I think I remember a post from trbarry saying that the effective resolution of ATSC HDTV is 1280x720, so I'd advice you to IVTC and downsize to that. You can fit twice the content, won't lose much quality and it will be easier on the player.