DVB-T - encoder from hell?

[savage747]

Hello,

this isn't supposed to be a "DVB-T sucks" rant or anything like this. Most broadcasters seem to offer usable quality over DVB-T. However, there seems to be one notable exception here in Germany: ProSiebenSat1. Their DVB-T channels usually look worse than others and I can't really explain why.

Here are 3 short samples:

http://rapidshare.de/files/10085323/dvb-t.zip.html (sorry, german page. Just click "Free", wait until the countdown expired (20 seconds) and type in the three randomly chosen characters displayed)

horrible.m2v: average bitrate ~3600 kbit/s, peak ~4200 kbit/s

Hi-motion scene. This really is a bit difficult to encode.

horrible2.m2v: average bitrate ~2900 kbit/s, peak > 4000 kbit/s

Well, this one should be much more easy as there is little motion. However, artifacting is cleary visible. What's most distracting: The audience seems to "recover" with every keyframe.

horrible3.m2v: average bitrate ~2680 kbit/s, peak ~2900 kbit/s

Very soft (but color-saturated background) and a fair bit of movement. Again, not exactly easy to encode...


Perhaps I'm a bit picky but I think I've seen much better encodes at similiar bitrates with MPEG2. I am aware that those encodes are most likely done on dedicated hardware with realtime constraints and that they won't give optimal results. Having toyed around with libavcodec, full PAL resolution, vbitrate=2000 and keyframe interval forced to 10 I come to the conclusion that the encoder used by this broadcasting network isn't exactly state-of-the art, though. Other networks deliver better quality so I wonder if some encoder-from-hell was used here...

So... is it just me?

[bond]

if they use a crappy encoder quality will be worse at the same bitrate than if they would use a better encoder (or a better format, like avc)

[sillKotscha]

horrible.m2v: average bitrate ~3600 kbit/s, peak ~4200 kbit/s

well, that one really isn't very nice...

but my guess is you should spend some money in a better aerial/antenna. For example with an integrated amplifier.

I don't know where you are located but here in Berlin I got satisfactory results - always keeping in mind that DVB-T isn't top notch.

this is my example of SAT.1 (http://s40.yousendit.com/d.aspx?id=3JZTXXR7M38CS02P9ZLAQA7XF0) - not really comparable but to get an impression of my aerial reception...

Sill

P.S.: don't bother about the extract - it's just a quick capture and not my favorite tv show :D

P.P.S.: and always remember... perhaps a crappy encoder is used (as bond said) and it's on the fly encoding

EDIT: if you demux the *.pva file you'll get a stereo AC-3 soundfile ;) and of course 5.1 AC-3 if it is broadcasted... I love my DVB-T capture card !!

[savage747]

but my guess is you should spend some money in a better aerial/antenna. For example with an integrated amplifier.


Uh, DVB-T is digital broadcasting (as you know). If there are no bitstream errors (and in those clips there are none) quality will be exactly the same no matter what antenna was used.

Edit: I'm in DVB-T region "Ruhrgebiet", btw. Your clip is average 2200 kbit/s and indeed looks rather nice for that bitrate.

[sillKotscha]

no matter what antenna was used.

I'd like to invite you to a coffee over to my house and show you the opposite...

because I gave up arguing - I've already convinced 2 friends of mine just by showing them a real life scenario.

[savage747]

I'd like to invite you to a coffee over to my house and show you the opposite...


Well, I'm afraid I'm living a bit too far away - but thanks for the offer :)


because I gave up arguing - I've already convinced 2 friends of mine just by showing them a real life scenario.


Sorry, but there is no way different antennas can give different picture quality as long as the signal is good enough to enable the error-correction to give bit-identical results to what has been broadcasted.

If the antenna is bad enough to give uncorrectable bit-errors there obviously will be *horrible* artifacts - but those are usually very very distinct from usual quantization artifacts in badly encoded MPEG2 streams.

However, my antenna is usually good enough to enable reception without bit errors passing the error correction.

status 00 | signal ffff | snr 0000 | ber 00000000 | unc 00000000 |
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a9a9 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a8a8 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a7a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a6a6 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a8a8 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a5a5 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a6a6 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal ffff | snr a4a4 | ber 00000000 | unc 00000000 | FE_HAS_LOCK

(unc = unrecoverable bit errors)

[sillKotscha]

Sorry, but there is no way different antennas can give different picture quality as long as the signal is good enough to enable the error-correction to give bit-identical results to what has been broadcasted.

signal is a good starting point... you will recieve a better signal with an amplifier, hence a better picture (at least concerning errors - blocking artifacts etc.) - isn't the visual impression and the stream any better if you'll have less artifacts??

However, my antenna is usually good enough to enable reception without bit errors passing the error correction.

well, than save your money and be a lucky guy.

as I said... I gave up arguing, seeing is believing ;)

happy new year

P.S.: if your signal is ok for you, then you'll have to live with the fact that crappy encoders are used and that every transmission is on the fly encoded, as already mentioned

[savage747]

signal is a good starting point... you will recieve a better signal with an amplifier, hence a better picture (at least concerning errors - blocking artifacts etc.) - isn't the visual impression and the stream any better if you'll have less artifacts??


heh :)

Well, the signal the antenna is receiving is used to transport a bitstream (that's MPEG transport stream + error correction code and whatnot). If your antenna provides a good enough signal to noise ratio this bitstream can be decoded, the error codes can be applied and you have an error-free bitstream to feed your MPEG decoder with.

There's no way to improve picture quality beyond that (zero error stays zero error even if you further increase signal/noise ratio by using a higher-gain antenna).


well, than save your money and be a lucky guy.


Sure :)


as I said... I gave up arguing, seeing is believing ;)


Well, I'm still wondering what you don't want to argue about ;)

You proposed a better antenna for my situation. My setup gives error-free bitstreams, though...

If you'd argue that another increase in antenna-signal quality would boost my picture quality then I'd have to disagree. Otherwise there indeed is nothing to argue about ;)


happy new year


Thanks alot, same for you :)



P.S.: if your signal is ok for you, then you'll have to live with the fact that crappy encoders are used and that every transmission is on the fly encoded, as already mentioned

That seems to be the sad fact :-/

It's just annoying that some TV networks don't seem to care for quality even if there are better on-the-fly encoders available. Seems quality of content correlates with quality of transmission ;)

(luckily ARD and ZDF deliver much better quality)

[Latexxx]

If there isn't enough error correction information present, the picture will start to degrade by blocking etc. When there are too many errors, the signal will break completely but there certainly is a twilight zone when it comes to mpeg-2. Some other codecs will just break but mpeg-2 can interpolate the picture to some extend even if there isn't enough error correction information.

So you are both right.

[bcn_246]

Here in the UK (Freeview DVB-T) some channels broadcast as low as 2000kbps. Especially the junk ones like quiz channels. I thought live broadcasts had to be CBR and only pre-recorded could be VBR encoded. As far as I know most of the stuff I get captured from my DVB-T card is CBR. The samples dont look to bad compared to what we get here, I agree that "horrible.m2v" looks very bad, very noticably around frame 100 when the camera moves. I would be thankful you are getting the full 720x576 (interlaced) resolution, many channels here only do 544x576 (interlaced) resolution. The only solution would be to get DVB-S, the sat. stations normally broadcast at higher bitrates as you are paying far more money. I doubt it is a poor arial, when signal is low you tend to get skips in audio and video, or picture break up, not poor quality.

[DrP]

Anything can be encoded CBR or VBR, its up to the network operator, being live or not doesn't come into it.

What you may find is that some of the channels are CBR to allow for non-recoding switch in / switch out of programs - ie switch from a network wide program to a local only one taking the network wide version direct and not recoding it to keep the quality up. That lets the network operator save on link costs by only sending out a X Mbit stream to each area instead of a far higher rate which would be needed if they were going to recode the video and expect to keep reasonable quality.

There's a lot of different ways to do this sort of stuff.

[Mug Funky]

2000 kbps???

there's your quality problem.

DVB-T in australia averages around 5-6mbps for SD channels. i forget what they are for HD channels (12-15?).

at work i don't let the encoder do anything below 4000, and then it's only for trailers that i don't care about (we keep low and high bitrate versions of trailers in case there's not enough space on a disc...). preferred min bitrate is 4500 kbps. and that's minimum.

perhaps some broadcasters could sacrifice some channels (that probably play crap anyway) for better quality on the good channels.