DTT bitrates and MPEG4 encoders

Apogee

RTÉ (or is it RTÉNL?) seem to be making some recent changes to the bitrate of the DTT channels. In December, they were averaging about 3-3.6Mbps. More recently, they brought this down to 2.3-3.0Mbps. However, in the past week or so, they seem to have adopted a fixed bitrate of 2.7Mbps for each of the channels.



That leaves a substantial 9.7Mbps of unused bandwidth for "experimenting".

I haven't seen anything in press releases or newspaper articles, but does anyone know what type of MPEG4 encoders they are using?

gtg60

That is odd and a bit dissapointing, with the bandwidth they have they should be trying to make the channels as good as possible.

What are you using to be able to measure the bitrate like that?

STB

Thats parse info generated by TSReader gtg60.

Apogee why is it generating MPEG2 stats? Audio ?

At a guess - the encoder could be a Tandberg or perhaps something from Harris given RTE NL's Monitoring Centre is a Zandar Predator II multiviewer.

The Cush

RTÉNL published an Invitation to Tender (ITT) for provision of a Digital Multiplex Head-end System Donnybrook in Feb capable of encoding and multiplexing.
Indicative Min-Max Bitrate per video channel 1-3 Mbps.

RTÉNL wishes to tender for a digital head-end system capable of encoding and multiplexing:
1. 8 SD video services including the embedded audio and teletext components
2. 8 radio services
3. 1 MHEG-5 data service for digital teletext
4. Provision of a PSI/SI server for generating PSI/SI tables and EPG data
5. Provision of components for System Software Updates

fat-tony

STB wrote: »

Apogee why is it generating MPEG2 stats? Audio ?

I'll answer that Apogee if you're not about!

The transport stream is defined by MPEG-2 - it's a standard TS, so that's whats being reported overall.
The compression used in the video streams is, of course, H.264 so that bit rate refers to the compressed video, which would be much higher quality that the equivalent MPEG-2 compressed stream at the same bit rate. However, the fact remains that Apogee measured higher bit rates earlier on in the tests, so that implies that either RTE have reduced the quality by increasing compression or perhaps are using more efficient compressors.

They've also increased the audio bitrate in the last week or so. It was going out at 192 Kbps from launch until about Christmas when it was dropped down to 128. It's now back at 192.

Apogee

The MPEG2 stats heading is most likely a hangover when transmissions were only MPEG2, and the software author hasn't got round to changing it to something generic like "Stream Statistics".

The Cush wrote: »

RTÉNL published an Invitation to Tender (ITT) for provision of a Digital Multiplex Head-end System Donnybrook in Feb capable of encoding and multiplexing.
Indicative Min-Max Bitrate per video channel 1-3 Mbps.

Interesting, but what have they been using up until now? And are the encoders being used now for SD the same ones used for the "HD Ireland" and Leinster finals?

watty

Any commercial/Professional head-end for broadcast will use variable bit-rate statistical Multiplexing. These rather expensive real time encoders have various advantages over CBR as typically used on streaming/IPTV or VBR as used in files.

1) Bitrate varies according to scene and movement complexity
2) If the other channels don't need so much bitrate, then higher bitrate can be used for more complex scenes or more motion
3) High and low bitrates can be interleaved to an extent.

This is because unlike DVD, BluRay, DivX file, IPTV or YouTube streaming up to 10 or more channels can be sharing the same overall bit stream.

This is one reason why similar bit rate broadcasts will always be better that the IPTV version. On the IPTV resolution has to drop with sudden scene change. IPTV can even suffer pixelation at "same" bit rate.

A more expensive realtime encoder even on constant bitrate can deliver better quality than a cheaper one too. Only the most expensive "stat-mux" head-end real time encoders can rival multipass VBR off-line encoding for DVD/BD.

Typically MPEG4 H.264 used to use half the bit rate as MPEG2 for 720x576 (SD) for same quality. The latest encoders (at about $25,000 per channel) claim to do even lower bit rate for same quality.

With "offline" encoding I found that 800kbps average VBR h.264 could outperform 2Mbps CBR realtime DivX MPEG4 easily and look similar to DVD quality.

Apogee

Bitrates up to 2.46Mbps. Interesting addition to the test channels EPG info:

radiospan

Apogee wrote: »

Bitrates up to 2.46Mbps. Interesting addition to the test channels EPG info:

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Didn't someone here see their TV reporting 1920x1080 for a blank screen on a test channel a few months back?

watty

2.46Mbps is still only SD.

HD is 5Mbps to 25Mbps

Apogee

They are doing some sort of HD tests this morning. Although the software is still reporting the same resolution for the RTENL testcard, they are pumping out a very high bitrate of 8.2Mbps while the amount of spare capacity on the mux has been reduced from ~10 Mbps to 3.09Mbps.

watty

They could be simply testing bitrates.

STB

In conjuction with capacity testing, perhaps

Apogee

I've just posted some info on constantly updating service type flags, but one that caught my eye was the HD flag for the RTÉNL Testcard.

fat-tony

DVBViewer is reporting the frame rate on the RTENL test channel as 50fps, as opposed to the normal 25fps of the others. As reported by Apogee also, I have noted the flagging of RTENL channel with the HD flags, although it's still 720x576 rather than any usual HD resolution.

Apogee

Current bitrates varying between 2-3Mbps for the TV channels.



RTÉ1, TV3, TG4, RTÉ News Now - 544x576
RTÉ2 - 704x576

All the DAB radio stations now have EPG data.

They are tagging the TV channels as "digital television service". Did someone in RTÉNL buy themselves a Panasonic Freesat TV for xmas?

fat-tony

I've noticed that DVBviewer is not auto switching to 4:3 mode on DTT when a non-widescreen programme is being broadcast. Works fine on Satellite. Do you have a similar problem Apogee?

fat-tony

fat-tony wrote: »

I've noticed that DVBviewer is not auto switching to 4:3 mode on DTT when a non-widescreen programme is being broadcast. Works fine on Satellite. Do you have a similar problem Apogee?

I think my version is the same.

Apogee

I don't use DVBViewer so can't comment either way.

fat-tony

Apogee wrote: »

I don't use DVBViewer so can't comment either way.

I know that you don't use DVBviewer, but was asking if your software auto-switched to 4:3?

It looks like all channels are fixed for widescreen on DTT rather than switching correctly. Just wanted to eliminate DVBviewer software as a potential source of the problem.

dahak

I looked at the Ireland vs. France match in November on a mac with EyeTV. I was also streaming the video around a LAN so I was keeping an eye on the bitrates.

The bitrates were above 7Mbps anytime the camera moved quickly and I think they got close to 8Mbps at the peaks.

Apogee

Video only:

RTÉ1 (0x044d) - 2.37Mbps
RTÉ2 (0x044e) - 4.84Mbps
TG4 (0x0450) - 1.88Mbps
RTÉ News Now (0x0451) - 3.00Mbps
RTÉNL1 (0x044f) - 1.78Mbps
RTÉNL2 (0x0452) - 2.05Mbps
Null pids (0x1fff) - 3.02Mbps

This is just a snapshot as bitrates and relative share of bandwidth per channel vary over time.

Apogee

This first snapshop was taken when only the HD testcard was displayed at 1.97Mbps




This is taken with the HD video content running at 6.85Mbps.



All of the SD channels have been squeezed down. Only 1.1 Mbps of spare bandwidth to play with.

Sam Russell

I assume TV3, if they had to pay for it, would take more bandwidth than the testcard. Just.:)

Apogee

When the HD video is playing, the TC (which is the likely placeholder for TV3) is reduced from 1.94Mbps to 1.09Mbps (RTÉNL1 - 0x044f). RTÉ1 (0x044d) was running to 1.87Mbps, so you wouldn't necessarily need a huge amount of extra bandwidth to cover video on TV3.

You also have to factor in that RTÉ2 (0x044e) at 2.19Mbps is running in parallel with the HD channel in the test, whereas when it rolls out proper, it will be the one channel, so you'll have a bit more bandwidth to play around with.

Sam Russell

If we have the choice, RTE2 in HD or TV3, I know which I would go for.

stwome01

I've been reading this information and I find it interesting. I have a few basic questions though. What is the maximum Mbps per Mux? I presume this is dependent on the transmission format ie. DVB-T Is the number of television channels per mux only limited by the bandwidth available and encoding type used? When a tv say H.264/AVC MP@L3, MP@L4.0, HP@L4.0 does this mean that is can decode any signal upto 25Mbsp or are these different encoding types. Ie. if a signal is transmitted at HP@L3.0 will the tv fail to decode the signal? forgive my ignorance. Thanks.

watty

The Mbps depend on

• Channel Width (fixed at 8MHz)
• QAM used (More gives More, but needs much stronger Signal. Can be QPSK to 64 realistically for Radio Waves, 256 on ordinary Coax and even 2048 on some specialist systems) Typical DTT is 16 for long range or 64 for capacity. I Think 64 is choice.
• Guard timing. Typically wastes about 10% of capacity (1/16th?). So that mobile works and typical Multipath (Ghosting on Analogue) can increase SNR (Eb/No). Possible values are 1⁄4, 1⁄8, 1⁄16, 1⁄32 for DVB-T
• Overhead of FEC. (Forward Error Correction). Satellite can be as high as 1/2, 2/3 or as low as 7/8 for feeds (big dish). Cable/DOCSIS is typically 10% FEC. I'm not sure how much DTT is using. More means less useful data, but greater interference and noise immunity.
• FFTs used. This is complex, but has a minimal impact on bitrate. 2K DTT (2048 FFTs) is about 1700 carriers. DVB-T is COFDM with 2K or 8K FFT. The new DVB-T2 takes advantage of more DSP power to have 1k, 2k, 4k, 8k, 16k, 32k FFTs.

My impression is that after all system overheads, the actual maximum MPEG2-TS rate is about 25Mbps per Multiplex. Cable TV can run close to 50Mbps in 8MHz and Satellite needs about four times the transponder spectrum to achieve the similar data rate to a terrestrial Multiplex.

Average Bit rates or Constant bitrates are only really meaningful on DVD or IPTV on DSL. The best use of Transport stream on Broadcast is to do encoding of each channel in a relationship using Statisical Multiplex. This uses extremely variable Variable Bit Rates (perhaps 0.2Mbps to 6Mbps for SD and 0.2Mbps to 20Mbps for HD) with as low an average so that over time all the channels add up to the Multiplex rate. This allows sudden peaks in bitrate for quality scene changes and movement avoiding the nasty pixellation on such content on CBR encoding typical on DSL IPTV. Simplistically the peak data (rapid changes) is "interleaved" with lack of change (very low bit rate) on other channels. There is obviously a trade off between increased latency and more flexible multiplexing/Encoding.

The State of the Art MPEG4 Encoders rather than x2 compression compared with MPEG2, can deliver similar or even better perceived quality in average bitrate of 1.5Mbps using Variable Bit Rate encoding in a Stat Mux, compared to 6Mbps CBR MPEG2 needed for IPTV on DSL. It will exceed quality of 2.5Mbps CBR MPEG4 on IPTV DSL as it can have very high peak rate.

"H.264/AVC MP@L3, MP@L4.0, HP@L4.0 does this mean that is can decode any signal upto 25Mbsp"

No, it's not related.
The bitrate of the transport stream (MPEG2-TS) can be very high. Each system has limits. Then the bitrate of a video in it is a separate issue.
""H.264/AVC MP@L3, MP@L4.0, HP@L4.0" are encoding/decoding standards, codecs. Actual bitrate is a separate issue, the range of allowable bitrates can depend on platform.

stwome01

Thanks Watty for the very comprehensive reply. Your response really makes me realise the balancing act involved to the best out of a small frequency bandwidth. I presume RTENL are basing some of their decisions on experiences other European countries are having with their DTT setups. Do you think RTENL will stick with DVB-T for the foreseeable furture?

One big question that occur to me while reading your email is why satelite tramission is cheaper then terrestial?

I was guessing that "H.264/AVC MP@L3, MP@L4.0, HP@L4.0" were some short of codecs but I had never heard of the these variations for H.264. Is the RTÉ transmission pure MPEG4 or is it some variation of H.264?

STB

stwome01 wrote: »

I was guessing that "H.264/AVC MP@L3, MP@L4.0, HP@L4.0" were some short of codecs but I had never heard of the these variations for H.264. Is the RTÉ transmission pure MPEG4 or is it some variation of H.264?

These are dvb hardware requirements found in hardware.

MP is the Main Profile Level. For instance L(evel) 3 is SD.
HP is the Hgher Profile Level. [EMAIL="HP@L4.0"]HP@L4.0[/EMAIL] isused for HD.

MPEG4/AVC is H264 Part 10.