OTA HD: CBR vs VBR

[Megahurts]

I capture OTA HD programs and convert to mpeg with HDTV2MPEG2.

Bitrate Viewer reports ABC, FOX, PBS as VBR and NBC, CBS as CBR. I checked a CBS football clip and Bitrate Viewer says average bitrate is 13.2mbps and peak is 16.6mbps. Sounds like VBR???

Does NBC & CBS use CBR?

BTW: GSpot reports all ts files converted to mepgs are VBR. Who do you trust???

Chas

[aedipuss]

well bitrate viewer is about 5 years old and has a known issue with anything having a bitrate that goes over 10mbps............

[edDV]

I capture OTA HD programs and convert to mpeg with HDTV2MPEG2.

Bitrate Viewer reports ABC, FOX, PBS as VBR and NBC, CBS as CBR. I checked a CBS football clip and Bitrate Viewer says average bitrate is 13.2mbps and peak is 16.6mbps. Sounds like VBR???

Does NBC & CBS use CBR?

BTW: GSpot reports all ts files converted to mepgs are VBR. Who do you trust???

Chas

All that is determined by your local broadcaster. Give them a call. They get the feed from CBS at ~45 Mb/s and sample down to approx 14-19Mb/s. 19Mb/s is the total ATSC bandwidth. They are probably puttting out a second SD channel.

[Megahurts]

Thanks for the responses.

I would like to here from those who capture NBC/CBS HD files and verify if Bitrate Viewer says it's CBR or VBR.

Also, is there any other program that determines the type of bitrate of an mpeg file?

Chas

[edDV]

Here is a sample broadcast encoder and multiplexer. They do far more than simple VBR. The multiple muxed broadcasts statistically share bandwidth based on the relative motion of the various subchannels factored by priority set in preferences by the local brodacaster. For instance, the weather channel might get starved for bitrate to favor the HD footaball game.
http://www.harmonicinc.com/view_csd_product_group.cfm?classID=503
http://www.harmonicinc.com/view_csd_product_group.cfm?classID=2411

This PDF introduces the concept of statistical multiplexing.
http://www.harmonicinc.com/stageone/files/harmonic/collateral/DiviTrackXE%5FWP%5F022503%2Epdf

[Megahurts]

edDV, thanks for your concern.

What I would like to know is if a network downloads a 45Mbs program, do the affiliates have the option to broadcast their reduced HD transport stream at CBR or VBR.

Just courious as to the other NBC and CBS affiliates whats broadcast, if it's CBR or VBR, on their HD channel.

Chas

[edDV]

edDV, thanks for your concern.

What I would like to know is if a network downloads a 45Mbs program, do the affiliates have the option to broadcast their reduced HD transport stream at CBR or VBR.

Just courious as to the other NBC and CBS affiliates whats broadcast, if it's CBR or VBR, on their HD channel.

Chas

They have the option. Small market stations like Redding are probably spending as little as possible on their DTV channel until 2009. CBR is probably the cheapest way to go if all they are doing is reprocessing the network feed into the full 19Mb/s bandwidth of their digital channel.

Each station also needs a SD to HD upscaler to play 480i commericals and local breaks. Their broadcast day on the DTV channel is a switch between the network feed (HD or upscaled SD) and their local SD upscale. The network upscalers are usually much better than those used locally.

Very few stations are originating HD locally but that is beginning to change this season. The San Francisco and Sacramento stations are beginning to experiment with local HD origination and HD commercial inserts. KABC in LA began a local HD news broadcast last year using techniques similar to ESPN's "SprortsCenter" for mixed 720p, 480i and 480p.

Larger market stations and most PBS mid to large stations are doing multiple subchannel broadcasts sharing the same 19Mb/s main channel. It is in this mode that statistical multiplexing makes a huge difference in picture quality. It looks to the viewer like VBR but it is more. Read the PDF above. I suppose there is some less sophisticated equipment available that does DVD style VBR.


PS: In case the above isn't clear, we are talking about sophisticated hardware encoders and multiplexers here. If you are curious what equipment is used at each station, give the chief engineer a call. They usually will be up front on such things and may discuss future plans for HD.

[Soopafresh]

Too bad the muxing algos aren't very good. It is ridiculous when you're watching Leno and the musical guest turns into macro blocks because of a fluctuation of the stage lights. Then you find out there's "Doppler Radar" on other PIDs and some infommercial of yet another PID - all of that bitrate could have been allocated to the main program, but nooooooooooooo......

[jagabo]

They know who pays the bills.

[edDV]

This where cable and DBS sometimes get better bitrate than what goes out over the air. Some DTV stations feed each subchannel separately at higher bitrate over fiber to the cable company and DBS uplink. Others feed the same mux that goes to air.

Quality also differs by how many SD channels they broadcast along with the HD main channel and how they weight the bitrates among subchannels.

[Soopafresh]

edDV - thanks for that info, you solved a mystery for me