Posted by: rodhew 11-26-03 12:21 PM
I finally got around to looking at the Voom DBS service with my TSReader application (
http://www.coolstf.com/tsreader) and thought some of you might be interested.
Voom operates from the 61.5 west orbital location with 11 transponders officially allocated and STA for two more. All other frequencies at 61.5 west are allocated and used by Echostar and Dominium. DBS transponders in North America are 24MHz wide with around 22MHz usable.
Voom's carriers use 8PSK modulation with a symbol rate of 22.000 Mega-Symbols per second and three-bits per symbol giving a raw rate of 66 Mbps per transponder. Inner FEC is turbo-code at a 5/6 and outer FEC is Reed-Solomon. This results in a usable bandwidth of around 51 Mbps per transponder.
In comparison, Dish Network's QPSK carrier used for HBO-HD at 61.5 has a rate of around 27 Mbps and their 8PSK carrier at 61.5 with 2/3 code-rate gets around 41 Mbps.
The transport stream carried on their transponders is MPEG-2 standard. Voom uses Motorola's Digicipher II system for both conditional access and systems (things like channel names, transponder locations, program guide etc). DCII was developed many years ago by General Instrument before they were aquired by Motorola. They were heavily involved in the Grand Alliance that came up with the ATSC specifications. Needless to say, DCII and ATSC are very similar systems.
All video in the system is encoded in variable bit rate MPEG-2. From observing bandwidth used by each PID (shown by TSReader) I see an average of around 2-3 Mbps for SD channels and around 12 Mbps for HD. The highest I've seen is around 16 Mbps. This compares with about 13-14 Mbps for Showtime and HBO HD and 17+ Mbps for HDNet on Dish Network.
All audio in the DCII system uses Dolby's AC3 system. SD channels and Music Choice run 192Kbps and HD channels 384Kbps.
Virtually all the programming on Voom is scrambled. The exception being their barker channel (Inside Voom) and the Music Choice channels. The barker channel is encoded at 1440x1080 (2/3 horizontal resolution) and then interpolated out by the receiver. This is very similar to the way in which most DBS service providers encode SD material at 2/3 resolution (see
http://www.coolstf.com/mpeg for more info).
The Music Choice channels also have associated on-screen graphics while the channel is tuned. These are delivered as MPEG-2 still pictures at very low rate and refresh every few seconds. These are encoded at 544x480 progressive.
In the MPEG-2 transport model, when the encoders multiplexed together don't have enough data to fill the bandwidth of the channel (a satellite transponder in this case), null packets are inserted into the stream. By measuring the bitrate of these null packets one can see the available room left in the channel for more data.
Taking a look through each transponder used by Voom, some are already quite full with about 2% null packets. This is very typical for VBR encoded DBS services - Dish Network is pretty much the same way. Quite a few transponders (6 or more - don't remember the exact number) had 30 to 40% null packets, so there's plenty of room left in the system for more channels.
That's about it. Real pitty that just about everything there is scrambled. Would have hoped during their free preview to customers they'd keep the CA turned off.
Oh, must also nominate the guy on their barker channel as Biggest Douchebag in the Universe - quite clearly stealing the title from John Edward
Cheers,
Rod
