DVB Basics discussion thread

[Doom9]

I have just completed a lenghty article on DVB. I spend the entire day looking for info and I hope I didn't commit too many blunders, but it isn't a really scientific work so I'd appreciate comments and corrections. I chose to have a separate thread in order not to have the sticky too cluttered. I'll collect info from this thread to adapt the sticky in due time.

[Pyscrow]

Doom 9,
Very good technical article, perhaps I could suggest an addition for the common man? (Info based on my experiences in Australia with our "unique" DVB-T setup)

1. Standard defintion Digital TV is basically DVD quality.
IE 704 x 576 picture
roughly 7 to 8000000 Bits/sec
256 kbs sound stereo or AC3
25 fps PAL


So if wishing to record and burn to DVD for use in a DVD player (I personally prefer to do this and watch the "timeshifted" program on my widescreen TV, rather than the computer - which is useless if the family want to watch a show together.), this is the channel to record, not High Definition. High definition here varies, but the better stuff is 50fps, and to high a bitrate for compliant DVD use.

2. If you are wanting to record a show specifically for a purpose, "shop about" within the broadcasts within the transport stream to suite your needs. Typically here in Australia each TV company broadcasts several SD versions of their channel, one HD version and a digital radio station or two within their transport stream

IE if you have a wizbang 5.1 stereo setup connected to your TV, and want to record music shows for later viewing through the TV, make sure the channel you record has AC3 sound, not plain stereo (I find this real handy, as my favorite music shows here in Australia runs typically from midnight through to 6:00am). If you only have a standard 2 channel stereo, record a channel with only stereo sound.

[chipsugar]

It is a very good article. Your guide states that internet access can only be gained through the satellite cards though but here in the uk AIUI the cable cards can act as a broadband modem.

[Joe Fenton]

Not bad, but you had one part wrong.

So, now we know why a DVB-S receiver can't handle DVB-C/-T broadcasts and the other way round.

Systems made from LSI chips use the exact same circuitry for all three. In fact, it is exactly the same circuitry as used for LSI based DVD players with the addition of a chip to handle the Cable/Satellite/Antenna replacing the chip that handles the DVD.

Motorola has a similar chipset, but DVB-T requires one extra chip. The only difference between cable boxes and satellite boxes is generally just a jumper on the board. The LSI "tuner" chip automatically handles the difference between the frequencies and modulations of the different formats. You just tie some lines high or low to tell it what it should be looking for.

[no_operation]

Nice and really informative.

Could use some more details on LNB access however. A few bits that I know:

Some of the stuff said about multiple satelites also applies to single satelites. For example, the european ASTRA's at 19.2E transmit in two bands (high band and low band) and with horizontal/vertical polarization. Astra transmits at 10.6-11.6GHz (low band) and 11.6-12.7GHz (high band). The LNB maps the signal down to 950-2050MHz for cable distribution, lowering the frequency by the LOF (local oscillator frequency) which is usally 9.75GHz for low band and 10.6GHz for high band.

On one cable, only one of the 4 combinations (hor/low, vert/low, hor/high, vert/high) can be transmitted, selected by LNB power supply level (14/18V) for H/V switching and a modulated 22KHz tone for selecting the high band. Because of that, using the loop-through of a receiver usally doesnt work and may be even dangerous, if the 14V and 18V power supplies collide.

There are different LNB's available for ASTRA reception: Old analogue-only and newer high-band capable 'digital' one's. Simple 1, 2, 4 or 8 port LNB's (Single, Twin, Quad and Octo) have a built-in switching matrix (multiswitch) and can serve one receiver per connector. And there are 2 or 4 ported LNB's (Dual and Quattro) that output the 2 (lowband) or 4 (low+high) signals directly. These require a multiswitch connected to them, direct receiver connections wont work.

[Latexxx]

Originally posted by Pyscrow
Doom 9,
Very good technical article, perhaps I could suggest an addition for the common man? (Info based on my experiences in Australia with our "unique" DVB-T setup)

1. Standard defintion Digital TV is basically DVD quality.
IE 704 x 576 picture
roughly 7 to 8000000 Bits/sec
256 kbs sound stereo or AC3
25 fps PAL

Unique? I don't think so. The Finnish version has some channels with very high quality (I once managed to play a movie from DVD simultaneously with the DVB-T broadcast and when changed the source the only thing that revealed DVB was the font of the subtitles) but on the other hand there are two low budged channels broadcasting at 1000000 - 3000000 bits per second.
There are also working versions of DVB-T running at least in Berlin and UK.

[Doom9]

@psycrow: I've added audio bitrates and a lot of info regarding frame sizes, framerates and aspect ratios both for SDTV and HDTV.

@Joe Fenton: Do you know of any receiver / card where you can actually flick a switch that makes it work with another DVB standard?

@chipsugar: You have a link to such a card and a cable provider offering these services? I thought cable providers used the DocSIS standard for broadband Internet access... I'd suppose that a standard uniquely created for data transmission is more suitable than DVB (I read something about a 3mbit/s maximum.. and I know cable providers may want to go higher than that so another standard seems to be more suitable).

@no_operation: thanks for the info.. I've already added it.

[Stux]

Great article :)

I thought I'd just throw a few more points in about Australian DVB-T

It operates on a 7Mhz band, I think this is due to historical reasons (ie the old analogue system was 7Mhz too)

Also, I believe 576p is a valid 'HDTV' resolultion 720x576x50 progressive I think.

A Question, is low band/high band (in satellite terms) the same as Ku-Band and C-Band?

[Doom9]

@stux: 720x576p @ 50 fps is already in my document, so it is indeed supported.

Is there a specs addendum describing terrestrial transmission in a 7 MHz band? I've not found anything about larger bands so far (except for the Japanese standard).
Also, what's the most commonly used DVB standard down under? I keep hearing about DVB-T, but in most other parts of the world, DVB-S is the most widely used DVB standard and DVB-T is just making an entrance.

[Joe Fenton]

Originally posted by Doom9
@Joe Fenton: Do you know of any receiver / card where you can actually flick a switch that makes it work with another DVB standard?


LSI makes reference boards for Professionals using their chipsets. Back when DVB-S/C was new, they had a reference board which plugged into the SCSI (I think) and gave DVB-S/C decoding under computer control. It was, however, priced for "professionals", i.e., about $3000 USD. I wasn't able to find the info on the board on their website anymore, so they are probably discontinued. They have moved on to bigger and better chips, including DVB-T decoding and DVD/HDTV real-time encoding/decoding chips. That is what they are pushing now.

Given that someone made the pcHDTV card for Linux, someone could probably use the chips out to make a card that did all formats for not much more. That would be an interesting project for the folks here - to design a card to handle the DVB formats and the software to use it. Someone could even do the circuit board layout and have the gerber data online, plus a list of websites of companies which do custom boards for a price, and websites for parts suppliers. Kind of a do-it-yourself project for the more technically inclined. Someone could even put together small numbers of boards for a fee for those not so technically inclined.

The basic design would be a tuner section which handled the different standards, then the section that decoded the transport stream and handled the encryption (one chip, usually has the smart card support as well), then you would either have another chip that did the MPEG2(or better) decoding, or just feed the data to the computer for software decoding. The first puts less load on the system, and the second is cheaper.

One problem is that here in the US, DirecTV is suing anyone who purchases smartcard programmers on the basis that they must be pirates. ANYONE! They are as bad as the RIAA. People with a legitimate need for the things have been forced to buy the programmers from out of the country to avoid DirecTV's lawyers.