symbol rate to bps

[kami2k]

Hi
i want to know how i can convert symbol rate to bps ?
thx

[kami2k]

any one...!!!?

[TheCoder]

It depends on the type of modulation.

For instance, QAM64 is a system that uses 6 bits per symbol so the bitrate would be 6 * symbol rate. QAM256 uses 8 bits per symbol, so bitrate would be 8 * symbol rate.

QPSK is very similar to QAM4 in that only 2 bits are encoded per symbol, so bitrate is 2 * symbol rate. Interestingly, the 27,500,000 symbol rate seen on a lot of satellites translates to a QAM256 rate of 6,875,000 which is seen on quite a few cable networks.

In all cases, if you know the number of bits per symbol then the bitrate is a simple multiplication.

[kami2k]

so a 47,000 symbol rate will be an aravage of 4mbps?
how it can be then 1080i? since buray are true hd and have 30mbps, which is quite huge as compared to 1080i?

[TheCoder]

so a 47,000 symbol rate will be an aravage of 4mbps?

As I say, it depends on the modulation used. The symbol rate on its own means nothing - you have to know how may bits are encoded into each symbol to be able to work out the bitrate.

For sat, a 27500 symbol rate (its quoted in Ksyms/sec - so actually 27,500,000 symbols/sec) using QPSK would work out at a bitrate of around 55Mbit/sec.

Your 47,000 would actually be 47,000,000 symbols/sec so, if QPSK is being used (which may not be the case if the sat is dvb-s2 rather than dvb-s) then your going to be getting a bitrate of 94Mbits/sec. I'm not sure where your sourcing this data from but, tbvh, such a high symbol rate seems excessive. Most transponders couldn't handle such a high datarate.

Note that the transducer bitrate is usually shared between multiple services. On a 55Mbit/sec datastream you would likely get 10 or more standard TV channels and all the attendant control data. Knowing the bitrate of a transducer tells you nothing about the bitrate of the underlying channels.

how it can be then 1080i? since buray are true hd and have 30mbps, which is quite huge as compared to 1080i?

Well, if it is 47,000 kSymb/sec (and a 94Mbit/sec datastream) then it could likely handle 8-10 HD channels. You have to remember that the transmitted data is highly compressed, probably in Mpeg4 so a typical HD channel may require only 7Mbit/sec of bandwith yet de-compress to a picture bitstream of over 25Mbit/sec. To see the real bitrate of a particular channel you really need to view the decompression statistics of the codec being used. Quite a few boxes will tell you the resolution and instant bitrate as part of the channel banner. Remember though that the channel bitrate is highly variable depending on what is changing within the picture.

[kami2k]

thanks for detail answer, here is an example of tp on 100E

12642 V |National Geographic Channel HD Asia | DVB-S2 |MPEG-4/HD |Conax |40700-2/3 |8PSK |

so it have average of 12megabytes/s ?

[TheCoder]

ok, the modulation is 8PSK so that encodes 3 bits per symbol rather than the 2 bits per symbol used by QPSK (ie, its a more efficient modulation system as it encodes 8 possible states per phase change rather than the 4 states of QPSK)

So, with a 40700Ksymbol/sec symbol rate the basic transducer bitrate will be 40700000 * 3, or 122100000 or 122.1Mbits/sec. Some of this basic bit-rate will be used for things like error correction - in this case the FEC is 2/3 so that tells you that 1/3rd of the basic bitrate is error correction info and 2/3 is actual data. So, actual bitrate out of the demodulator/error correction circuitry will be around 91.6Mbits/sec

Now, that bitrate will be divided between all services on the transponder. That could easily be 4-5 HD channels plus some audio channels plus any control data necessary (epg data, emm/ecm encryption data, teletext, subtitles etc, etc....).

You cant really say anything about the final bitrate of any particular channel as the data for those channels will be highly compressed using mpeg4. If you have 7Mbps going into a codec, you could quite easily have a decompressed 25Mbps datastream coming out but even that doesn't really tell you very much. Mpeg4 is usually used with variable bitrate datastreams - at any time the bitrate can vary between just a few kbps to many Mbps almost instantly, depending on how much action is occuring on the screen. With a static picture you dont need any change so the bitrate could be a theoretical 0. Changing pictures obviously need more information to describe those changes.......

What info are you actually looking for ?

If its the bitrate of a particular HD channel then its impossible to say as (as I say above) the bitrate is variable between 0 and the upper limit of the codecs compression abilities (the compressor will be at the channels headend, your box contains the codec decompressor).

If all you want is the basic transducer bitrate then 122.1Mbits/sec is the answer you want.

If your trying to store a whole captured TS on disk then your more likely to be needing to save around 12Mbytes/sec of data to disk.

If your just trying to save a single channel, then its probably going to be around 1Mbyte/sec of data (you'll be storing it in its compressed Mpeg4 state).

Note the last figure is a highish estimate - the actual rate will vary depending on the content of the video stream. The numbers just give an idea of what you might expect for content with a fair amount of motion.

[kami2k]

ooooooooh ic, now i got the point, thanks very much