<---- -- akhirnya resmi jg keluar dari kampuzz inih, horey horey ... LOLOS euy

The first that im do in the morning, i woke up. after that i get for pray? like usualy then go to my computer and get the reminder.


Oh, i missed if this day are the new year in hijri calendar,
oh, god forgive me about ay false in the previous years, beside that i miss with my mother at home, i pray for my beloved father..

<in progress>

What's MCPC?

MCPC stands for Multiple Channel Per Carrier. Given an average satellite transponder with a bandwidth of 27MHz, typically, the highest symbol rate that can be used is SR 26MS/s. Obviously, with this large bandwidth, multiple video or audio channels can be transmitted via the transponder at the same time.

MCPC uses a technique called Time Division Multiplex to transmit the multiple programs at the same time. As one can expect from the name, TDM sends data for one channel at a certain time and then data for another channel at another time.

Many encoder manufacturers are currently experimenting with statistical multiplexing of MPEG-2 data. Using this technique, channels that need high data rate bursts in order to prevent pixelization of the picture (such as live sports events), will obtain the bandwidth as they need it by reducing the data rate for other services that don't.

Statistical multiplexing should improve perceived picture quality, epecially on video that changes rapidly and has the advtange of requiring no changes in the receiver equipment.

What's SCPC?

SCPC stands for Single Channel Per Carrier. In the case of this type of transmission, only a part of the available transponder is used for the signal. The satellite operator can sell the remaining space on the transponder to other up linkers. SCPC is typically used for feeds rather than for direct programming. SCPC has the advantage over MCPC that the signals up linked to the same transponder can be transmitted up to the satellite from different locations (SNG trucks for example), but has the disadvantage of not being quite as efficient as MCPC because of "guard bands" which keep the SCPC signals on the same transponder separated from each other.

NBC uses SCPC MPEG-2 for its back haul feeds and is able to use up to four SCPC transmissions on a single satellite transponder (GE-1 Ku-Band). Microspace uses the same type of transponder on the same satellite, but in MCPC format and is able to transmit six video channels and a few audio channels in the same space.

What are PIDs?

MPEG-2 transmits its data in packets of 188 bytes each. At the start of each packet is a package identifier (or PID) that tells the receiver what to do with the packet. Because the MPEG-2 data stream might be in MCPC mode, the receiver has to decide which packets are part of the current channel being watched and therefore pass them onto the video decoder for further processing. Those packets that aren't part of the current channel are simply discarded.

There are typically four types of PIDs used by satellite receivers. The VPID is the PID for the video stream and the APID is the audio stream. Occasionally, a PCR PID (program clock reference) is used to synchronize the video and audio packets, however, most of the time, this data is embedded into the video stream. The forth data PID is used for data such as the program guide, information about other frequencies that make up the total package etc. This data is called the System Information and uses a PID value of between 0000 and 0014 (hex).

The System Information stream

SI packets tell the receiver about the format of the transmission along with information such as multiple language selections, program guide information and other transponders that are related to the current transponder.

The primary reason that MPEG-2/DVB receivers cannot handle Digicipher 2 and ATSC signals is because the SI packets are totally different between the two standards. In theory, it should be possible to make an MPEG-2 receiver receive DCII/ATSC, however, this would either require access to the source code of the MPEG-2 receiver's firmware (and probably a license from General Instrument) or the DCII/ATSC signal being transmitted using both DCII/ATSC and MPEG-2/DVB SI packets. This is possible (see the ATSC technical documentation page for information on how this is done), however, the audio will either have to be sent twice or the receiver will need to handle both Musicam and Dolby AC3 as this is another big difference between the systems.

Quadrature Amplitude Modulation is the cable version of QPSK. Using many different symbol phases (the initial standard for the US is 64 different phases), a given 6MHz of cable bandwidth will be able to carry the same amount of data as a single 30MHz transponder. Given a 125 channel cable system, this means that they will be able to carry 625 video and audio programs assuming compression levels where five video services are sent on a single RF channel.

Vestigal Sideband modulation (otherwise known as VSB-8) is the technique that will be used in the US for terrestrial ATSC transmission. VSB-8 uses AM transmission with phase information within the sideband. The other sideband is almost totally surpressed and a pilot carrier is inserted to help receivers initially acquire the signal. VSB-8 uses eight phases with 3 bits encoded per phase which are then reduced to two bits in the receiver. I could try to explain how it works, but Harris Semiconductor has written a much better explanation which is linked at the bottom of this page.

Satellite transponders are rather noisy communications channels are are therefore subject to a large number of errors when a signal is sent through them. Because satellite transmissions are broadcast, the receiver cannot send a message to the transmitter to say "I didn't get that last piece of information, please re-transmit it". As a result, Forward Error Correction is used, where the transmitter sends error correction information along with the actual signal so that should errors occur, the receiver can re-generate the bit stream.

FEC when used with QPSK modulation uses two forms of error correction. The first, called convolutional coding with the Viterbi algorithm code is quoted as a fraction, for example, 2/3. The fraction defines the amount of the symbol rate that's used for real data, with the remainder used error correction purposes.

After the convolutional error correction code has been removed and used as needed, a second error form of error correction is used called the Reed-Solomon code. This correction results in 188 bytes out for every 204 bytes coming in with the remainder used as parity bits to help correct any remaining errors. Additionally, the FEC scheme also uses interleaving of the data stream to prevent noise bursts from interrupting the flow of data in much the same way that CDs use it to prevent scratches from causing drop-outs.

Consider the following message:

This is a sample message

If interleaved, it might look like:

eTs haais mgi smeaesp l

Should an error occur and say wipe out the 'mgi' part of the message, the de-interleaved message will now read

This *s a sa*ple messsa*e

As a result, only single characters are missing from the message (shown here as asterix), rather than an entire word missing in the case of non-interleaved data.

As a final step, the QPSK symbols are scrambled to ensure that long runs of the same symbol value don't cause a lack of change in phase of the carrier. Since the QPSK demodulator obtains its signal clock from directly from the signal, there must be a large number of phase changes in order to re-generate the clock and of course scrambling results in this. Note: this form of scrambling is not the same as scrambling of the decoded signal.