FEATURE Phase Shifts in Digital TV I/Q Vectors Swap •how to detect phase shifts automatically •reversing inverted phase shifts •finding the synchronization byte •how a constellation diagram shows swapped vectors 140 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 03-04/2013 — www.TELE-audiovision.com www.TELE-audiovision.com — 03-04/2013 — TELE-audiovision International — 全球发行量最大的数字电视杂志 141 FEATURE Phase Shifts in Digital TV Jacek As satellite signal analyzers become more and more affordable, many satellite enthusiasts decide to buy and use them. When they start playing with their new instruments, they sometimes encounter terms not so obvious to everybody. Transponder frequency, symbol rate, FEC or polarization are commonly used and most of the users have no problem in apprehending their meaning. But I/Q vectors can be a puzzle for some of the fans. You can see “I/Q Normal” and “I/Q Inverted” (or “I/Q Swapped”) options in some analyzer screens. What does it mean? In fact, it is not anything complex and we will explain it in a simple way in this feature article. Let’s consider the simplest form of modulation used in satellite TV – QPSK. In this modulation, the sinusoidal signal amplitude remains unchanged but its phase can change at regular intervals. For example, if we have a transponder broadcasting with a symbol rate of 27.5 Ms/sec, its phase can change 27.5 million times in a second. Or we can say that one symbol lasts for sec (about 36 nanoseconds). There are four phase shifts allowed in QPSK what cor- Phase shift 45° 135° 225° 315° Symbol 00 01 11 10 responds to four different symbols. In the figures below (graph.1-3), you can see an example of a QPSK modulated carrier with all four possible phase shifts in the order: 45°, 135°, 225°, 315°. In this example, there are ■ graph 1. ■ graph 2. ■ graph 3. 142 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 03-04/2013 — www.TELE-audiovision.com four symbols sent: 00, 01, 11 and 10. Just to remind you, in QPSK, a symbol is a pair of subsequent bits. Phase shifts are produced by summing a carrier signal with the auxiliary signal of the same frequency but shifted in phase by 90°. A QPSK modulated signal can be defined as: The resulting y(t) is also a sine function but its amplitude and phase depends on the I and Q values. In QPSK modulation I and Q can be equal either to 1 or to -1. Therefore we have four different possibilities for y(t): or or or A pair of bits is assigned to each possible state of y(t) in QPSK. This is shown graphically in a constellation. (graph 4.) In other words, if the in- coming signal is shifted 45° in phase, , your receiver understands that two zero bits are being sent to it. If the signal is shifted by 135°, your box assumes that bits 1 and 0 have arrived and so on. And what will happen if we swap the I and Q vectors? This may happen if somebody unintentionally sets up the headend in a wrong way or simply will not take into account the natural vector swap that takes place in some frequency conversions. In such situation the constellation will look differently – see the graph 5. The 45° and 225° shifts produce the same bits as previously but the remaining two: 135° and 315° are swapped. So, in a continuous flow of bits, some pairs of bits will stay undistorted (00 and 11) but the other pairs will take reverse values 10 will change to 01 and vice versa. That’s the effect of inverted I/Q modulation. Some old timers can still remember the first generation of satellite receivers that in their transponder data required the user to define I/Q Normal or I/Q Inverted. More recent receiver can automatically detect I/Q inversion and reverse the operation of their demodulators accordingly. But how is it possible to detect a I/Q swap? The transport stream consists of fixed length data packets. For example the DVB standard requires the packet to have 204 bytes. The very first byte in every packet is always the same 0x47 in hexadecimal notation or simply 01000111 in binary format. It is called the sync byte as it is used for synchronization. Your receiver right after tuning to a new transponder starts looking for the 0x47 bytes to find the ones located every 204 bytes in a stream. Only in this way it can start decoding the content of the packets. If it is impossible to find regularly spaced 0x47 bytes, it is a clear indication that I/Q vectors are swapped. So, the receiver also swaps I/Q signals in its demodulator ■ graph 4. 144 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 03-04/2013 — www.TELE-audiovision.com because one inversion and another inversion recreates the normally modulated signal again. The principle described above applies also to more complex modulations like 8PSK or QAM. The only difference is that I and Q can take more values than 1 and -1 as in QPSK what results in more phase shifts and amplitude values of y(t). The effect of I/Q swap is the same: some bits remain unchanged, the others are reversed (0 becomes 1 and vice versa). However, as you already know now, it is not so difficult to detect such situation and take countermeasures - simply apply additional I/Q swaps in a receiver. Signal analyzer can detect I/Q swap on the same basis as your receiver does. QPSK modulators usually offer in their menu a possibility to invert I and Q vectors. Today, it does not make any difference to your receiver whether a transponder transmits with normal or inverted I/Q vectors. And the viewer cannot sense it in any way either. ■ graph 5.