MSB MODULATION DOUBLES CABLE TV CAPACITY Harold R. Walker and Bohdan Stryzak Pegasus Data Systems ( 5/12/06) pegasusdat@aol.com Abstract: Ultra Narrow Band Modulation ( Minimum Sideband Modulation ) makes it possible to insert a 12 Mb/s digital channel between each of the present analog channels on a cable TV system. A 100 channel analog system then becomes a 200 channel system, with 100 analog and 100 digital channels. This unique phase modulation method has no Bessel sidebands to interfere with adjacent analog channels. The spectrum consists of a single frequency that is inserted between the aural carrier of one channel and the video carrier of the channel above. All spectral spread is at such low levels that it is considerably below the noise level of the cable system. The presently accepted signal and noise levels for cable TV are easily met. The 12 Mb/s data rate can be increased if necessary to more than 20 Mb/s without ill effect. The method is also useful for FM-SCA at very high data rates. Applications for the method include Cable Internet uses, as well as digital TV. MPEG2 and MPEG4 encoded digital video can be transmitted with excellent motion and definition. The additional digital channels added by the MSB method described can be used for pay per view TV, Internet downloads, high speed data transfer and distribution, digital audio, or any other use for which digital data modulation can be used. MSB makes it possible to use the latest MPEG4, or H.264 technology, which gives excellent quality video at 2.00 Mb/s, or higher, on the added digital channels. The present hardware utilizes data rates up to 20 Mb/s transmitted on a single phase modulated frequency that has no useful sidebands. Digital data has been transferred using this method, and work is underway to transmit HDTV using this method with H.264 encoding. Table 1 shows TV resolution and data rates using H264. Ultra Narrow Band Modulation is a radically different modulation method that confines the useful modulation to a single frequency spectral line. The modulation method is a phase modulation method that utilizes abrupt phase change modulation together with zero group delay filtering to preserve the instantaneous phase shifts. The phase shifts are imposed on the carrier alone. Sidebands are not required. This differs radically from the commonly used CPFSK methods presently in use with Nyquist bandpass filters. The phase change is accompanied by some very weak Fourier sinx/x spectral components seen in Figure 1 at +- 6 MHz that can be ignored, since they are below FCC minimums for most services and can be completely submerged below system background noise. The detected phase change for the NRZMSB variation shown in Figure 1 is +-45 degrees, the same as for GMSK modulation. Another MSB variation utilizes +- 90 degree modulation. The spectrum of MSB modulation, which is the result of the patented proprietary encoding and modulation method, has all skirt peaks at -45 db below the modulation peak f c, which is 10-15 db below the adjacent video carrier, for a total of -55 db. This is approximately 10 db below the system background noise level. Cable TV standards require that adjacent channel interference, or the background noise, be - 45 db below the channel being viewed. The spectrum of the MSB channel after one stage of head end 1
narrow band filtering is shown in Fig 1. Figure 1 shows the spectrum of the MSB modulated signal with one stage of zero group delay filtering at the transmitter ( head end ). The MSB carrier frequency is approximately 1.25 MHz below the video carrier above, which is far enough below so that it does not interfere with the channels above or below. The baseband code for Figure 1 is the standard NRZ baseband input to the modulator with 90 degree phase modulation. There are alternate MSB codes that have a slightly different spectrum while still meeting applicable specifications. Tests over a cable system have shown that a single frequency type of modulation, such as that using MSB, can be inserted in the region between the aural carrier of a channel and the video carrier of the channel above, as shown in Fig. 2, without interference. Fig. 2 shows the position and signal strengths of the carriers for Channels 5 and 6. Other channels can also be used. Cable TV photos and spectral plots for insertion between channels 2 and 3 are appended. Figure 1 shows a signal between channels 2and 3. The minimum Sideband carrier is seen at the exact center. The video carrier of channel 6 is immediately above and the sound carrier of channel 5 immediately below. The injection level is the same as the channel 5 sound carrier. calculates to be 250 KHz wide. 81.999125 MHz was chosen for development purposes. Tests using the MSB spectrum show that the injected signal will not cause interference to either Ch. 5 or Ch. 6, or to any other channel, if the inserted signal f c is at the power level of the aural carrier of Channel 5. The injected MSB signal is at 81.999125 MHz, adjusted so as not to cause herringbone in the adjacent video channels. Fig. 3. The same applies to all other channels where the MSB carrier is inserted between the aural and video carriers. Examples showing the MSB signal between channels 2 and 3 are shown as well. A Cable TV head end is normally used to receive and add together a number of TV channels so that they may be distributed over the cable. The power levels of the individual channels are equalized so that they have a uniform power level at the receiving end. Regulations require that any noise or added interfering signals be at -45 db below the video level. The injection level to the cable using MSB, as shown here in Fig. 2, is -15 db relative to the video, so the RMS, or mean power level of the lower spikes and Fourier 'grass' in Fig. 1, needs to be -35 db relative to the MSB signal peak, or below. This is easily achieved with MSB modulation and special zero group delay filtering. It has been demonstrated that low levels of amplitude modulation can be used on top of the present single frequency MSB phase modulation. This amplitude modulation can be used for service messages such as welcome, good bye, please wait etc. The method meets the FCC and Cable TV industry requirements regarding interference, while enabling the transmission of digital data at very high data rates, without increasing the overall spectrum used, thus effectively multiplying the cable capacity by 2, that is, 1 analog channel plus 1 digital channel at 6 to 12 Mb/s per analog channel. It has been shown that this can be increased to as high as 20 Megabits/second for HDTV using H.264. The space available for additional channels, as seen in Fig. 2, is the region about 50 khz above the aural carrier of Channel 5 at 81.75 MHz and about 50 khz into Channel 6 from 82 MHz. That is, from 81.80 to 82.050 MHz, a region that Figure 3 is a photograph of a portion of the TV screen when using an MSB signal inserted between the aural carrier of channel 5 and the video carrier of Channel 6. The program is the "Home Shopping Network", which was advantageous for this illustrative purpose because it leaves large expanses of white areas to look for
the herringbone pattern that comes from an interfering signal. As can be seen, there is no visible interference. A photo showing the unwanted herringbone is attached. Some individual analog channel space can be taken over exclusively by MSB signals. A single 6 MHz analog channel can theoretically take 80-100 MSB channels. Thus a 100 channel analog cable system could carry 200 MSB channels at 12 Mb/s and 99 analog channels of normal analog TV programming. Two way cable systems can employ MSB for the back haul, or up link. In this application, the channels are independent, occupying frequencies between 10 and 50 MHz. Channels can be separated 60-100 khz apart on the backhaul, transmitting data on each channel at 3 to 6 Mb/s. The data rates are lower than for the down haul, but not noticeably so. patent applications and PCT filings have been made. Further details including schematics and performance measurements are available from: www.vmsk.org Table 1. Courtesy Silicon Valley Tech and Mobilygen. Using H.264 coding, VHS or DVD quality video is said to be obtainable at data rates as low as 944 kb/s and HDTV quality at 4-6 Mb/s and up. For internet users, data can be sent down line at 12.0 Mb/s. Up stream, the rate can be 2-4 Mb/s per user. See table 1. MSB operates with a much lower signal to noise level than QAM. QAM 64 requires about 34 db S/N for 10-6 BER. MSB will operate at 10-12 db S/N error free. The detected phase shift angle is 90 degrees, the same as for GMSK, but the filter noise bandwidth is much narrower. The receiver IF filter noise BW is only 2-3 khz wide at the 3 db points. This allows the large number of MSB channels to be used, since they need not be at high power levels. FM Supplementary Carriers ( FM-SCA) can also use Minimum Sideband Modulation. The frequencies involved are above those for channel 6, so the same high data rates can be expected. Digital video, even HDTV, can easily be carried on such a supplementary carrier. The physical principle behind MSB was published in 1939. It did not become a practical method for transmitting digital phase modulation until the zero group delay filters were devised in 1996. An earlier version utilizing single sidebands was introduced in 1996 ( US Pat. 5,930,303 ). The present carrier altering method was introduced in 2001. ( U.S. Pat 6,445,737 ). Several other
Figure 3. The NRZMSB signal shown in Figure 1injected at the same level as the aural carrier of Channel 5. The grass hump seen in Fig. 1 is 10 to 15 db below the normal system background noise level ( system grass ). 2
Figures 4 and 5. Figure 4 shows the spectrum without the MSB modulation and Figure 5 is with the MSB modulation at the exact center. It is a single frequency line bearing phase modulation at the level of the sound carrier below. 3
Figure 6. The spectrum showing the MSB signal between Channels 2 and 3 at the left. The MSB carrier is at 59.98 MHz. The video carrier for Channel 4 is seen at the right. There is a slight interference pulse above the Channel 3 audio caused by a harmonic from the 59.98 MHz signal. This is of no consequence and can be removed. In a multiple MSB channel system, another MSB signal equal to the Channel 3 sound carrier would be inserted. There is no interference from low level harmonic spurs if the new signal is more than 12 db stronger. Interference pulses of this nature have a16 nanosecond duration and normally do not affect the video due to the video bandpass filter group delay.
Figure 7. When using a repeating fixed data pattern as opposed to random data, which would not normally be the case, and with the injected level too high, a herringbone pattern results. This is not seen with proper injection levels and random data. This example was a 10100000 pattern injected at the same level as the video ( 10-15 db above the normal aural level ) Figure 8. Normal injection level and random data. The MSB Signal is at 59.98 MHz. This is a channel 2 news photo taken 12/20/05. Note the absence of herringbone from the MSB
signal injected between channels 2 and 3. Figure 9. Channel 3 with normal signal injection level. There is no noticeable change in the picture when the MSB signal is turned on and off. When the data rate is 6 Mb/s, as in the channel 2-3 example, there are sinx/x spikes at 66, 72, 78, 84 and 90 MHz. They fall within the video band of channels 5 and 6. Very careful observation with the signal being turned on and off showed no sign of interference. TV Set Comcast Cable from street Tap Spectrum Analyzer 1 Isolation Amp. 2-47dBm video -60 dbm MSB Modulator Data Source MSB Detector Decoder D C D C Cable TV Test Setup Figure 10. Test Setup. MSB modulation is added to signals from cable, then separated and detected.